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| Earth Resources Observation and Science (EROS) |
DEM Data Record Format
Record type A, element 1 has been changed to require certain information in specified byte locations. A new element, element 2, record type A, has been defined to record the NMD organization from which the DEM was authorized. The element counts of old record type A, elements 2-15, have been incremented by one to elements 3-16 An attempt has been made to keep the old and new DEM formats compatible; therefore, although the element counts have changed, the byte positions and information content of these fields (old elements 2-15, new elements 3-16) remain the same. This change should be transparent to old DEM applications programs. The new element 2, the mapping center of DEM origin is named in record A, bytes 141-144. Valid codes are MAC (Mapping Application Center, GPM2 (specific to MAC Gestalt Photo Mapper II auto correlator), MCMC (Mid Continent Mapping Center), RMMC (Rocky Mountain Mapping Center), WMC (Western Mapping Center) and FS (U.S. Forest Service). Codes indicating other sources of DEM's are defined when required. Also, new data elements 17-29 have been appended to the end of the type A record. These elements are contained in the end of the previously blank filled portion of the 1,024 byte record.
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 bands called profiles. Therefore, the number of complete profiles covering the DEM area is the same as the number of type B records in the DEM. In a UTM structured DEM, an occasional profile exists within the bounds of the DEM quadrilateral but is void of elevation grid points and 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 at the respective DEM corner.) The type C record contains statistics on the accuracy of the data in the file.
The following special conventions shall be observed for the population of data fields in the A, B, and C record elements:
123456789012345678901234 (byte position, left justified) " .000000000000000D+00"| Standard format specified is " 0.0" | D24.15. Zero values listed are " 0.000000000000000D+00"| common machine dependent numeric " .000000000000000" | defaults for zero values.
Sample DEM Type A Logical Records-Reno, Nevada-California, Quadrangle (West Half) (1 degree)
NJ11-01W
3 1 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 3
2 4 -0 42840000000000D+06 0.140400000000000D+06 0.428400000000000D+.06
0.144000000000000D+06 -0.42480000000000D+06 0.14400000000000D+06
-0424800000000000D+06 0.14040000000000D+06 0.999000000000D+03
0.264100000000000D+04 0.0 00.300000E+010.300000E+010.100000E+01 1
1201
Data
Element Content Explanation
1 NJ11-01W Quadrangle name field (144 characters); NJ11-01W
is the designation for the DEM covering the west half
of the Reno, Nevada-California, sheet.
2 3 DEM level code; 3 reflects processing by DMA which
includes registration to planimetric features
appearing on the source 1-degree.
3 1 Pattern code; 1 indicates a regular elevation
pattern.
4 0 Planimetric reference system code; 0 indicates
geographic coordinate system.
5 0 Zone code; there are no zones in the geographic
system. Therefore, the zone code is set to zero.
6 0.0 Map projection parameters; all 15 fields are set to
(15 sets of 0.0) zero for the geographic system and should be ignored.
Presence of non-zero parameters are not related to
the geographic coordinate system and should also be
ignored.
7 3 Units code; 3 represents arc-seconds as the unit of
measure for ground planimetric coordinates throughout
the file.
8 2 Units code; 2 represents meters as the unit of
measure for elevation coordinates throughout the
file.
9 4 Number (n) of sides in the polygon which defines the
coverage of the DEM file (usually equal to 4).
10 -0.4284...D+06, A 4,2 array containing the ground coordinates of
0.1404...D+06 the four corners of the DEM. In this case
........ translation from arc-seconds to degrees, minutes
........ and seconds yields: -119 00 0, 39 00 00; -119 00
-0.4248...D+06 00, 40 00 00; -118 00 00, 40 00 00; -118 00 00.
0.1404...D+06 39 00 00.
11 0.9990...D+06 Minimum and maximum elevations for the DEM.
0.2641...D+04
12 0.0 Counterclockwise angle from the primary axis of
ground planimetric referenced to the primary
axis of the DEM local reference system.
13 0 Accuracy code; 0 indicates that a record of
accuracy does not exist and that no record type
C will follow.
14 0.300000E+01 A three-element containing DEM spatial
0.300000E+01 resolution (x,y, z); set to 3, 3, 1; or 3, 6, 1;
0.100000E+01 or 3, 9, 1 depending on the latitude of the
product.
15 1 1201 A two-element array containing the number of
rows and columns of profiles in the DEM. The
row value m is usually set to 1 as an indication
that the arrays are actually one-dimensional
profiles. The column value n set to 1,201
indicates that the DEM file consists of a total
of 1,201 profiles.
DEM Type B Logical Record-Reno, Nevada California, Quadrangle (West Half) (1 degree)
1 1 1201 1 -0.42480000000000D+06 0.1404000000000D+06 0.0
0.121000000000D+04 0.177200000000D+04 1538 1539 1539 1534 1529 1520
1513 1508 1501 1493 1488 1479 1475 1474 1473 1472 1469 1467
1466
1464
Data
Element Content Explanation
1 1 1 Row and column identification number of the
profile contained in this record; 1, 1
represents row 1, column 1, in the DEM data set.
2 1201 1 Number of rows (elevations) and columns in this
profile; 1201, 1 indicates there are 1,201
elevations and 1 column in this profile.
3 -0.4284...D+06 Translated to the decimal, -428400.0 and
0.1404...D+06 140400.0 are the ground planimetric coordinates
(arc-seconds) of the first elevation in the
profile, thus computed equal to -119 and 39
degrees.
4 0.0 Elevation of local datum for the profile.
Always zero for 1-degree DEM, the reference is
mean sea level.
5 0.1212...D+04 Minimum and maximum elevations for the profile.
0.1772...D+04
6 1538 1539... An array of m x n elevations (m=1201, n=1) for
....... the profile expressed in units of measure of
meters (record A, element 8, indicates meters
as units of measure) above or below the local
datum (record a, element 4).
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 16 1 6 Code 1 = available
availability of 0 = unavailable
statistics in
data element 2
2 RMSE of file's INTEGER*2 3I6 7 24 In some units as
datum relative indicated by elements
to absolute 8 and 9 of logical
datum (x, y, z) record type A.
3 Sample size on INTEGER*3 I6 25 30 If 0, then accuracy
which statistics will be assumed to be
in data element estimated rather than
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 INTEGER*2 3I6 37 54 In same units as
data relative indicated by elements
to file's 8 and 9 of logical
datum (x, y, z) record type A.
6 Sample size on INTEGER*2 I6 55 60 If 0, then accuracy
which statistics will be assumed to
in element 5 be estimated rather
are based than computed.
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