| Section | Page |
|---|---|
| Introduction | 1 |
| 7.5-minute Digital Elevation Models | 2 |
| Characteristics | 2 |
| Data production | 3 |
| 1-degree Digital Elevation Models | 5 |
| Characteristics | 5 |
| Data production | 6 |
| 30-minute Digital Elevation Models | 7 |
| Characteristics | 7 |
| Data production | 7 |
| 15-minute Alaska Digital Elevation Models | 8 |
| Characteristics | 8 |
| Data production | 8 |
| 7.5-minute Alaska Digital Elevation Models | 9 |
| Characteristics | 9 |
| Data production | 9 |
| Geometry | 10 |
| Accuracy | 10 |
| Classification levels | 14 |
| Level 1 | 14 |
| Level 2 | 14 |
| Level 3 | 14 |
| Digital Elevation Model caveats | 15 |
| Data records | 15 |
| Sample data records | 16 |
| Sample applications | 16 |
| Appendix A. Digital Elevation Model data elements--logical record type A | 29 |
| Appendix B. Digital Elevation Model data elements--logical record type B | 35 |
| Appendix C. Digital Elevation Model data elements--logical record type C | 37 |
| Appendix D. Sample quadrilateral coordinates | 38 |
| Appendix E. Codes for State plane coordinate zones | 39 |
| Appendix F. Universal Transverse Mercator zone locations and central meridians | 41 |
| Appendix G. Parameters required for definition of map projections | 42 |
| Appendix H. National and international datums used fordigital elevation data | 47 |
| Figure | Page |
|---|---|
| 1. Structure of a 7.5-minute Digital Elevation Model, UTM meter grid | 4 |
| 2. Structure of a 1-degree Digital Elevation Model, arc-second grid | 6 |
| 3. Computation of a first data point in a 7.5-minute Digital Elevation Model west of the central meridian | 11 |
| 4. Computation of first data point in a 7.5-minute Digital Elevation Models east of the central meridian | 12 |
| 5. Geometry and nomenclature of the Digital Elevation Models file | 13 |
| 6. Preeruption isometric plot of Mount St. Helens Digital Elevation Models | 25 |
| 7. Posteruption isometric plot of Mount St. Helens Digital Elevation Models | 25 |
| Table | Page |
|---|---|
| 1. Sample Digital Elevation Model type A logical records--Mannboro, Virginia, quadrangle (7.5 minute) | 17 |
| 2. Sample Digital Elevation Model type B logical records--Mannboro, Virginia, quadrangle (7.5 minute) | 20 |
| 3. Sample Digital Elevation Model type C logical records--Mannboro, Virginia, quadrangle (7.5 minute) | 21 |
| 4. Sample Digital Elevation Model type A logical records--Reno, Nevada-California,quadrangle (west half) (1 degree) | 22 |
| 5. Sample Digital Elevation Model type B logical records--Reno, Nevada-California, quadrangle (west half) (1 degree) | 24 |
The Earth Science Information Center (ESIC) distributes digital cartographic/geographic data files produced by the U.S. Geological Survey (USGS) as part of the National Mapping Program. Digital cartographic data files may be grouped into four basic types. The first of these, called a Digital Line Graph (DLG), is the line map information in digital form. These data files include information on base data categories, such as transportation, hypsography, hydrography, and boundaries. The second type, called a Digital Elevation Model (DEM), consists of a sampled array of elevations for a number of ground positions at regularly spaced intervals. The third type is Land Use and Land Cover digital data which provides information on nine major classes of land use such as urban, agricultural, or forest as well as associated map data such as political units and Federal land ownership. The fourth type, the Geographic Names Information System, provides primary information for all known places, features, and areas in the United States identified by a proper name.
The digital cartographic data files from selected quadrangles currently available from ESIC include the following:
The digital data are useful for the production of cartographic products such as plotting base maps and for various kinds of spatial analysis. A major use of these digital cartographic/geographic data is to combine them with other geographically referenced data, enabling scientists to conduct automated analyses in support of various decision making processes.
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Manuscript approved for publication August, 1993.
This document describes five distinct digital elevation products that are distributed by the USGS in the standard DEM format:
The UTM-based DEM's (7.5-minute DEM) and the geographic-based DEM's (30-minute, Alaska, and 1-degree DEM's) are identical in logical data structure but differ in sampling interval, geographic reference system, areas covered, and horizontal and vertical accuracy. Knowledge of all of these properties is essential to ensure that the user does not exceed the useful limits of the data for required applications. The 7.5-minute UTM DEM's are available for selected quadrangles, which are indicated on a status graphic published biannually by USGS. The 1-degree DEM's are available for all of the contiguous United States, Hawaii, and portions of Alaska, Puerto Rico, and the Virgin Islands. Many of the original 1-degree DEM's are being replaced with more accurate digital models through a cooperative regridding project with DMA, scheduled for completion in 1995. As they become available, these 1-degree DEM's will replace their corresponding product. The 30-minute and Alaska DEM's are new DEM series and are available on a limited basis as projects are completed.
A 7.5-minute DEM has the following characteristics:
Profiles for 7.5-minute DEM's are generated by using a UTM cartesian coordinate system as a base. The profiles are clipped to the straight-line intercept between the four geographic corners of the quadrangle--an approximation of the geographic map boundary (neatline) as shown in figure 1. The resulting area of coverage for the DEM is a quadrilateral, the opposite sides of which are not parallel. (See appendix D for an example of UTM coordinates describing a 7.5-minute quadrilateral figure.)
The UTM coordinates of the four corners (bounds) of the DEM's are listed in the type A record, as shown in table 1, data element 11; the UTM coordinates of the starting points of each profile are listed in the type B record (profiles), table 2, data element 3. These coordinates describe the shape of the quadrilateral and the variable x, y starting position of each profile. Because of the variable orientation of the quadrilateral in relation to the UTM coordinate system, profiles intersect the east and west neatlines as well as the north and south neatlines as shown in figure 1. In addition, DEM's have profile easting values that are continuous from one DEM to the adjoining DEM only if the adjoining DEM is contained within the same UTM zone.
The 7.5-minute DEM data are produced in 7.5- x 7.5-minute blocks either from map contour overlays that have been digitized, or from automated or manual scanning of National Aerial Photography Program (NAPP) quarter quad-centered photographs or from the National High-Altitude Photography Program (NHAP) quad-centered photographs. The NHAP program was formally discontinued in 1988, however limited production using this scale source is permitted. The data are processed to produce a DEM having a 30-m sampling interval. The structure of a 7.5-minute DEM data file is shown in figure 1. (See tables for sample data records.)
Figure 1.--Structure of a 7.5-minute Digital Elevation Model, UTM meter grid.
The USGS has used four processes to collect the digital elevation data for production of 7.5-minute DEM's: (1) the Gestalt Photo Mapper II (GPM2), (2) manual profiling from photogrammetric stereomo- dels, (3) stereomodel digitizing of contours, and (4) derivation from DLG hypsography and hydrography categories or pseudo-DLG's (tagged vector contours).
The GPM2 (now discontinued) was an automated photogrammetric system designed to produce ortho- photographs, digital terrain data, and contours. An electronic image correlation component of the GPM2 measured the parallax of 2,444 points within each 9- x 8-mm area of a photogrammetric stereomodel. Of these 2,444 correlated points, subunits of 576, 1,024, or 1,600 points were collected for inclusion in the elevation model. These subunits were called patches, and the patch size was selected to accommodate various terrain conditions. The horizontal (x and y) spacing of the elevation points within each patch was approximately 182 mm at photographic scale (equivalent to a ground distance of approximately 47 ft when using photographs at 1:80,000 scale). Each of the two NHAP stereomodels used to cover a standard 7.5-minute quadrangle contained over 500,000 correlated points; these were regridded to form a DEM in the standard format. Before discontinuance, approximately 15,000 DEMs were added to the NDCDB using this autocorrelation system.
The manual profiling process uses stereoplotters, equipped with three-axis electronic digital profile recording modules, for scanning of stereomodels along successive terrain profiles. High-altitude aerial photographs are used as source material. The scan speed and distance between profiles are selected by the operator to accommodate steepness in topographic slope. The most commonly used profile separation is approximately 90-m, with elevations normally recorded every 30 m along each profile. The profiled elevation data are reformatted and regridded using a weighted four-nearest-neighbor interpolation to a regular 30-m UTM spacing, written in standard DEM format, and tested for vertical accuracy before placement in the National Digital Cartographic Data Base. Digital profile data of this type are collected as companion products during the production of orthophotographs. For stereo model digitizing of contours (now deactivated), digital contours were acquired in digital form on stereoplotters equipped with three-axis digital recording modules. Digital data were acquired as the contours were stereocompiled for 7.5-minute quadrangle maps. The contours were assigned elevation values (attributes) during the acquisition phase. The contour data were processed into profile lines, and the elevation matrix was computed at a 30-m spacing using a bilinear interpolation.
Derivation of DEM's from DLG's or pseudo-DLG's is a process that involves the use of hardware such as scanners, manual digitizers, and (or) semiautomated line followers. The hypsography and hydrography categories of DLG or pseudo-DLG data are required as input to DEM processing. Contours and spot elevations in three-dimensional coordinates are required from the hypsographic category. Lake and shoreline data are required from the hydrographic category for water body flattening. Drainage data are also extracted from the hydrography file. Elevations are computed where contours merge with the hydrographic data. Ridge lines, the opposite of drainage lines, are normally interpolated automatically by computer programs using trends within the hypsographic overlay. If necessary, the delineation of ridge lines may be accomplished by manual inputs or sophisticated medial axis computation algorithms. The DLG or pseudo-DLG are reformatted as tagged vector line strings and are input to gridding software to interpolate gridded elevations for the DEM. A subsequent processing step involves trimming the grid along the quadrangle perimeter and formatting the data into a DEM file structure. The DEM is finally processed by the DEM Editing System to validate and test the accuracy before entry into the National Digital Cartographic Data Base.
A 1-degree DEM has the following characteristics:
The majority of the 1-degree Digital Elevation Models are produced by DMA from cartographic and photographic sources. Under a cooperative agreement, selected 1-degree DEM's are being regridded by USGS from 7.5-minute DEM's and 30-minute DEM's. These data sets will be available upon completion of quality control and exchange between DMA and USGS.
Elevation data from cartographic sources are collected from
any map series 7.5 minute through 1 degree
(1:24,000 scale through 1:250,000 scale). The hypsographic
features (contours, drain lines, ridge lines,
lakes, and spot elevations) are first digitized and
then processed into the required matrix form and interval
spacing. The structure of a 1-degree DEM data file is shown in
figure 2 (see tables for sample data
records).
Figure 2.--Structure of a 1-degree Digital Elevation Model, arc-second grid.
Elevation data from photographic sources are collected by using manual and automated correlation techniques. Elevations along a profile are collected at 80 to 100 percent of the eventual point spacing. The raw elevations are weighted with additional information such as drain, ridge, water, and spot heights during the resampling process in which final elevations are determined for the required matrix form and interval spacing.
The digital elevation models distributed within the Department of Defense cover 1- x 1-degree blocks but are called Digital Terrain Elevation Data Level 1 (DTED-1). These blocks are referenced by their southwest corner coordinates rather than by the name of a corresponding map sheet. The header records and profile records are considerably different in their structure than those in the corresponding USGS-distributed version; however, in reformatting the product, the USGS does not change the basic elevation information.
The accuracy of the DMA product, together with the data spacing, adequately support computer applications that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at map scales not larger than 1:250,000. The plotting of contours from the 1-degree DEM at scales larger than 1:250,000, or reliance on the elevation heights without incorporating the National Map Accuracy Standard (NMAS) horizontal error tolerance, will lead to less reliable results.
Note that DMA 1-degree DTED-l data and USGS-distributed 1-degree DEM's are gridded by using the WGS 72 or WGS 84 datum, which is significantly different than the NAD 27 datum used by the USGS for gridding 7.5-minute DEM's. At the present time, policy issues concerning the transformation algorithms and procedures to convert to or from NAD 27 and WGS 72 to WGS 84 and (or) NAD 83 are being developed. See appendix H for additional information related to these datums.
A 30-minute DEM has the following characteristics:
The USGS uses two processes to collect the digital elevation data for production of 30-minute DEM's: (1) derivation from DLG contours of any map series 7.5 minute to 30 x 60 minute (1:24,000 scale to 1:100,000 scale), (2) resampling from digital elevation models with a source spacing equal to or less than 2-arc second sampling interval (if the data is resampled from preexisting DEMs, it is arbitrarily archived as level 1). DEM data for this series are tested according to level 2 (see Level 2, p. 14) accuracy specifications.
The accuracy of the DEM data, together with the data spacing, adequately support computer applications that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at map scales not larger than 1:100,000. The plotting of contours from 30-minute DEM data at scales larger than 1:100,000, or reliance on the elevation heights without incorporating the NMAS horizontal error tolerance, will lead to less reliable results.
15-minute Alaska DEM's have the following characteristics:
| 15 x 36 minutes | State of Alaska north of 68° N latitude |
|---|---|
| 15 x 30 minutes | Between 62° N and 68° N latitude |
| 15 x 22.5 minutes | Between 59° N and 62° N latitude |
| 15 x 20 minutes | State of Alaska south of 59° N latitude |
The 15-minute Alaska DEM data are produced to match the spatial format of the 1:63,360-scale source contours. The primary process used for production of Alaska DEM's is to combine digitized hypsographic and hydrographic data from 1:63,360-scale graphics. Processing can include all of the scanning, resampling, and contour interpolation programs previously mentioned. Data production for this series, if derived from hypsography overlays, is classified according to level 2 specifications.
The accuracy of the DEM data, together with the data spacing, adequately support computer applications that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at map scales not larger than 1:63,360. The plotting of contours from the 15-minute Alaska DEM's at scales larger than 1:63,360, or reliance on the elevation heights without incorporating the NMAS horizontal error tolerance, will lead to less reliable results.
7.5-minute Alaska DEM's have the following characteristics:
| 7.5 x 18 minutes | State of Alaska north of 68° N latitude |
|---|---|
| 7.5 x 15 minutes | Between 62° N and 68° N latitude |
| 7.5 x 11.25 minute | Between 59° N and 62° N latitude |
| 7.5 x 10 minutes | State of Alaska south of 59° N latitude |
The 7.5-minute Alaska DEM data are produced to match the spatial format of the 1:24,000- and 1:25,000- scale source contours. The collection of elevation data is primarily by raster-to-vector digitizing of map separates and gridding the resultant vectors. Data production for this series, if derived from hypsography overlays, is classified according to level 2 specifications.
The accuracy of the DEM data, together with the data spacing, adequately support computer applications that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at map scales not larger than 1:24,000. The plotting of the contours from the Alaska DEM's at scales larger than 1:24,000, or reliance on the elevation heights without incorporating the NMAS horizontal error tolerance will lead to less reliable results.
Profiles are the basic building blocks of DEM's and are defined as one-dimensional arrays, that is, arrays of dimension in m rows x 1 column, where m is the length of the profile (variable length for 7.5-minute DEM's or fixed length for 30-minute, Alaska, and 1-degree DEM's).
Figure 3 provides an example of the computation for the first data point inside the quadrilateral representing a 7.5-minute DEM west of the UTM central meridian. Figure 4 provides a similar example for a quadrangle east of the central meridian.
Figure 5, formula 1, illustrates the internal horizontal relationship (xp, yp) of elevations ordered as profiles in which the spacing of the elevations along each profile is ‘y and the spacing between profiles is ‘x. Figure 5, formula 2, relates the internal array structure to actual ground coordinates (xgp, ygp) based on an origin of the DEM at the lower left corner (xgo, ygo), and a rotation angle measured from quadrangle north, if any. The rotation angle of 7.5-minute DEM's is normally set to zero (see record A, element 13 in table 1). The rotation angle for a 1-degree DEM is always set to zero (see record A, element 13, in table 4). In general the mathematics is simpler for a 1-degree DEM. Each 1-degree DEM profile is composed of the same number of elevations per profile and the array is a square or rectangle. Therefore, the equations of figure 5 are greatly simplified.
The accuracy of a DEM is dependent upon the level of detail of the source and the grid spacing used to sample that source. The primary limiting factor for the level of detail of the source is the scale of the source materials. The proper selection of grid spacing determines the level of content that may be extracted from a given source during digitization. For example, 1:250,000-scale topographic maps are the primary source of 1-degree DEM's. Larger scale maps, such as 1:100,000 and 1:24,000, are used to generate higher accuracy DEM's. Scales smaller than 1:250,000 have not been used as a DEM source.
Another factor is the horizontal and vertical dimension of the DEM. Horizontal accuracy of DEM data depends on the horizontal spacing of the elevation matrix. Within a standard DEM, most terrain features are generalized by being reduced to grid nodes spaced at regular intersections in the horizontal plane (7.5-minute DEM interval is 30 m; 1-degree DEM interval is 3 arc-seconds). This generalization reduces the ability to recover positions of specific features less than the internal spacing during testing and results in a de facto filtering or smoothing of the surface during gridding.
Vertical accuracy of DEM data depends on the spatial resolution
(horizontal grid spacing), quality of the
source data, collection and processing procedures, and digitizing
systems. As with horizontal accuracy, the
entire process, beginning with project authorization, compilation
of the source data sets, and the final
gridding process, must satisfy accuracy criteria customarily
applied to each system. Each source data set
must qualify to be used in the next step of the process. Errors
are compounded in each step of the
process. For this reason, significant effort is expended in each
phase of the production process to
minimize errors.
Figure 3.--Computation of first data point in a 7.5-minute Digital Elevation Model west of the central meridian.
Figure 4.--Computation of first data point in a 7.5-minute Digital Elevation Model east of the central meridian.
Figure 5.--Geometry and nomenclature of the 7.5-minute Digital Elevation Model file.
The method of determining 7.5-minute DEM accuracy involves computation of the root-mean- square error (RMSE) for linearly interpolated elevations in the DEM and corresponding "true" elevations from the published maps. Test points are well distributed, are representative of the terrain, and have "true" elevations well within the DEM accuracy criteria.
Test points are located on contour lines, bench marks, or spot elevations. A minimum of 28 test points per DEM is required (20 interior points and 8 edge points). Collection of test point data and comparison of the DEM to the quadrangle hypsography are conducted by the quality-control units within the USGS.
The broad DMA production objective for a 1-degree DTED-1 is to satisfy an absolute horizontal accuracy (feature to datum) of 130 m, circular error at 90-percent probability; and an absolute vertical accuracy (feature to mean sea level) of ±30 m, linear error at 90-percent probability. The relative horizontal and vertical accuracy (feature to feature on the surface of the elevation model), although not specified, will in many cases conform to the actual hypsographic features with higher integrity than indicated by the absolute accuracy.
The following is a description of the general data characteristics used to classify DEM's into one of three levels of quality. There are varying methods of data collection and degrees of editing available for DEM data. Classification levels are indicated in the DEM record A (appendix A).
Level 1 DEM's are elevation data sets in a standardized format. The intent is to reserve this level for 7.5- minute DEM's or equivalent that are derived from scanning National High-Altitude Photography Program, National Aerial Photography Program, or equivalent photography.
A vertical RMSE of 7 m is the desired accuracy standard. A RMSE of 15 m is the maximum permitted. The intent for 7.5-minute DEM data at this level is that an absolute elevation error tolerance of 50 m (approximately three times the 15-meter RMSE) be set for blunders for any grid node when compared to the true elevation, or that an array of points not encompass more than 49 contiguous elevations and be in error by more than 21 m (three times the 7-m RMSE). Systematic errors within the stated accuracy standards are tolerated at this level.
DEM data acquired photogrammetrically by using manual profiling or image correlation techniques are restricted to the level 1 category. DEM's with a RMSE of from 7 to 15 meters in elevation are being retained as an intermediate product and eventually will be replaced by higher accuracy DEM's. The DEM record C (appendix C) contains the RMSE accuracy statistics acquired during quality control.
A 30-minute DEM product has been produced by regridding level 1 or level 2 source 7.5-minute DEM data. These grid derivative DEM's are arbitrarily labeled as level 1 DEM's and carry a measured RMSE in record C. A tolerance has not been set for this RMSE, as a minimum level of accuracy has been satisfied previously with the data sets origin in the 7.5-minute DEM program. Alternatively, 30-minute DEM's are labeled as level 2 if gridded from 1:100,000-scale hypsography and hydrography stable base separates. The tolerance for level 2, 30-minute DEM's, is one-half contour interval (see level 2 specifications, below).
Level 2 DEM's are elevation data sets that have been processed or smoothed for consistency and edited to remove identifiable systematic errors. DEM data derived from hypsographic and hydrographic data digitizing, either photogrammetrically or from existing maps, are entered into the level 2 category after review on a DEM Editing System. An RMSE of one-half contour interval is the maximum permitted. There are no errors greater than one contour interval in magnitude. The DEM record C contains the accuracy statistics acquired during quality control.
Level 3 DEM's are derived from DLG data by using selected elements from both hypsography (contours, spot elevations) and hydrography (lakes, shorelines, drainage). If necessary, ridge lines and hypsographic effects of major transportation features are also included in the derivation. A RMSE of one-third of the contour interval is the maximum permitted. There are no errors greater than two-thirds contour interval in magnitude. The DEM record C contains the accuracy statistics acquired during quality control.
The majority of the 7.5-minute DEM's produced to date are level 1. The current priority for authorization of 7.5-minute DEM production is now oriented to level 2, interpolation from digital contours. The USGS does not currently produce level-3 DEM data.
All 30-minute DEM's derived from contours are level 2. All 30-minute DEM's derived from 7.5-minute DEM's are level 1.
All 1-degree DMA DTED-l's have been classified as level 3 because the hypsographic information, when plotted at 1:250,000 scale is consistent with the planimetric features normally found on 1:250,000-scale topographic maps. Inconsistencies may exist, but these are regarded as isolated cases to be tempered by the 90-percent confidence level for the overall product. NOTE: The USGS classification of "level 3" for 1-degree DEM's is not to be confused with the DMA'S "DTED level 1." In the DMA, the term "level" is related to the spatial resolution of the data and not to the source of the data. For example, DTED level 2 (which the USGS does not distribute) would have an array spacing of 1 x 1 arc-second for latitudes south of 50° N.
A DEM file is organized into a series of three logical records are formatted as shown in appendixes A, B, and C. Because of the restructuring of new elements within what was formerly defined as element 1, record type A, the element counts of old record type A elements 2-15 (see table 1) have been incremented by one to elements 3-16 (see appendix A). A one-to-one correspondence still exists between the informational content of byte positions of elements in the old and new records. Therefore, DEM programs that expected elements 2-15 in specific byte positions in the DEM file will find the same information in those byte positions in the new files. In essence, this change is transparent to old DEM applications programs. Also, new data elements 17-29 have been appended to the end of the record A (see appendix A). These elements are contained in the end of the previously blank filled portion of the 1,024 byte record. This change is also expected to be transparent to existing DEM programs.
Appendixes D-G consist of code definitions that are needed to interpret various data elements in the three records. The type A record contains information defining the general characteristics of the DEM, including descriptive header information relating to the DEM's name, boundaries, units of measurement, minimum and maximum data values, number of type B records, and projection parameters. There is only one type A record for each DEM file, and it appears as the first record in the data file. The type B record contains elevation data and associated header information. All type B records of the DEM files are made up of data from one-dimensional arrays called profiles. The number of complete profiles covering the DEM area is synonymous with the number of type B records in the DEM. In a UTM structured DEM, an occasional profile that exists within the bounds of the DEM quadrilateral but is void of elevation grid points is not represented in the DEM. (This is called the "missing profile condition" and occurs occasionally as the first or last hypothetical profile of the DEM.) The type C record contains statistics on the accuracy of the data in the file.
The physical structure of the DEM distributed to the user is as follows:
Following are sample sets of A, B, and C records, corresponding to a typical 7.5-minute DEM (tables 1, 2, and 3), and sample sets of A and B records for a typical 1-degree DEM (tables 4 and 5). Included in these samples are literal ASCII listings of records directly from DEM distribution tapes. Following the literal listings are tabular explanations of each element in the type A, B, and C records. The tabular explanations may be used as direct references between the literal listings and the logical record type formats shown in appendixes A-C. Appendixes D-G consist of code definitions that are needed to interpret various data elements in the three records.
The DEM files may be used in the generation of graphics such as isometric projections displaying slope, direction of slope, and terrain profiles between designated points. They may also be combined with other data types such as stream locations and weather data to plan forest fire control, or with remote sensing data to classify vegetation. Many nongraphic applications such as modeling terrain gravity data for use in locating energy resources, determining the volume of proposed reservoirs, calculating the amount of material removed during strip mining, and determining landslide probability have also been developed. Figures 6 and 7 show two graphic applications of DEM's.
Table 1.--Sample DEM Type A Logical Record--Mannboro, Virginia,Quadrangle (7.5 minute)
MANNBORO,VA Free form information field
------------|Blank Filler field
-----------------------------------|
3 EMC 2 1 1 18 0.0 0.0
0.0 0.0 0.0
0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 2
2 4 0.244998676000000D+06 0.412627656700000D+07
0.245420930000000D+06 0.414014832600000D+07
0.256491863000000D+06 0.4
13981850700000D+07 0.256087907000000D+06 0.412594681300000D+07
0.470000000000000D+02 0.114000000000000D+03 0.0
10.300000E+020.300000E+020.100000E+01 1 383 00
10163008908I5 0 2 1 1
| Data Element | Content | Explanation |
|---|---|---|
| 1 | MANNBORO, VA | Quadrangle name field. |
| TEXT | Bytes 41-80, free format textual information | |
| FILLER | Bytes 81-135 reserved for future use. | |
| PROCESS | 3 | Indicates DEM was made from a DLG hypso overlay |
| FILLER | Byte 137, blank fill | |
| SECTIONAL INDICATOR | Bytes 138 -140 specific to 30-minute DEM, left blank for 7.5-minute DEM | |
| 2 | EMC | Mapping Center origin code indicating that DEM was created in Eastern Mapping Center. |
| 3 | 2 | DEM level code indicating DEM level 2, synonymous with file created from DLG to DEM interpolation software. |
| 4 | 1 | Pattern code indicating either a regular or random elevation pattern; 1 indicates a regular pattern. |
| 5 | 1 | Planimetric reference system code; 1 indicates UTM coordinate system. |
| 6 | 18 | Code defining the zone in the ground planimetric reference system (i.e., UTM zone 18). |
| 7 | 0.0 | Map projection parameters; all 15 fields are set to zero for the UTM (15 sets of 0.0) coordinate system and should be ignored. |
| 8 | 2 | Units code; 2 represents meters as the unit of measure for ground planimetric coordinates throughout this file. |
| 9 | 2 | Units code; 2 represents meters as the unit of measure for elevation coordinates throughout this file. |
| 10 | 4 | Number (n) of sides in the polygon which defines the coverage of the DEM file (usually equal to 4). |
| 11 | 0.2449986...D+06 | A 4,2 array containing the ground coordinates of the four corners of the DEM. |
| ................ | Translation to decimal format yields: 244998.7, 4126276.6; 245420.9, | |
| ................ | 4140148.3; 256491.9, 4139818.5; 256087.9 4125946.8. | |
| 12 | 0.47.....D+02 | Minimum and maximum elevations for the DEM. |
| 0.114....D+03 | ||
| 13 | 0.0 | Counterclockwise angle (in radians) from the primary axis of ground planimetric reference to the primary axis of the DEM local reference system. |
Figure 6.--Preeruption isometric plot of Mount St. Helens
7.5-minute Digital Elevation
Models generated from data obtained from
July 15, 1979, photographs. View
is from the northeast at a 45° altitude
angle; vertical exaggeration is 3:1.
Figure 7.--Posteruption isometric plot of Mount St. Helens
7.5-minute Digital Elevation
Models generated from data obtained from
September 6, 1980, photographs.
View is from the northeast at a 45° altitude
angle; vertical exaggeration is 3:1.
APPENDIXES
APPENDIX A.--Digital Elevation Model Data Elements
Logical Record Type A
_________________________________________________________________
_________________________________________________________________
_____________________________________
Type
Physical Record Format
Data (FORTRAN
ASCII Starting Ending
Comment
Element Notation)
Format byte byte
_________________________________________________________________
_________________________________________________________________
_____________________________________
1 File name ALPHA
A40 1 40
Authorized DEM quadrangle name.
Free Format Text ALPHA
A40 41 80
Free format descriptor field, contains useful information related
to digital process such as digitizing instrument, photo codes,
slot
widths, etc.
Filler ---
--- 81 135
Process Code ALPHA
A1 136
Code 1=GPM
2=Manual Profile
3=DLG2DEM (includes any DLG type process such
as CTOG or LINETRACE)
4=DCASS
Filler ---
--- 137
Sectional Indicator ALPHA
A3 138 140
This code is specific to 30-minute DEM's. Identifies 1:100,000-
scale sections. Formatted as XNN, where X is "s"=7.5-minute,
"F"=15-minute, and NN is a two-digit sequence number.
2 MC origin code ALPHA
A4 141 144
Mapping Center origin Code. Valid codes are EMC, WMC,
MCMC, RMMC, FS, GPM2.
3 DEM level code INTEGER*2
I6 145 150
Code 1=DEM-1
2=DEM-2
3=DEM-3
4 Code defining INTEGER*2
I6 151 156
Code 1=regular
elevation pattern
2=random is reserved for
(regular or random)
future use.
APPENDIX A.--Digital Elevation Model Data Elements
Logical Record Type A--continued
_________________________________________________________________
_________________________________________________________________
_____________________________________
Type
Physical Record Format
Data (FORTRAN
ASCII Starting Ending
Comment
Element Notation)
Format byte byte
_________________________________________________________________
_________________________________________________________________
_____________________________________
5 Code defining INTEGER*2
I6 157 162
Code 0=Geographic
ground planimetric
1=UTM
reference system
2=State plane
For codes 3-20, see appendix F.
Code 0 represents the geographic (latitude/longitude) system
for 30-minute, 1-degree and Alaska DEM's. Code 1 represents
the current use of the UTM coordinate system for 7.5-minute
DEM's.
6 Code defining INTEGER*2
I6 163 168
Codes for State plane and UTM coor-
zone in ground
dinate zones are given in appendixes
planimetric
D and E for 7.5-minute DEM's. Code
reference system
is set to zero if element 5 is also set to zero defining data as
geographic.
7 Map projection REAL*8
15D24.15 169 528
Definition of parameters for various projections
parameters (see
is given in appendix F. All 15 fields of this
appendix F)
element are set to zero and should be ignored when geographic,
UTM, or State plane coordinates are coded in data element 5.
8 Code defining unit INTEGER*2
I6 529 534
Code 0=radians
of measure for
1=feet
ground planimetric
2=meters
coordinates through-
3=arc-seconds
out the file
Normally set to code 2 for 7.5-minute DEM's. Always set to
code 3 for 30-minute, 1-degree, and Alaska DEM's.
APPENDIX A.--Digital Elevation Model Data Elements
Logical Record Type A--continued
_________________________________________________________________
_________________________________________________________________
_____________________________________
Type
Physical Record Format
Data (FORTRAN
ASCII Starting Ending
Comment
Element Notation)
Format byte byte
_________________________________________________________________
_________________________________________________________________
_____________________________________
9 Code defining unit INTEGER*2
I6 535 540
Code 1=feet
of measure for
2=meters
elevation coordinates
Normally code 2, meters, for
throughout the file
7.5-minute, 30-minute, 1-degree, and Alaska DEM's.
10 Number (n) of sides INTEGER*2
I6 541 546
n=4.
in the polygon which
defines the coverage
of the DEM file
11 A 4,2 array con- REAL*8
4(2D24.15) 547 738
The coordinates of the quadrangle
taining the ground
corners are ordered in a clockwise
coordinates of the
direction beginning with the
four corners for
southwest corner. The array is
the DEM
stored row-wise as pairs of eastings and northings.
12 A two-element array REAL*8
2D24.15 739 786
The values are in the unit of
containing minimum
measure given by data element 9 in
and maximum eleva-
this record.
tions for the DEM
13 Counterclockwise REAL*8
D24.15 787 810
See figure 5. Set to zero to align
angle (in radians)
with the coordinate system specified
from the primary
in element 5.
axis of ground
planimetric refer-
ence to the pri-
mary axis of the DEM
local reference
system
APPENDIX A.--Digital Elevation Model Data Elements
Logical Record Type A--continued
_________________________________________________________________
_________________________________________________________________
_____________________________________
Type
Physical Record Format
Data (FORTRAN
ASCII Starting Ending
Comment
Element Notation)
Format byte byte
_________________________________________________________________
_________________________________________________________________
_____________________________________
14 Accuracy code for INTEGER*2
I6 811 816
Code 0=unknown accuracy
elevations
1=accuracy information is given
in logical record type C.
15 A three-element array REAL*4
3E12.6 817 852
These elements are usually set to; 30, 30, 1 for
of DEM spatial resolu-
7.5-minute DEM's; 2, 2, 1 for 30-minute DEM's;
tion for x, y, z. Units of
3, 3, 1 for 1-degree DEM's.
measure are consistent
2, 1, 1 for high resolution DEM's in Alaska
with those indicated by data
3, 2, 1 for low resolution DEM's in Alaska.
elements 8 and 9 in this
7.5-minute DEM'S will eventually be converted
record
to geographics, i.e., 1,1,1.
16 A two-element array INTEGER*2
2I6 853 864
When the row value m is set to 1 the
containing the num-
n value describes the number of
ber of rows and
columns in the DEM file. Raw GPM
columns (m,n) of
data files are set to m=16, n=16.
profiles in the DEM
Note: Old format stops here
-----------------------------------------------------------------
-----------------------------------------------------------------
-----------------------------------------------------------------
-----------------------------------------------
17 Largest primary INTEGER*2
I5 865 869
Present only if two or more
contour interval
primary intervals exist.
18 Source contour INTEGER*1
I1 870
Corresponds to the units of the map
interval units
largest primary contour interval
0=N.A., 1=feet, 2=meters.
19 Smallest primary INTEGER*2
I5 871 875
Smallest or only primary contour interval
20 Source contour INTEGER*1
I1 876
Corresponds to the units of the map
interval units
smallest primary contour interval.
1=feet, 2=meters.
APPENDIX A.--Digital Elevation Model Data Elements
Logical Record Type A--continued
_________________________________________________________________
_________________________________________________________________
_____________________________________
Type
Physical Record Format
Data (FORTRAN
ASCII Starting Ending
Comment
Element Notation)
Format byte byte
_________________________________________________________________
_________________________________________________________________
_____________________________________
21 Data source date INTEGER*2
I4 877 880
YYMM two-digit year and two-digit month
MM = 00 for source having year only.
22 Data inspection/ INTEGER*2
I4 881 884
YYMM two-digit year and two-digit month.
revision date
23 Inspection/ ALPHA*1
A1 885
"I" or "R"
revision flag
24 Data validation INTEGER*1
I1 886
0= No validation performed.
flag
1=TESDEM (record C added) no quali-
tative test (no DEM Edit System
[DES] review).
2=Water body edit and TESDEM run.
3=DES (includes water edit) no
qualitative test (no TESDEM).
4=DES with record C added, qualita-
tive and quantitative tests for
level 1 DEM.
5=DES and TESDEM qualitative and
quantitative tests for levels 2
and 3 DEM's.
25 Suspect and void INTEGER*1
I2 887 888
0=none
area flag
1=suspect areas
2=void areas
3=suspect and void areas
APPENDIX A.--Digital Elevation Model Data Elements
Logical Record Type A--continued
_________________________________________________________________
_________________________________________________________________
_____________________________________
Type
Physical Record Format
Data (FORTRAN
ASCII Starting Ending
Comment
Element Notation)
Format byte byte
_________________________________________________________________
_________________________________________________________________
_____________________________________
26 Vertical datum INTEGER*1
I2 889 890
1=local mean sea level
2=National Geodetic Vertical
Datum 1929 (NGVD 29)
3=North American Vertical
Datum 1988 (NAVD 88)
27 Horizontal datum INTEGER*1
I2 891 892
1=North American Datum 1927 (NAD 27)
2=World Geodetic System 1972 (WGS 72)
3=WGS 84
4=NAD 83
5=Old Hawaii Datum
6=Puerto Rico Datum
7=NAD 83 Provisional (shifts in
horizontal coordinates are
computed, but old DEM nodes
are not resampled)
28 Data Edition INTEGER*2
I4 893 896
01-99 Primarily a DMA specific field.
29 Percent Void INTEGER*2 I4 897 900 If
element 25 indicates a void, this
field
(right justified) contains the
percentage of nodes in the file set
to
void (-32,767).
_________________________________________________________________
_________________________________________________________________
_____________________________________
APPENDIX B.--Digital Elevation Model Data Elements
Logical Record Type B
_________________________________________________________________
_________________________________________________________________
_____________________________________
Type
Physical Record Format
Data (FORTRAN
ASCII Starting Ending
Comment
Element Notation)
Format byte byte
_________________________________________________________________
_________________________________________________________________
_____________________________________
1 A two-element array INTEGER*2
2I6 1 12
See figure 5. The row/column
containing the row
numbers may range from 1 to m and
and column identifi-
1 to n. The row number is normally
cation number of the
set to 1. The column identification
DEM profile con-
is the profile sequence number.
tained in this
record
2 A two-element array INTEGER*2
2I6 13 24
See figure 5. The first element in the field
containing the number
corresponds to the number of rows and columns
(m, n,) of elevations
of nodes in this profile. The second
in the DEM profile
element is set to 1, specifying 1 column per B record.
3 A two-element array REAL*8
2D24.15 25 72
See figure 5.
containing the
ground planimetric
coordinates (Xgo,Ygo) of
the first elevation in
the profile
4 Elevation of local REAL*8
D24.15 73 96
The values are in the units of
datum for the
measure given by data element 9,
profile
logical record type A.
APPENDIX B.--Digital Elevation Model Data Elements
Logical Record Type B--continued
_________________________________________________________________
_________________________________________________________________
_____________________________________
Type
Physical Record Format
Data (FORTRAN
ASCII Starting Ending
Comment
Element Notation)
Format byte byte
_________________________________________________________________
_________________________________________________________________
_____________________________________
5 A two-element array REAL*8
2D24.15 97 144
The values are in the unit of
of minimum and
measure given by data element 9 in
maximum elevations
logical record type A.
for the profile
6 A m,n array of INTEGER*2
mn(I6) 6x(146 or 170)
See data element 15 in appendix A.
elevations for the
146 = max for
A value in this array would be
profile. Elevations
first block. 170 = max
multiplied by the spatial resolution
are expressed in
for subsequent blocks
value and added to the elevation of the
units of resolution
local elevation datum for the element profile (data element 4 in
this record) to obtain the elevation for the point. The
planimetric ground coordinates of the point Xgp, Ygp, are
computed according to
the formulas in figure 5.
_________________________________________________________________
_________________________________________________________________
_____________________________________
APPENDIX C.--Digital Elevation Model Data Elements
Logical Record Type C
_________________________________________________________________
_________________________________________________________________
_____________________________________
Type
Physical Record Format
Data (FORTRAN
ASCII Starting Ending
Comment
Element Notation)
Format byte byte
_________________________________________________________________
_________________________________________________________________
_____________________________________
1 Code indicating INTEGER*2
I6 1 6
Code 1=available
availability of
0=unavailable
statistics in data
element 2
2 RMSE of file's datum INTEGER*2
3I6 7 24
In same units as indicated by
relative to absolute
elements 8 and 9 of logical record
datum (x, y, z)
type A.
3 Sample size on which INTEGER*3
I6 25 30
If 0, then accuracy will be assumed
statistics in data
to be estimated rather than
element 2 are based
computed.
4 Code indicating INTEGER*2
I6 31 36
Code 1=available
availability of
0=unavailable
statistics in data
element 5
5 RMSE of DEM data INTEGER*2
3I6 37 54
In same units as indicated by
relative to file's
elements 8 and 9 of logical record
datum (x, y, z)
type A.
6 Sample size on INTEGER*2
I6 55 60
If 0, then accuracy will be assumed
which statistics in
to be estimated rather than
element 5 are based
computed.
_________________________________________________________________
_________________________________________________________________
_____________________________________
APPENDIX D.--Sample Quadrilateral Coordinates
Geographic Coordinates UTM Coordinates
Quad Corner
No. Latitude Longitude Easting Northing
1 35°30' 107°37‘30" 261897 3931463
2 35°37‘30" 107°37‘30" 262267 3945330
3 35°37‘30" 107°30‘ 273590 3945036
4 35°30‘ 107°30‘ 273238 3931169
APPENDIX E.--Codes for State Plane Coordinate Zones
Alabama, East (AL) 0101
Alabama, West 0102
Alaska (AK) 5001
thru 5010
Arizona, Central (AZ) 0203
Arizona, East 0201
Arizona, West 0202
Arkansas, North (AR) 0301
Arkansas, South 0302
California (CA) 0401
thru 0407
Colorado, Central (CO) 0502
Colorado, North 0501
Colorado, South 0503
Connecticut (CT) 0600
Delaware (DE) 0700
District of Columbia (DC) 1900
Florida, East (FL) 0901
Florida, North 0903
Florida, West 0902
Georgia, East (GA) 1001
Georgia, West 1002
Hawaii (HI) 5101
thru 5105
Idaho, Central (ID) 1102
Idaho, East 1101
Idaho, West 1103
Illinois, East (IL) 1201
Illinois, West 1202
Indiana, East (IN) 1301
Indiana, West 1303
Iowa, North (IA) 1401
Iowa, South 1402
Kansas, North (KS) 1501
Kansas, South 1502
Kentucky, North (KY) 1601
Kentucky, South 1602
Louisiana, North (LA) 1701
Louisiana, Off Shore 1703
Louisiana, South 1702
Maine, East (ME) 1801
Maine, West 1802
Maryland (MD) 1900
Massachusetts, Island (MA) 2002
Massachusetts, Mainland 2001
Michigan, Central (MI) 2102
Michigan, Central (Lambert) 2112
Michigan, East 2101
Michigan, North (Lambert) 2111
Michigan, South(Lambert) 2113
Michigan, West 2103
Minnesota, Central (MN) 2202
Minnesota, North 2201
Minnesota, South 2203
Mississippi, East (MS) 2301
Mississippi, West 2302
Missouri, Central (MO) 2402
Missouri, East 2401
Missouri, West 2403
Montana, Central (MT) 2502
Montana, North 2501
Montana, South 2503
Nebraska, North (NE) 2601
Nebraska, South 2602
Nevada, Central (NV) 2702
Nevada, East 2701
Nevada, West 2703
New Hampshire (NH) 2800
New Jersey (NJ) 2900
New Mexico, Central (NM) 3002
New Mexico, East 3001
New Mexico, West 3003
New York, Central (NY) 3102
New York, East 3101
New York, Long Island 3104
New York, West 3103
North Carolina (NC) 3200
North Dakota, North (ND) 3301
North Dakota, South 3302
Ohio, North (OH) 3401
Ohio, South 3402
Oklahoma, North (OK) 3501
Oklahoma, South 3502
Oregon, North (OR) 3601
Oregon, South 3602
Pennsylvania, North (PA) 3701
Pennsylvania, South 3702
Rhode Island (RI) 3800
South Carolina, North (SC) 3901
South Carolina, South 3902
South Dakota, North (SD) 4001
South Dakota, South 4002
Tennessee (TN) 4100
Texas, Central (TX) 4203
Texas, North 4201
Texas, North Central 4202
Texas, South 4205
Texas, South Central 4204
APPENDIX E.--Codes for State Plane Coordinate Zones--continued
Utah, Central (UT) 4302
Utah, North 4301
Utah, South 4303
Vermont (VT) 4400
Virginia, North (VA) 4501
Virginia, South 4502
Washington, North (WA) 4601
Washington, South 4602
West Virginia, North (WV) 4701
West Virginia, South 4702
Wisconsin, Central (WI) 4802
Wisconsin, North 4702
Wisconsin, South 4803
Wyoming, Zone I, East (WY) 4901
Wyoming, Zone II, East
Central 4902
Wyoming, Zone III, West
Central 4903
Puerto Rico (PR) 5301
Virgin Islands, St. Croix (VI) 5202
Virgin Islands, St. John,
St. Thomas 5201
American Samoa (AS) 5300
Guam (GU) 5400
________________
source: Federal Information Processing Standard
(FIPS) Publication 70-1, 1986
APPENDIX F.--Universal Transverse Mercator Zone Locations
and Central Meridians
Zone C.M. Range Zone C.M. Range
01 177W 180W-174W 31 003E 000E-006E
02 171W 174W-168W 32 009E 006E-012E
03 165W 168W-162W 33 015E 012E-018E
04 159W 162W-156W 34 021E 018E-024E
05 153W 156W-150W 35 027E 024E-030E
06 147W 150W-144W 36 033E 030E-036E
07 141W 144W-138W 37 039E 036E-042E
08 135W 138W-132W 38 045E 042E-048E
09 129W 132W-126W 39 051E 048E-054E
10 123W 126W-120W 40 057E 054E-060E
11 117W 120W-114W 41 063E 060E-066E
12 111W 114W-108W 42 069E 066E-072E
13 105W 108W-102W 43 075E 072E-078E
14 099W 102W-096W 44 081E 078E-084E
15 093W 096W-090W 45 087E 084E-090E
16 087W 090W-084W 46 093E 090E-096E
17 081W 084W-078W 47 099E 096E-102E
18 075W 078W-072W 48 105E 102E-108E
19 069W 072W-066W 49 111E 108E-114E
20 063W 066W-060W 50 117E 114E-120E
21 057W 060W-054W 51 123E 120E-126E
22 051W 054W-048W 52 129E 126E-132E
23 045W 048W-042W 53 135E 132E-138E
24 039W 042W-036W 54 138E 138E-144E
25 033W 036W-030W 55 147E 144E-150E
26 027W 030W-024W 56 153E 150E-162E
27 021W 024W-018W 57 159E 156E-162E
28 015W 018W-012W 58 165E 162E-168E
29 009W 012W-006W 59 171E 168E-174E
30 003W 006W-000E 60 177E 174E-180W
APPENDIX G.--Parameters Required for Definition of Map
Projections
_________________________________________________________________
_________________________________________________________________
_____________________________________
(00)* (01)**
(02) (03)
(04)
Parameter Geographic Universal
State Albers
Lambert
Transverse
Plane Conical
Conformal
Mercator (UTM) Equal Area
_________________________________________________________________
_________________________________________________________________
_____________________________________
1 *** Longitude of
any point *** Semimajor axis
of ellipsoid. If this field
within the zone
is left blank (=0),
the value for Clarke's
1866 spheroid in
meters will be assumed.
2 *** Latitude of any
point *** Eccentricity
squared of ellipsoid (e2).
within the UTM
zone If field is zero,
this will indicate a sphere. If the field is 1, this field will
be
interpreted as
containing the semiminor axis of the ellipsoid.
3 *** ***
*** Latitude of 1st
standard parallel
4 *** ***
*** Latitude of 2d
standard parallel
5 *** ***
*** Longitude of
central meridian
6 *** ***
*** Latitude of
projection's origin
7 *** ***
*** False easting in
the same units of measure as the semimajor axis of ellipsoid
8 *** ***
*** False northing in
the same units of measure as the semimajor axis of ellipsoid
9-15 (not used for projections on this page)
_________________________________________________________________
_________________________________________________________________
_____________________________________
* Projection code number.
** For the Northern Hemisphere, supplying UTM zone will result
in ignoring any given projection parameters.
*** Parameter is not applicable to projection.
Note: All angles (latitudes, longitudes, or azimuth) are required
in degrees, minutes, and arc-seconds in the packed real number
format
+DDDMMSS.SSSSS.
APPENDIX G.--Parameters Required for Definition of Map
Projections--continued
_________________________________________________________________
_________________________________________________________________
_____________________________________
(05)
(06) (07)
(08)
Parameter Mercator
Polar Polyconic
Equidistant Conic
Stereographic
Type A Type B
_________________________________________________________________
_________________________________________________________________
_____________________________________
1 Semimajor axis of ellipsoid.
If this field is left blank (=0), the value
for Clarke's 1866 spheroid in meters will be assumed.
2 Eccentricity squared of ellipsoid (e).
If field this is left blank (=0), this will
indicate a sphere.
If the field is 1, this field will be
interpreted as containing the semiminor axis of the ellipsoid.
3 *** ***
***
Latitude of Latitude of 1st
standard parallel standard parallel
4 *** ***
***
*** Latitude of 2d
standard parallel
5 Longitude
Longitude directed . . . . . .
Longitude of central meridian . . . . . .
of Central
straight down below
Meridianpole of map
6 ***
Latitude of true scale . . . . . .
Latitude of projection's origin . . . . .
7 . . . . . False easting in the same units of measure
as the semimajor axis of ellipsoid . . . . . .
8 . . . . . False northing in the same units of
measure as the semimajor axis of ellipsoid . . . . .
9 *** ***
***
Zero Any non-zero number
10-15
(not used for projections on this page)
_________________________________________________________________
_________________________________________________________________
_____________________________________
APPENDIX G.--Parameters Required for Definition of Map
Projections--continued
(09) (10)
(11) (12)
(13) (14)
Parameter Transverse Stereographic
Lambert Azimuthal Azimuthal
Gnomonic Orthographic
Mercator
Equal-Area Equidistant
1 Same as . . . . . .
. . . . . . . . . .Radius of the sphere of reference . . . . . . .
. . .
projections
If this field is left blank, the value
03 thru 08
6370997.0 meters will be assumed
2 Same as ***
***
*** *** ***
projections
03 thru 08
3 Scale factor ***
***
*** *** ***
at central
meridian
4 *** ***
***
*** *** ***
5 Longitude of . . . . . .
. . . . . . . . . . . . . . . .Longitude of center of projection .
. . . . . . . . . . .
central
meridian
6 Latitude . . . . . .
. . . . . . . . . . . . . . . Latitude of center of projection . .
. . . . . . . . .
of origin
7 False easting in the same units of measure as
the semimajor axis or radius of the sphere
8 False northing in the same units of measure
as the semimajor axis or radius of the sphere
9-15
(not used for projections on this page)
APPENDIX G.--Parameters Required for Definition of Map
Projections--continued
_________________________________________________________________
_________________________________________________________________
_____________________________________
(15)
(16) (17)
(18) (19)
Parameter General Vertical
Sinusoidal Equirectangular
Miller Cylindrical Van Der Grinten I
Near-Side Perspective
(Plate Caree)
_________________________________________________________________
_________________________________________________________________
_____________________________________
1 . . . . . . . . . . . . . . . . . . . . .
Radius of the sphere of reference . . . . . . . . . . . . . . .
If this
field is left blank, the value
6370997.0
meters will be assumed.
2 ***
*** ***
*** ***
3 Height of perspective
*** ***
*** ***
point above sphere
4 ***
*** ***
*** ***
5 Longitude of center of
. . . . . . . . . Longitude of central meridian . . . . . .
. . . . .
projection
6 Latitude of center of
*** ***
*** ***
projection
7 . . . . . . . . False easting in the same
units of measure as radius of the sphere . . . . . . .
8 . . . . . . . . False northing in the same
units of measure as radius of the sphere . . . . . .
9-15
(not used for projections on this page)
_________________________________________________________________
_________________________________________________________________
_____________________________________
APPENDIX G.--Parameters Required for Definition of Map
Projections--continued
(20)
(20)
Parameter Oblique Mercator
Parameter Oblique Mercator
(Definition Format A)
(Definition Format B)
(Definition Format A) (Definition Format B)
1 Same as for projections 03 thru 09
9 Longitude of
first ***
point defining central
2 Same as for projections 03 thru 09
geodetic line of
projection
3 Scale factor at the center of
projection
10
Latitude of first ***
4 ***
Angle of azimuth east
point defining central
of north for central
geodetic line of
line of projection
projection
5 ***
Longitude of point 11
Longitude of second ***
along central line of
point defining central
projection at which
geodetic line of
angle of azimuth is
projection
measured
6 Latitude of origin of projection
12
Latitude of second ***
point defining central
7 Same as for projections 03 thru 19
geodetic line of
projection
8 Same as for projections 03 thru 19
13 Zero
Any non-zero
number
14 and 15
(not used for
this
projection)
Two types of horizontal datums are presently in use for DEM data distributed by the USGS, the civilian North American Datum (NAD) and military World Geodetic System (WGS). The NAD 27 datum is currently used to define positions on USGS topographic maps and 7.5-minute DEM's. Plans are to convert to the new NAD 83 for these applications. The WGS 72 is currently used to define positions for 1-degree DMA DEM's and DTED's. DMA is converting these data to the new WGS 84. The NAD 83 and WGS 84 datums are being phased into the mapping community at different rates or where resources are available. For the conterminous United States these new datums are considered to be functionally the same; however, the two have been defined separately since they were designed to serve different segments of the mapping community, primarily civilian and military. The following information will help clarify the relationship between these datums.
Unlike local surveys, which treat the Earth as a plane, the precise determination of the latitude and longitude of points over a broad area must take into account the actual shape of the Earth. To achieve the precision necessary for very accurate location, the Earth cannot simply be assumed to be a sphere. Rather, the Earth's shape more closely approximates an ellipsoid (oblate spheroid): flattened at the poles and bulging at the Equator. Thus the Earth's shape, when cut through its polar axis, approximates an ellipse.
Geodetic surveying, which takes into account variations in the shape of the Earth, is based on a reference ellipsoid to the geoid, the actual shape of the Earth, that is selected as a best fit over a limited area. The ellipsoid used to define a datum is a mathematical surface upon which computation of position can be based, as opposed to the actual surface of the Earth on which surveys are conducted. The geoid, which approximates the sea level surface, is an equipotential surface of the Earth's gravity field. It can be thought of as a continuous sea-level surface extended beneath the continents. It is the "level" surface of reference for astronomic observations and geodetic leveling, but because of undulations that respond to the Earth's mass distributions, it is not a useful computational surface for horizontal surveys.
The NAD 27 is defined with an initial point at Meades Ranch, Kansas, and by the parameters of the Clarke 1866 ellipsoid. The location of features on USGS topographic maps, including the definition of 7.5-minute quadrangle corners, are referenced to the NAD 27.
Using recent measurements with modern geodetic, gravimetric, astrodynamic, and astronomic instruments, the Geodetic Reference System 1980 (GRS 80) ellipsoid has been defined as a best fit to the worldwide geoid. Unlike NAD 27, which is based on an initial point (Meades Ranch, Kansas), NAD 83 is an Earth-centered datum and uses the GRS 80 ellipsoid. Because the NAD 83 surface deviates from the NAD 27 surface, the position of a point based on the two reference datums will be different.
The definition of DMA DEM's, as presently stored in the USGS data base, references the WGS 72 datum. Like NAD 83, WGS 72 is an Earth-centered datum. The WGS 72 datum was the result of an extensive effort extending over approximately three years to collect selected satellite, surface gravity, and astrogeodetic data available through 1972. The combination of the data was performed using a unified WGS solution (a large- scale least squares adjustment). Such an adjustment was made possible in part because of the availability of adequate computers and software.
The WGS 84 datum was developed as a replacement for WGS 72 by the military mapping community as a result of newer, more accurate instrumentation and more comprehensive control networks. It is an improvement over WGS 72 in several respects. New and more extensive data sets and improved software were used in the development. A more extensive file of Doppler-derived station coordinates was available and for many more local geodetic systems; improved sets of ground-based Doppler and laser satellite-tracking data and surface gravity were available; and geoid heights were deduced from satellite radar altimetry (a new data type) for oceanic regions between 70° north and south latitude (approximately). This system is described in "World Geodetic System 1984," Department of Defense DMA TR 8350.2, September 1987.
DMA has recomputed the 1-degree DTED's for the contiguous United States and has made a copy of the data set available to the USGS.
The methods available for transformation from NAD 27 to NAD 83 can result in inconsistencies. Therefore, a single method of conversion has been adopted by the USGS. The method involves the use of 7.5-minute grid intersection tables developed by the National Ocean Service and the software program NADCON, which is available as an interactive PC-based program or adapted to batch processing on PC or mainframe computers. Bilinear interpolation of the shifts derived for the four quadrangle corners results in a uniform horizontal translation of values which are applied to all points interior to and including the edges of the quadrangle.
The datum applicable to a given DEM data set can generally be determined by the following criteria:
All USGS DEM's lacking the new record A, elements 16-29,
are NAD 27.
All DMA DEM's and DTED's lacking datum descriptors are WGS
72.
All DEM's having the new record A elements have datums as
indicated in record A, element 28.
| Difference (Meters) | |||
|---|---|---|---|
| Degrees | Latitude | Longitude | Height |
| 90 N | 0.0 | 0.0 | 4.1 |
| 85 | 0.4 | 1.5 | 4.1 |
| 80 | 0.8 | 3.0 | 4.0 |
| 75 | 1.3 | 4.4 | 3.9 |
| 70 | 1.7 | 5.9 | 3.8 |
| 65 | 2.1 | 7.2 | 3.6 |
| 60 | 2.4 | 8.6 | 3.4 |
| 55 | 2.8 | 9.8 | 3.2 |
| 50 | 3.1 | 11.0 | 3.0 |
| 45 | 3.4 | 12.1 | 2.7 |
| 40 | 3.6 | 13.1 | 2.4 |
| 35 | 3.9 | 14.0 | 2.0 |
| 30 | 4.1 | 14.8 | 1.7 |
| 25 | 4.2 | 15.5 | 1.3 |
| 20 | 4.4 | 16.1 | 1.0 |
| 15 | 4.4 | 16.5 | 0.6 |
| 10 | 4.5 | 16.5 | 0.2 |
| 5 N | 4.5 | 17.1 | -0.2 |
| 0 | 4.5 | 17.1 | -0.6 |
| 5 S | 4.4 | 17.1 | -1.0 |
| 10 | 4.4 | 16.9 | -1.4 |
| 15 | 4.2 | 16.5 | -1.8 |
| 20 | 4.1 | 16.1 | -2.1 |
| 25 | 3.9 | 15.5 | -2.5 |
| 30 | 3.7 | 14.8 | -2.8 |
| 35 | 3.5 | 14.0 | -3.1 |
| 40 | 3.3 | 13.1 | -3.4 |
| 45 | 3.0 | 12.1 | -3.7 |
| 50 | 2.7 | 11.0 | -3.9 |
| 55 | 2.4 | 9.8 | -4.2 |
| 60 | 2.1 | 8.6 | -4.3 |
| 65 | 1.7 | 7.2 | -4.5 |
| 70 | 1.4 | 5.9 | -4.7 |
| 75 | 1.1 | 4.4 | -4.8 |
| 80 | 0.7 | 3.0 | -4.8 |
| 85 | 0.4 | 1.5 | -4.9 |
| 90 S | 0.0 | 0.0 | -4.9 |
*Applies only when proceeding directly from WGS 72 coordinates to WGS 84 coordinates; does not contain the effect of the WGS 84 Earth gravitational model and geoid, nor the effect of local geodetic system-to-WGS 84 datum shifts being better than local geodetic system-to-WGS 72 datum shifts.