There are 184 ice motion products, hence 184 time intervals
over which ice deformation is computed. Each data file
has 736 lines - 4 lines per ice motion product (4 x 184 = 736).
Here are the first 4 lines of the file for the 50 x 50 km box:
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. This is not needed
for normal purposes, but it allows the source data to be easily
identified in case a problem is detected later.
The second line is the date and time of the first image,
and the latitude and longitude of the ship. Specifically,
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 ship. Same format
as the second line.
NOTE: The position of the ship is taken from
this table on the
RGPS web site. The positions in the table are derived from the
SAR images themselves, and may be subject to geolocation error.
To obtain accurate ship positions, 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
where 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 ship was near the
edge of an image, not all the points in the vicinity of the
ship could be tracked, and therefore some cells might 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 box, n_cells = 0.
That's because no points could be tracked within the 50 x 50 km box
centered at the ship, 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.
