Digital Elevation Model (DEM)
Table of Contents
Background
The USGS Digital Elevation Model (DEM) data files
are digital representations of cartographic
information in a raster form.
DEMs consist of a sampled array of elevations for a number of ground positions
at regularly spaced intervals. These digital cartographic/geographic data files
are produced by the U.S. Geological Survey (
USGS) as part
of the National Mapping Program and are sold in 7.5-minute, 15-minute, 30-minute
(also known as 2-arc-second), and 1-degree units. The 7.5- and 15-minute DEMs
are included in the large scale category while 30-minute DEMs fall within the
intermediate scale category and 1-degree DEMs fall within the small scale
category.
Digital Elevation Model (DEM)
- Large Scale
- The DEM data for 7.5-minute units correspond to the USGS 1:24,000- and
1:25,000-scale topographic quadrangle map series for all of the United States
and its territories. Each 7.5-minute DEM is based on 30- by 30-meter data
spacing with the Universal Transverse Mercator
(UTM)
projection. Each 7.5- by 7.5-minute block provides the same coverage as the
standard USGS 7.5-minute map series.
The 7.5-minute Alaska DEM data correspond to the USGS 1:24,000- and
1:25,000-scale topographic quadrangle map series of Alaska by unit size. The
unit sizes in Alaska vary depending on the latitudinal location of the unit.
The 7.5-minute Alaska DEM data consist of a regular array of elevations
referenced horizontally to the geographic (latitude/longitude) coordinate
system of the North American 1927 Datum
(NAD 27) or
the North American 1983 Datum
(NAD 83).
The spacing between elevations along profiles is 1-arc-second in latitude by
2-arc-seconds of longitude.
The 15-minute DEM data correspond to the USGS 1:63,360-scale topographic
quadrangle map series of Alaska by unit size. The unit sizes in Alaska vary
depending on the latitudinal location of the unit. The 15-minute DEM data
consist of a regular array of elevation referenced horizontally to the
geographic (latitude/longitude) coordinate system of
NAD 27. The
spacing between elevations along profiles is 2-
arc
seconds of latitude by 3-arc-seconds of longitude.
- Intermediate Scale
- The 30-minute DEM data cover 30-minute by 30-minute areas which correspond
to the east half or west half of the USGS 30- by 60-minute topographic
quadrangle map series for the conterminous United States and Hawaii. Each
30-minute unit is produced and distributed as four 15- by 15-minute cells. The
spacing of elevations along and between each profile is 2 arc seconds.
- Small Scale
- The 1-degree DEM (3- by 3-arc-second data spacing) provides coverage in 1-
by 1-degree blocks for all of the contiguous United States, Hawaii, and most
of Alaska. The basic elevation model is produced by or for the Defense Mapping
Agency (DMA), but is distributed by the USGS, in DEM data record format. In
reformatting the product, the USGS does not change the basic elevation
information. The 1-degree DEMs are also referred to as 3-arc-second or
1:250,000-scale DEM data.
- The EROS Data
Center (EDC) also concatenated the 1- by 1-degree blocks for the contiguous
United States in the Land Analysis System
(LAS)
environment using the elevation data from the photographic sources. This is
referred to as the 1-degree DEM mosaic data set. Nine strips of concatenated
imagery comprise the data set.
Extent of Coverage
- Large Scale
- The UTM-based 7.5-minute DEM data are available for most of the contiguous
United States, Hawaii, and Puerto Rico. Data availability for the contiguous
United States may be viewed on a status coverage
map. The
geographic-based 15-minute Alaska DEM data are available for most of Alaska.
- The 7.5-minute DEM data are available for portions of Alaska.
- Intermediate Scale
- The 30-minute DEM data are available for portions of the contiguous United States and Hawaii.
- Small Scale
- The 1-degree DEM data are available for all of the contiguous United
States, Hawaii, and most of Alaska. The concatenated images (1-degree mosaic
data) cover only the contiguous United States.
Acquisition
Processing Steps
- Large Scale
- The 7.5-minute DEM data are produced in 7.5- by 7.5-minute blocks either
from digitized cartographic
map contour overlays or from scanned National Aerial Photography Program (NAPP) photographs.
- Four processes have been used to generate DEM data for 7.5-minute units.
The first three processes have been discontinued:
- The Gestalt Photo Mapper II (GPM2) is an automated photogrammetric
system designed to produce orthophotos, digital terrain data, and contours
in subunits known as patches.
- Manual profiling from photogrammetric stereomodels uses stereoplotters
equipped with three-axis electronic digital profile recording modules for
scanning stereomodels along successive terrain profiles.
- Elevations are recorded by stereomodel digitizing of contours, in which
digital contours are acquired on stereoplotters equipped with three-axis
digital recording modules.
- Interpolation from digital line graph (DLG)
hypsographic
and
hydrographic
data. All data now being generated use this process.
- 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 DEMs 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. If
derived from hypsography overlays, data production for this series is
classified according to
Level 2
specifications.
- Intermediate Scale
- The USGS uses two processes to collect the digital elevation data for
production of 30-minute DEMs: (1) derivation from DLG contours of any map
series 7.5-minutes to 30- by 60- minutes (1:24,000-scale to 1:100,000-scale);
and (2) resampling from digital elevation models with a source spacing equal
to or less than a 2-arc-second sampling interval (if the data are resampled
from preexisting DEMs, they are arbitrarily archived as
Level 1). The
DEM data for this series are tested according to
Level 2
accuracy specifications.
- Small Scale
- The majority of the 1-degree DEMs are produced by DMA from cartographic
and photographic sources. Elevation data from cartographic sources are
collected from any map series 7.5-minutes through 1-degree (1:24,000-scale
through 1:250,000-scale). The topographic features (e.g., contours, drain
lines, ridge lines, lakes, and spot elevations) are first digitized and then
processed into the required matrix form and interval spacing.
- 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 drainage, ridge, water, and spot
heights during the interpolation process in which final elevations are
determined for the required matrix form and interval spacing.
- The DEMs distributed within the Department of Defense cover 1- by 1-degree
blocks and are called Digital Terrain Elevation Data Level 1 (DTED-1). In
reformatting the DMA product to create the DEMs, the USGS restructures the
header records and data but, does not change the basic elevation information.
- The DMA 1-degree DTED-1 data and the USGS-distributed 1-degree DEMs are
gridded by using the World Geodetic System 1972
(WGS 72) or
World Geodetic System 1984
(WGS 84).
All 1-degree DEMs were converted to WGS 84.
- The EDC concatenated the 1- by 1-degree blocks for the contiguous United
States in the LAS environment using the elevation data from the photographic
sources. The starting line and sample of each 1- by 1-degree image was
calculated from the projection coordinates, contained in the
data descriptor
record, to form nine strips of data. The coordinates of each strip, along
with their number of lines (rows) and samples (columns), are:
| Strip
| Coordinate Range
| Line/Sample
|
| 1 |
50N 125W / 31N 113W |
22801/14401 |
| 2 |
50N 117W / 31N 106W |
22801/13201 |
| 3 |
50N 110W / 28N 100W |
26401/12001 |
| 4 |
50N 104W / 25N 95W |
30001/10801 |
| 5 |
50N 99W / 25N 90W |
30001/10801 |
| 6 |
50N 94W / 29N 83W |
25201/13201 |
| 7 |
50N 87W / 24N 79W |
31201/ 9601 |
| 8 |
50N 83W / 24N 75W |
31201/ 9601 |
| 9 |
50N 79W / 33N 66W |
20401/15601 |
Data Characteristics
- Large Scale
- Each 7.5-minute unit of DEM coverage (based on the 7.5-minute quadrangle)
consists of a regular array of elevations referenced horizontally in the
UTM
projection coordinate system. Elevation units are in meters or feet relative
to National Geodetic Vertical Datum of 1929 (NGVD 29) in the continental U.S.
and local mean sea level in Hawaii and Puerto Rico. The data are ordered from
south to north in profiles that are ordered from west to east.
- These horizontally referenced data may be
NAD 27, NAD
83, Old Hawaiian Datum (OHD), or Puerto Rico Datum (PRD) of 1940.
- The 7.5-minute Alaska DEMs consist of a regular array of elevations
referenced horizontally to the geographic (latitude/longitude) coordinate
system of NAD 27 or NAD 83. The data are ordered from south to north in
profiles that are ordered from west to east. The unit of coverage corresponds
to four basic quadrangle sizes for 1:63,360-scale graphics (depending on
latitude):
-
-
-
-
- Cell size limits
-
- 7.5- by 18-minutes -- State of Alaska north of 68°N latitude
-
- 7.5- by 15-minutes -- Between 62°N and 68°N latitude
-
- 7.5- by 11.25-minutes -- Between 59°N and 62°N latitude
-
- 7.5- by 10-minutes -- State of Alaska south of 59°N latitude
- The 15-minute Alaska DEMs consist of a regular array of elevations
referenced horizontally to the geographic (latitude/longitude) coordinate
system of NAD 27 or NAD 83. The data are ordered from south to north in
profiles that are ordered from west to east. Elevation units are in meters or
feet relative to NGVD 29. The unit of coverage corresponds to four basic
quadrangle sizes for 1:63,360-scale graphics (depending on latitude):
-
-
-
-
- Cell size limits
-
- 15- by 36-minutes -- State of Alaska north of 68°N latitude
-
- 15- by 30-minutes -- Between 62°N and 68°N latitude
-
- 15- by 2.5-minutes -- Between 59°N and 62°N latitude
-
- 15- by 20-minutes -- State of Alaska south of 59°N latitude
- Intermediate Scale
- The 30-minute DEM data consist of a regular array of elevations referenced
horizontally to the geographic (latitude/longitude) coordinate system of NAD
27 or NAD 83. The unit of coverage is a 30- by 30-minute block. Saleable units
are four 15-minute DEMs covering a 30- by 30-minute area. Elevation data on
the integer minute lines (all four sides) correspond to the same profiles on
the surrounding eight blocks. Elevations are in meters or feet relative to
NGVD 29.
- Small Scale
- The 1-degree DEM consists of a regular array of elevations referenced
horizontally on the geographic (latitude/longitude) coordinate system of the
WGS 72 (converted to WGS 84). The information content is approximately
equivalent to that which can be derived from contour information represented
on 1:250,000 scale maps.The unit of coverage is a 1- by 1-degree block.
Elevation data on the integer degree lines (all four sides) overlap with the
corresponding profiles on the surrounding eight blocks.
- Elevations are in meters relative to NGVD 29 in the continental U.S. and
local mean sea level in Hawaii. DEM accuracy information is provided in the
Appendix.
- The 1-degree DEM mosaic data set is characteristically the same as the
source 1- by 1-degree DEM unit of coverage.
Spatial Resolution
- Large Scale
- The 7.5-minute DEM data are stored as profiles in which the spacing of the
elevations along and between each profile is 30 meters. The number of
elevations in a profile will vary because of the variable angle between the
quadrangle's geographic boundary (neatline) and the UTM coordinate system. DEM
data of low-relief terrain or generated from contour maps with intervals of 10
feet or less are recorded in feet while DEM data of moderate to high-relief
terrain or generated from maps with terrain contour intervals greater than 10
feet are generally recorded in meters.
UTM Meter Grid for 7.5 DEM
- The 15-minute DEM data are collected with a 2- by 3-arc-second spacing in
latitude, and longitude, respectively. The first and last data points along a
profile are at the integer degrees of latitude. Elevation data on the
quadrangle neatlines (all four sides) share edge profiles with the surrounding
eight quadrangles.
- Intermediate Scale
- Spacing of the elevations along each profile is 2-arc-seconds. The first
and last data points are at the integer 15-minutes of latitude. A 15-minute
profile will, therefore, contain 451 elevations.
- Small Scale
- Spacing of elevations along and between each profile of 1-degree DEMs is
3-arc-seconds with 1,201 elevations per profile. The exception is DEM data in
Alaska, where the profile spacing varies depending on the latitudinal location
of the DEM. Latitudes between 50 and 70 degrees north have spacings at
6-arc-seconds with 601 profiles per 1-degree unit and latitudes greater than
70 degrees north have spacings at 9-arc-seconds with 401 profiles per 1-degree
unit.
- The 1-degree mosaic data set spacing of elevation and profile data is the
same as the data of conterminous U.S. 1-degree DEM data set.
Arc Second Grid for 1-degree
DEM
DEM Accuracy
The method of determining 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 and representative of
the terrain. Collection of test point data and comparison of the DEM to the
quadrangle hypsography are conducted by USGS quality-control groups.
- Large Scale
- The vertical accuracy of 7.5-minute DEMs is equal to or better than 15
meters. A minimum of 28 test points per DEM is required (20 interior points
and 8 edge points). The accuracy of the 7.5-minute 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-scale.
Accuracy of the 15-minute DEMs is equal to or better than one-half of a
contour interval of the 15-minute topographic quadrangle map. The accuracy of
the 15-minute 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-scale. The plotting of contours from the 15-minute Alaska
DEMs at scales larger than 1:63,360, or reliance on the elevation heights
without incorporating the National Map Accuracy Standard (NMAS) horizontal
error tolerance, will lead to less reliable results.
- Intermediate Scale
- The 30-minute DEM accuracy is equal to or better than one-half of a
contour interval of the 30- by 60-minute topographic quadrangle map. The
accuracy of the 30-minute 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-scale. 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.
- Small Scale
- The 1-degree mosaic data set spacing of elevation and profile data is the
same as the 1-degree DEM contiguous U.S. data. The accuracy of the 1-degree
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 1:250,000-scale. 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 NMAS horizontal error tolerance, will lead to less reliable
results.
- For general information on accuracy, see
Horizontal
Positional Accuracy and
Vertical
Positional Accuracy.
Data Organization
A DEM file is organized into three logical records:
Contains information defining the general characteristics of the DEM,
including DEM name, boundaries, units of measurement, minimum and maximum
elevations,
projection
parameters, and number of type B records. Each DEM file has one Type A record.
Contains profiles of elevation data and associated header information.
Each profile has a Type B record.
Contains statistics on the accuracy of the data.
Examples of a DEM record format may be found in the
Appendix.
Entity and Attribute Information
A digital elevation model is composed of
integer
values representing a gridded form of a topographic map hypsography overlay.
Additional information on DEM specifications can be found in the USGS
National Mapping Program's
Standards for Digital
Elevation Model (DEMs).
Classification Levels
DEM data are organized in three classification levels. Level-1 DEMs are
elevation data sets in a standardized format. The intent is to reserve this
level for 7.5-minute DEMs which are created by scanning National High Altitude
Photography (NHAP)/NAPP photography. A vertical RMSE of 7 meters is the desired
accuracy standard. A RMSE of 15 meters is the maximum permitted.
Level-2 DEMs 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. A RMSE of one-half contour interval is the maximum permitted.
Level-3 DEMs are derived from DLG data by incorporating selected elements
from both hypsography (contours, spot elevations) and hydrography (lakes,
shorelines, drainage). A RMSE of one-third of the contour interval is the
maximum permitted.
Digital Elevation Model Caveats
- Large Scale
- The majority of the 7.5-minute DEMs produced to date are categorized as
Level-1 DEMs.
- Intermediate Scale
- All 30-minute DEMs derived from contours are Level 2. All 30-minute DEMs
derived from 7.5-minute DEMs are Level 1.
- Small Scale
- All 1-degree DMA DTED-1 data have been classified as Level 3.
Procedures for Obtaining Data
Information on DEM products and services can be found at:
Digital Elevation
Models (DEMs)
Applications and Related Data Sets
The DEM files may be used in the generation of graphics such as
isometric
projections displaying slope, direction of slope (aspect), and terrain profiles
between designated points. They may also be combined with other data types such
as stream location data and weather data to assist in forest fire control or
they may be combined with remote sensing data to aid in the classification of
vegetation. Applications include: (1) modeling terrain gravity data for use in
locating energy resources, (2) determining the volume of proposed reservoirs,
(3) calculating the amount of material removed during strip mining, (4)
determining landslide probability, and (5) developing parameters for hydrologic
models.
References
- Large Scale
- U.S. Geological Survey, 1992, US GeoData, [brochure]: [Reston, Virginia],
U.S. Geological Survey, 2 p. [fold out, oversized]
- U.S. Geological Survey, 1992, US GeoData data users guides and associated
documentation, order form: [Reston, Virginia], U.S. Geological Survey, 2 p.
- U.S. Geological Survey, 1993, Digital elevation models -- data users guide
5: Reston, Virginia, U.S. Geological Survey, 48 p.
- U.S. Geological Survey, 1993, US GeoData digital elevation model,
factsheet: [Reston, Virginia], U.S. Geological Survey, 2 p.
- U.S. Geological Survey, 1994, Cartographic technical instructions, price
list: [Reston, Virginia], U.S. Geological Survey, 2 p.
- U.S. Geological Survey, 1994, US GeoData available through the Internet,
factsheet: [Reston, Virginia], U.S. Geological Survey, 2 p.
- Intermediate Scale
- U.S. Geological Survey, 1993, Digital elevation models --data users guide
5: Reston, Virginia, U.S. Geological Survey, 48 p.
- Small Scale
- U.S. Geological Survey, 1993, Digital elevation models --data users guide
5: Reston, Virginia, U.S. Geological Survey, 48 p.
Appendix
A soft copy in ASCII text format of the individual records is
available at: National
Mapping Program Standards
DEM Accuracy
General Information
The accuracy of a DEM is dependent upon its source and the spatial
resolution, that is grid spacing, of the data profiles. One factor influencing
DEM accuracy is source data scale and resolution. A dependency exists between
the scale of the source materials and the level of grid refinement possible. The
source resolution is also a factor in determining the level of content that may
be extracted during digitization. For example, 1:250,000-scale topographic maps
are the primary source of 1-degree DEMs.
Another factor is the horizontal and vertical dimension of the DEM.
Horizontal accuracy of DEM data is dependent upon 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. 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 is dependent upon 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 have the effect of compounding for each step of the
process. Production personnel are directed to account for each production step
leading to the final DEM.
SDTS Format
The Spatial Data Transfer Standard (SDTS) distribution format is designed as
a mechanism for the transfer of spatial data between various computer systems.
The SDTS format is designed to transfer data with complete content transfer (no
loss of information). Data quality reports provide complete metadata and
documentation of processing. The SDTS is a
FIPS standard
intended to work between various platforms and various data models. There is
limited software available at this time which can accommodate the SDTS-formatted
data. For more in-depth information on SDTS, refer to the SDTS technical
documentation (available online).
UTM Meter Grid for 7.5 DEM
Arc Second Grid for 1-degree
DEM