The Level 3 georeferenced databases available on CD-ROM contain surface height measurements derived from altimetry data, ordered by geographic areas or "bins". Each database consists of a header file that gives the bin configuration and geographic limits of the data set, a data file that contains the actual data, and a bin directory giving the logical records to look to for each geographical bin. Each data record contains the geodetic position, revolution number, surface elevation relative to a reference ellipsoid, and slope correction. The file is structured so that the data records are ordered by bin and then by time within each bin. Following is a description of the data.
The surface elevation in the databases, Hdb, was calculated using the following
Hdb=Hsc - Halt - dHret - dHcg + dHion + dHtrop - dHtide
where Hsc = The height of the spacecraft above the reference ellipsoid
Halt = The altimeter measured range
dHret = The retracking correction
dHcg = The center of gravity correction to the range
dHion = The ionospheric delay correction to the surf ht
dHtrop= The tropospheric delay correction to the surf ht
dHtide= The solid tide value
A value for the slope correction, dHslp, is written on each database record and may or may not not been applied to Hdb. Check the mission status words to see if the slope correction has been applied. In order to obtain a slope corrected surface elevation, Hcor, use the following:
Hcor=Hdb-dHslp
The format of the georeferenced database files are as follows:
| BYTES | TYPE | DESCRIPTION |
|---|---|---|
| 1-4 | I*4 | Number of latitude rows in the database, NROWS |
| 5-8 | I*4 | Northwestern-most latitude of database in degrees North (x 105) |
| 9-12 | I*4 | Northwestern-most longitude of database in degrees East (x 105) |
| 13-16 | I*4 | Southeastern-most latitude of database in degrees North (x 105) |
| 17-20 | I*4 | Southeastern-most longitude of database in degrees East (x 105) |
| 21 to NROWS*4-1 | I*4 | Width of each latitude row in degrees (x 105), starting with the southernmost row. This is dimensioned by the number of latitude rows in the database. |
| next NROWS*4 | I*4 | The number of longitude divisions in each latitude row, starting with the southern-most row. This is dimensioned by the number of latitude rows in the database. |
| next 4 bytes | I*4 | Logical record number in database at which directory starts. |
| next 4 bytes | I*4 | Unused |
| next 4 bytes | I*4 | Maximum latitude of data (may not be equal to value in bytes 5-8 if data is subarea of the total bin configuration area) in deg N (x106) |
| next 4 bytes | I*4 | Minimum longitude of data (may not be equal to value in bytes 9-12 if data is subarea of the total bin configuration area) in deg E (x106) |
| next 4 bytes | I*4 | Minimum latitude of database (may not be equal to value in bytes 13-16 if data is subarea of the total bin configuration area) in deg N (x106) |
| next 4 bytes | I*4 | Maximum longitude of database (may not be equal to value in bytes 17-20 if data is subarea of the total bin configuration area) in deg E (x106) |
| next 20 bytes | C*20 | Character data description of orbit used in calculation of surface height in database |
| next 4 bytes | I*4 | YYMMDD database beginning time |
| next 4 bytes | I*4 | HHMMSS database beginning time |
| next 4 bytes | I*4 | YYMMDD database ending time |
| next 4 bytes | I*4 | HHMMSS database ending time |
| next 4 bytes | I*4 | mission word |
| next 4 bytes | I*4 | mission status word for Seasat |
| next 4 bytes | I*4 | mission status word for GEOSAT-GM |
| next 4 bytes | I*4 | mission status word for GEOSAT-ERM |
| next 4 bytes | I*4 | mission status word for TOPEX |
| next 4 bytes | I*4 | mission status word for ERS-1 |
| next 4 bytes | I*4 | mission status word for GEOS-C |
Subgroup 1: One logical record for each bin containing data
| BYTES | TYPE | DESCRIPTION |
|---|---|---|
| 1-4 | I*4 | Indicates the number of logical records which follow which are located in the bin |
| 5-32 | Unused | Blank |
Subgroup 2: One logical record for each data point in the bin
| BYTES | TYPE | DESCRIPTION |
|---|---|---|
| 1-4 | I*4 | North latitude of datum point in degrees (x 106) |
| 5-8 | I*4 | East longitude of datum point in degrees (x 106) |
| 9-12 | I*4 | Surface height relative to the ellipsoid in cm. |
| 13-16 | I*4 | Height sigma used in meters (x 105) |
| 17-20 | I*4 | Reserved for future use. |
| 21-24 | I*4 | Reserved for future use. |
| 25-28 | I*4 | Rev number |
| 29-32 | I*4 | Slope correction in meters (x 105) (-999999999 if unavailable) |
NOTE: Subgroups 1 and 2 are repeated for as many bins with data.
| BYTES | TYPE | DESCRIPTION |
|---|---|---|
| 1-4 | I*4 | Record number at which data for bin 1 starts |
| 5-8 | I*4 | Record number at which data for bin 2 starts |
| 9-12 | I*4 | Record number at which data for bin 3 starts |
| 13-16 | I*4 | Record number at which data for bin 4 starts |
| 17-20 | I*4 | Record number at which data for bin 5 starts |
| 21-24 | I*4 | Record number at which data for bin 6 starts |
| 25-28 | I*4 | Record number at which data for bin 7 starts |
| 29-32 | I*4 | Record number at which data for bin 8 starts |
NOTE: The directory contains as many 32-byte logical records as necessary to designate the record locations of all bins. Any geographical bin for which there is no data has a 0 put in as the starting record number
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Curator: Jairo Santana NASA Official: Dr. H. Jay Zwally Last Updated: 12/11/2006 |