You can get a raster version of the SeaWiFS bathymetry data in raw PGM format here.
https://oceancolor.gsfc.nasa.gov/SeaWiFS/bathymetry/SeaWiFS_median_depth.35N.35S.180W.180E.pgm.bz2
The file has been compressed with bzip2.
The 8-bit values in the PGM file have been produced as described in the following email message that I sent out at the time we were producing the data set.
> Subject: Finding shallow water with SeaWiFS > Date: Tue, 15 Oct 2002 17:41:47 -0400 > From: Norman Kuring <norman@seawifs.gsfc.nasa.gov> > > Some notes on the new SeaWiFS depth binning scheme: > > The software I have written takes a SeaWiFS level-2 > depth file (Rick Stumpf's latest algorithm and Bryan > Franz's implementation in MSl12) as input. Computed > depths are stored as floating point meters in these > files. Special values of -1 and -2 indicate spectrally- > determined land, and clouds or other masks, respectively. > (I believe that one of the other currently used masks > is stray_light. This will have the effect of eliminating > data within a pixel or two of land.) > > I eliminate the edgemost 200 pixels from each LAC scan > as a means of avoiding swath-edge artifacts. > > -1 becomes 1 (land) > -2 becomes 255 (cloud or other masking condition) > <0.2 becomes 2 > >100 becomes 254 > > I scale the rest of the depths logarithmically from > floating point to 8-bit unsigned integer as follows. > > pixval = (254-2)*log(depth/0.2)/log(100/0.2) + 2 + 0.5 > > Each pixel value is assigned to a 0.01 degree by 0.01 degree > bin of a Plate Carree projection of the Earth between 35 North > and 35 South (252,000,000 bins in all). > > After having accumulated these log-scaled depths in these bins, > I run another program to generate an image of the data. > This program ignores cloud or otherwise masked pixels > unless there are no other pixels in a given bin. I can > currently produce an image showing the mean, median, or mode > pixel value. > > . > . > .
So, using the above information, you could write a small program to print out a table of latitude, longitude, and depth. A sample program written in Perl follows.
my $depth_file = 'SeaWiFS_median_depth.35N.35S.180W.180E.pgm';
open F,$depth_file or die "Could not open $depth_file ($!)";
<F>; # skip header line
my($wid,$hgt) = split ' ',scalar <F>;
<F>; # skip another header line
$wid == 36000 or die "Unexpected width in depth file\n";
$hgt == 7000 or die "Unexpected height in depth file\n";
my $inc = 360/$wid;
my $log500 = log(100/0.2);
for($lat = 35 - $inc/2; $lat > -35; $lat -= $inc){
for($lon = -180 + $inc/2; $lon < 180; $lon += $inc){
read(F,$pixel,1) == 1 or die "Error reading $depth_file ($!)";
$pixel = unpack "C",$pixel;
printf "%7.3f %8.3f ",$lat,$lon;
if($pixel == 0){
print "no data\n";
}
elsif($pixel == 1){
print "land\n";
}
elsif($pixel == 255){
print "cloud or other masking condition\n";
}
else{
$depth = 0.2*exp($log500*($pixel - 2)/252);
printf "%6.2f\n",$depth; # depth in meters
}
}
}
You will also find a colored version of the PGM raster in PPM format here
https://oceancolor.gsfc.nasa.gov/SeaWiFS/bathymetry/SeaWiFS_median_depth.35N.35S.180W.180E.ppm.bz2
or in PNG format here
https://oceancolor.gsfc.nasa.gov/SeaWiFS/bathymetry/SeaWiFS_median_depth.35N.35S.180W.180E.color.png
The color look-up table used to convert the PGM image to the PPM image lives here.
https://oceancolor.gsfc.nasa.gov/SeaWiFS/bathymetry/depth_color_lut.txt
Additional websites describing some of the work we have done in collaboration with others are here.
Norman Kuring 10 March 2005