U.S. Geological Survey, 19790701, 7.5 minute Digital Elevation Models: U. S. Geological Survey, Reston, VA.
This is a raster data set. It contains the following raster data types:
Planar coordinates are encoded using row and column
Abscissae (x-coordinates) are specified to the nearest 30
Ordinates (y-coordinates) are specified to the nearest 30
Planar coordinates are specified in meters
The horizontal datum used is North American Datum 1927.
The ellipsoid used is Clark 1866.
The semi-major axis of the ellipsoid used is 6378206.4.
The flattening of the ellipsoid used is 1/294.9787.
DEM's can be used as source data for digital orthophotos, and, as
layers in geographic information systems, for earth science
analysis. DEM's 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.
U.S. Geological Survey, Unknown, digital contour lines: U.S. Geological Survey, Reston, VA.
U.S. Geological Survey, Unknown, photo ID number: U.S. Geological Survey, Reston, VA.
U.S. Geological Survey, Unknown, project control: U.S. Geological Survey, Reston, VA.
Data sources used in this process:
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 thatmay be extracted from a given source
during digitization.
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 which are selected as equal to or less
than intended horizontal RMSE error of the DEM.
The testing of horizontal accuracy of the source
materials isaccomplished 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.
The vertical RMSE statistic is used to describe the
vertical accuracy of a DEM, encompassing 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. These errors
are reduced in magnitude by editing but cannot be
completely eliminated. Blunder errors are those
errors of major proportions and are easily
identified and removed during interactive editing.
Systematic errors are those errors that follow
some fixed pattern and are introduced by data
collection procedures and systems. These error
artifacts include: vertical elevation shifts,
misinterpretation of terrain surface due to trees,
buildings and shadows, and fictitious ridges, tops,
benches or striations. Random errors result from
unknown or accidental causes.
DEM's are edited to correctly depict elevation
surfaces that correspond to water bodies of
specified size.
Level 1 DEM: A RMSE of 7-meters or less is the
desired accuracy standard. A RMSE of 15-meters
is the maximum permitted. A 7.5-minute DEM at
this level has an absolute elevation error
tolerance of 50 meters (approximately three
times the 15-meter RMSE) for blunder errors for
any grid node when compared to the true
elevation. Any array of points in the DEM can not
encompass more than 49 contiguous elevations
in error by more than 21 meters (three times the
7-meter RMSE). Systematic errors that are
within stated accuracy standards are tolerated.
Level 2 DEM: A vertical RMSE of one-half of the
contour interval, determined by the source map,
is the maximum permitted. Systematic errors
may not exceed one contour interval,
determined by the source map, is the maximum
permitted. Systematic errors may not exceed
one contour interval specified by the source
graphic. Level 2 DEMs have been processed or
smoothed for consistency and edited to remove
identifiable systematic errors.
The DEM is visually inspected for completeness on a DEM view
and edit system for the purpose of performing a final quality
control and if necessary edit of the DEM. The physical format
of each digital elevation model is validated for content
completeness and logical consistency during production quality
control and prior to archiving in the National Digital
Cartographic Data Base.
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. This condition occurs infrequently and is always the
first or last profile of the dataset.
Level 2 DEM: Level 2 DEM's may contain void areas due to
interruptions to contours in the source graphic or DLG. 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, " symbolized by
contours overprinted with photorevised or other surface
patterns.
The fidelity of the relationships encoded in the data
structure of the 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.
Are there legal restrictions on access or use of the data?
- Access_Constraints: None
- Use_Constraints:
None. Acknowledgement of the U.S. Geological Survey would be
appreciated in products derived from these data.
1-888-ASK-USGS (voice)
7.5-minute digital elevation models
Although these data have been processed successfully on a
computer system at the U.S. Geological Survey, no warranty
expressed or implied is made by the Geological Survey
regarding the utility of the data on any other system, nor shall
the act of distribution constitute any such warranty. USGS will
warrant the delivery of this product in computer-readable
format. and will offer appropriate adjustment of credit when
the product is determined unreadable by correctly adjusted
computer input peripherals, or when the physical medium is
delivered in damaged condition. Requests for adjustments of
credit must be made within 90 days from the date of this
shipment from the ordering site.
| Data format: |
USGS standard DEM: The standard USGS DEM can be described as an ASCII formatted elevation file preceded by a metadata header file which consists of one 1024 byte ACSII record. in format DEM Size: 1 |
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Note: This CD-ROM can be used with all computer operating systems that support CD-ROM as a logical storage device. All text files on the disc are in ASCII format. Data files are in ASCII or binary format. Cartridge (Density 3480 megabytes) (format ASCII; available unlabelled or with ANSI-standard labels; available block sizes are multiples of 1024 characters. To ensure compatibility, a maximum blocking factor of 31744 is recommended. For efficiency, blocking factors less than 16,000 are discouraged. ) Cassette (Density 8-mm High (4.5Gb) gigabytes) (format ASCII; available unlabelled or with ANSI-standard labels; available block sizes are multiples of 1024 characters. To ensure compatibility, a maximum blocking factor of 31744 is recommended. For efficiency, blocking factors less than 16,000 are discouraged. ) Cassette (Density 8-mm Low (2.3Gb) gigabytes) (format ASCII; available unlabelled or with ANSI-standard labels; available block sizes are multiples of 1024 characters. To ensure compatibility, a maximum blocking factor of 31744 is recommended. For efficiency, blocking factors less than 16,000 are discouraged. )
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The online copy of the data set (when available electronically) may be accessed without charge. For cartridge, cassette, and CD-ROM the costs are available at: URL: <http://edc.usgs.gov/glis/hyper/order_info/prices#7_MIN_DEM>
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