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Data Products

Data Products

Level 4: Elevation Grids

Level 4 data available on CD-ROMS are contour maps and gridded elevations. These grids give the surface topography of the ice sheets relative to the OSU91A geoid . The altimetric-derived heights from which the grid was created are described in the level 3 documentation. The procedure used to grid the data is explained in detail in Zwally et al, 1990. The grid points are on a polar stereographic projection with a nominal spacing of 5 km (ERS-1) 10 km (GEOSAT) and 20 km (Seasat). The gridded elevations are derived from the data by a weighted fitting of a bi-quadratic function (bi-linear where data distribution is poor) to the elevation data that fall within a certain radius of the grid location. The elevation data without the slope correction are used and the resultant gridded heights are slope corrected using the best available grids in an iterative manner. The format of the gridded data files is as follows:

Record #1:

Elevation Grid Header Record
BYTES TYPE DESCRIPTION
1-4 I*4 Number of i values in the grid
5-8 I*4 Number of j values in the grid
9-12 I*4 Starting north latitude of grid in degrees North (x 106) (this will be approximate for a polar stereographic grid)
13-16 I*4 Starting east longitude of grid in degrees East (x 106) (this will be approximate for a polar stereographic grid)
17-20 I*4 Ending north latitude of grid in degrees North (x 106) (this will be approximate for a polar stereographic grid)
21-24 I*4 Ending east longitude of grid in degrees East (x 106) (this will be approximate for a polar stereographic grid)
25-28 I*4 Status word for data used to generate grid.
29-32 I*4 Polar stereographic grid size conversion and scaling factor from half-inch grids on projection plane to the desired grid size (x 106)
33-36 I*4 The number of grids of desired size from the pole to the equator based on the grid size conversion and scaling factor (x 106)
37-40 I*4 Latitude of the map perimeter in degrees North (x 106)
41-44 I*4 Greenwich orientation in degrees (x 106)
45-48 I*4
 Polar stereographic switch  
    =0, grid has constant increment in latitude,longitude 
    =1, grid is in polar stereographic projection 
49-52 I*4 Number of I-axis divisions to the extent of the map perimeter
53-56 I*4 Number of J-axis divisions to the extent of the map perimeter
57-60 I*4 J coordinate of the projected pole
61-64 I*4 I coordinate of the projected pole
65-68 I*4 Minimum J index of the grid
69-72 I*4 Maximum J index of the grid
73-76 I*4 Minimum I index of the grid
77-80 I*4 Maximum I index of the grid
81-180 N/A padded to keep same size as data records

Records 2 to end:

Elevation Grid Data Records
BYTES TYPE DESCRIPTION
1-4 I*4 Condition number of the matrix used in the least squares solution to the function (x 106)
5-8 I*4 Capsize in degrees latitude - radius from grid location defining area from which data was used to define grid (x 106)
9-12 I*4 North latitude of grid point in degrees (x 106)
13-16 I*4 East longitude of grid point in degrees (x 106)
17-20 I*4 Height values of the grid at location relative to sea level in meters (x 105)
21-24 I*4 Number of data values that were used to calculate grid value
25-28 I*4 Number of parameters used to define function, NPT,(equals 0, 3, or 6) 0 indicates a non-defined grid value
29-52 I*4 Six gridding function coefficients. If NPT is less than 6 then the rest of the coefficients are initialized to zero. (x 105)
53-76 I*4 Set of null coefficients associated with any negligible singular values. If NPT is less than 6 then rest of coefficients are set to zero (x 106)
77-80 I*4 Distance in km from grid locations to closest data point (x 106)
81-84 I*4 North latitude of closest data point to grid location in degrees (x 106)
85-88 I*4 East longitude of closest data point to grid location in degrees (x 106)
89-92 I*4 Height associated with closest data point to grid location in meters (x 105)
93-96 I*4 Standard deviation of the data with respect to the gridding function in meters (x 106)
97-180 I*4 Correlation matrix from solution. This is a symmetrical 6 X 6 matrix so only the upper triangular portion is stored. The order of storage is elements 1-6 are the first row elements, 7-11 columns 2-6 of second row etc. (x 105)