[Personnel: Last_Name='USGS EROS CENTER', First_Name='INFORMATION SCIENTIST']
2-Arc-Second (30-Minute Digital Elevation Model) QuadrangleEntry ID: DEM_30
Abstract: Digital Elevation Model (DEM) is the terminology adopted by the USGS
to describe terrain elevation data sets in a digital raster form.
The standard DEM consists of a regular array of elevations cast on a
designated coordinate projection system. The DEM data are stored as
a series of profiles in which the spacing of the elevations along
and between each profile is ... in regular whole number intervals. The
normal orientation of data is by columns and rows. Each column
contains a series of elevations ordered from south to north with the
order of the columns from west to east. The DEM is formatted as one
ASCII header record (A-record), followed by a series of profile
records (B-records) each of which include a short B-record header
followed by a series of ASCII integer elevations per each profile.
The last physical record of the DEM is an accuracy record
(C-record). A 30-minute DEM (2- by 2-arc second data spacing)
consists of four 15-by 15-minute DEM blocks. Two 30-minute DEMs
provide the same coverage as a standard USGS 30- by 60-minute
quadrangle. Saleable units are 30- by 30-minute blocks, that is,
four 15- by 15-minute DEMs representing one half of a
map. Coverage includes the contiguous United States and Hawaii.
Purpose: DEMs can be used as source data for digital orthophotos, and for
earth science analysis as layers in geographic information systems.
DEMs can also serve as tools for volumetric analysis, for site
location of towers, or for drainage basin delineation. These data
were collected as part of the National Mapping Program.
The 30-minute ... DEM's rows and columns are based on the geographic
coordinate system. The use of this system results in a rectangular
DEM which shares a common edge and duplicate points with other
adjacent 30-minute DEMs. The 30-minute DEM is distributed as four
15-minute DEM units, and corresponds to the same coverage as the
east half or west half of a 1:100000-scale topographic series USGS
(Click for Interactive Map)
Data Set Citation
Dataset Originator/Creator: U.S. Geological Survey
Dataset Title: 2-Arc-Second (30-Minute DEM)
Dataset Release Place: Reston, VA, USA
Dataset Publisher: U.S. Geological Survey
Data Presentation Form: MapsOnline Resource: http://earthexplorer.usgs.gov
ISO Topic Category
The accuracy of a DEM depends upon the source level of detail and
the resolution of the data samples. The primary limiting factor for
the source level of detail 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. DEM
data accuracy is derived by comparing linear interpolation
in the DEM with corresponding map location elevations and
computing the statistical standard deviation or root-mean-square
error (RMSE). The RMSE is used to describe the DEM accuracy. For
15-minute DEMs derived from vector or DLG hypsographic and
hydrographic source data, an RMSE of one-half contour interval or
better is required. The 1-degree DEM data have and absolute
of 130 meters horizontally and 30 meters vertically.
The fidelity of the relationships encoded in the data structure of
DEM are automatically verified using a USGS software program upon
completion of the data production cycle. The test verifies full
compliance to the DEM specification.
The DEM is visually inspected for completeness on a DEM view and
system for the purpose of performing a final quality control, and,
necessary, edit of the DEM. The physical format of each digital
elevation model is validated for content completeness and logical
consistency, both during production quality control and prior to
archiving. Due to the variable orientation of the quadrilateral in
relation to the Universal Transverse Mercator (UTM) projection grid,
profiles that pass within the bounds of the DEM quadrilateral may be
void of elevation grid points and are not represented in the DEM.
condition occurs infrequently and is always the first or last
of the dataset. Level codes of data quality are assigned depending
the specific production process used. Level 1 DEMs are created by
correlation or manual profiling from aerial photographs. Level 2
are created from digital line graph contours. Level 2 DEMs may
void areas due to interruptions to contours in the source contours.
Void area elevation grid posts are assigned the value of -32,767. In
addition, suspect elevation areas may exist in the DEM but are not
specifically identified. Suspect areas can be located on the source
graphic as a disturbed surface and are symbolized by contours
overprinted with photorevised or other surface patterns.
The horizontal accuracy of the DEM is expressed as an estimated
root mean square error (RMSE). The estimate of the RMSE is based
upon horizontal accuracy tests of the DEM source materials that
are selected as equal to or less than the intended horizontal RMSE
error of the DEM. The testing of source material horizontal
accuracy is accomplished by comparing the planimetric (X and Y)
coordinates of well-defined ground points with the coordinates of
the same points as determined from a source of higher accuracy.
For 15-minute DEMs derived from vector or DLG hypsographic and
hydrographic source data, an RMSE of one-half of a contour
interval or better is required.
The vertical RMSE statistic is used to describe the vertical
accuracy of a DEM and encompasses both random and systematic
errors introduced during production of the data. The RMSE is
encoded in element number 5 of record C of the DEM. Accuracy is
computed by a comparison of linear interpolated elevations in the
DEM with corresponding known elevations. Test points are well
distributed, representative of the terrain, and have true
elevations with accuracies well within the DEM accuracy criteria.
Acceptable test points include, in order of preference: field
control, aerotriangulated test points, spot elevations, or points
on contours from existing source maps with appropriate contour
interval. A minimum of 28 test points per DEM is required to
compute the RMSE, which is composed of a single test using 20
interior points and 8 edge points. Edge points are those which are
located along, at, or near the quadrangle neatlines and are deemed
by the editor to be useful to evaluating the accuracy of the edge
of the DEM. Collection of test point data and comparison of the
DEM with the quadrangle hypsography are conducted by the quality
control units within the USGS. There are three types of DEM
vertical errors: blunder, systematic, and random. Blunder errors
are those errors of major proportions and can be easily identified
and removed during interactive editing. Systematic errors are
those errors that follow some fixed pattern and were introduced by
data collection procedures and systems. Random errors result from
unknown or accidental causes.
Access Constraints None.
Use Constraints There is no guarantee of warranty concerning the accuracy of these data. Users should be aware that temporal changes may have occurred since the data was collected and that some parts of these data may no longer represent actual surface conditions. Users should not use these data for critical applications without a full awareness of their limitations. Acknowledgement of the originating agencies would be appreciated in products derived from these data. Any user who modifies the data set is obligated to describe the types of modifications they perform. User specifically agrees not to misrepresent the data set, nor to imply that changes made were approved or endorsed by the U.S. Geological Survey. Please refer to http://www.usgs.gov/privacy.html for the USGS disclaimer.
Data Set Progress
Role: DIF AUTHOR
Email: lta at usgs.gov
Long Term Archive USGS EROS Center U.S. Geological Survey
City: Sioux Falls
Province or State: SD
Postal Code: 57198-0001
Creation and Review Dates
DIF Creation Date: 2001-06-01
Last DIF Revision Date: 2005-09-01
Future DIF Review Date: 2002-08-01