Peter W. Sloss NOAA/NGDC Code E/GC3 325 Broadway Boulder, CO 80305-3328 Peter.W.Sloss@noaa.gov How to find the correct scan line for a given latitude in the Mercator Images: There are three Mercator Image files of Red, Green, and Blue separates. There is also a color scale bar (2000 pix wide x 131 lines) with breaks indicated at 9000, 7000, 5000, 3000, 1500, 500, 200 meters below sea level, and 0, 200, 500, 1000, 2000, 3500, and 5000 meters above sea level. The endpoints of the scale are at 11000 meters below and 9000 meters above sea level. The image files are in the Mercator Projection on a sphere, spanning ±80¡ latitude, and from 90¡E around the world plus 90¡ to 120¡E. There are 3352 lines, each 4682 pixels long, in the image, including a blank line (all white) at the top of the image. (There is a white pixel at the beginning and end of each line.) There are also dotted lines every 10¡ of latitude and longitude. To find a specific latitude on the image, count lines away from the center, using to the following FORTRAN code to compute a lookup table: subroutine mparts(phid) c ** Compute meridional parts for pixels 5'x5' at equator c y = R*alog(tan(pi/4 + phi/2.)) c phi = pi/2. - 2*atan(exp(-y/R)) c At Equator, pixel width for 5' is 2*pi/(360*12) dimension phid(1676) data pi /3.141592654/ pi2 = pi/2.0 y5 = 2.*pi/(360.*12.) c ** Compute latitude ÒphiÓ for the 1676 equal y-steps from Equator to 80¡ do 10 i=1,1676 y = y5*i c ** This is the inverse Mercator conversion phi = pi2 - 2.0*atan(exp(-y)) 10 phid(i) = phi*57.295779 end The latitude offsets are symmetrical about the Equator (Line 1676), so the lookup table need not be repeated for negative (south) latitudes. Note that the latitude of a given raster line will almost never fall on a whole degree -- the nearest neighbor must be used. The computed latitudes ÒphidÓ are in degrees. If you have any further questions, please call Peter Sloss at 303.497.6119.