Decoding CM93 file name

[davidhills]

Hi everyone

I found this on the net below

but am struggling to under stand why
and what 0030:3 is and how it becomes +10 so -90 + 10 becomes -80
can some explain a bit more please

thanks
David


The tile 00300000 hence has the SW corner at lat from -90° + 0030:3 =-80°



The CM93 top directory contains a number of (144) subdirectories named from


"00300000" to "04501020", as well as six other files. These sub directories each cover a geographical area of 40° x 40° . The first four numbers describes the latitude and the last four, the longitude of the SW corner of the area covered.


The key to understanding the numbers is to realize that CM93`s coordinate system


of the world starts at the South Pole or to be exact at lat -90° long 0° and from there proceeds North a East with a factor of 3 for each degree of lat and long.


The tile 00300000 hence has the SW corner at lat from -90° + 0030:3 = -80° or 80°S to and longitude 0°E , and covers the area from 80°S latitude to 40°S and from 0° longitude to 40°E.


Looking at 04501020 it brakes down to lat -90° + 0450:3 = 60° and long 1020:3 = 340° subtracting 360° results in -20° or 20°W. So we have he SW corner at 60°N and 20°W .


One more example, the tile 03900840 has the SW corner at 40°N and 80°W.


Each of these sub-directories in turn contains directories with the individual charts. Ordered in scale from smallest to largest they are Z,A,B,C,D,E,F and G. Where Z contains overview charts and G contains harbor plans. Note that, generally, not all of these are present in each subdirectory.