Data Products and File Format

The files are in ASCII format. Since there are 184 ice motion products, there are 184 time intervals over which ice deformation is computed. Each file has 736 lines - 4 lines per time interval (4 x 184 = 736). Here are the first 4 lines of the file for the 50 x 50 km region:

R1000_97305002.LP       
   1997 305  16  20    75.7611  -143.9476
   1997 307  17   2    75.9258  -144.0467
   -0.141700   -0.001900    0.023114    2.029175   100
The first line is the file name of the RGPS ice motion product from which the deformation was computed, for purposes of cross-referencing.

The second line is the date and time of the first image, and the latitude and longitude of the SHEBA station. The numbers are:

year, day-of-year, hours, minutes, latitude, longitude
where the time is GMT, and latitude and longitude are in degrees. Note that west longitude is negative. In the example above, the date is November 1 (day 305) 1997 and the time is 16:20 GMT.
The third line is the date and time of the second image, and the latitude and longitude of the SHEBA station. Same format as the second line.

NOTE: The position of the SHEBA station is derived from the SAR images and therefore is subject to geolocation error. If accurate positions are needed, please refer to the SHEBA web site (scroll down to "Spline Data ASCII File").

The numbers in the fourth line are:

vorticity, divergence, shear, delta_t, n_cells
The three invariants are dimensionless, delta_t is the time interval in days over which the invariants are computed, and n_cells is the number of 5 x 5 km cells that went into the computation. Thus the area used in the computation is n_cells x 25 square kilometers. One would normally expect n_cells to be 100 for the 50 x 50 km deformation product, since 50 x 50 km = 10 x 10 cells = 100 cells. Similarly one might expect n_cells = 400 for the 100 x 100 km case, n_cells = 900 for the 150 x 150 km case, and n_cells = 1600 for the 200 x 200 km case. However, if the SHEBA station was near the edge of an image, not all the points in the vicinity of the station would be tracked, and therefore some cells would be missing. The n_cells parameter gives an indication of the completeness of the data that went into the computation of deformation.
NOTE: In one case (August 8-9) for the 50 x 50 km region, n_cells = 0. That's because no points could be tracked within the 50 x 50 km region centered at the station, although some points farther away could be tracked. In this one degenerate case, the vorticity and divergence and shear have been set to 999. Note that n_cells dips in August due to the difficulty of summer ice tracking. Interpretation of the ice deformation during this time must take into account the smaller sample sizes.