importing global data

Simon Cox simon at
Wed Aug 4 23:22:59 EDT 1993

Dear Grass-hoppers

I bought a copy of the NGDC_RELIEF CDRom, and am trying to import
some of the datasets into Grass.  This has revealed a deficiency
in my understanding of Grass raster maps.

The data I am first trying to import is the Global 5 minute elevation grid.
This is supplied in binary format, standard SUN 2-byte signed ints,
in 2160 rows x 4320 cols.  The first row represents the North Pole.
The last row represents 89:55:00S.

I set up a new location with North at 90N, South at 90S, West at 0,
East at 0, and a resolution of 5 minutes.  This gave the right number
of cells.  I then copied the file straight into a Grass cell directory,
made a few other support files in cellhd etc, and ran to
make the rest.  This worked fine except

(i) the negative values (bathymetry) have 65536 added to them
(ie -1m shows as 65535m elevation!)

(ii) d.what.rast shows that the cells are misregistered by 2.5 degrees
South and (?) East.  Does d.what.rast give coords of cell *centres* or
one of the corners?  if the former, then this suggests that the region
should be set 1/2 a pixel outside the data.  Is this true?  In which case
we are in trouble here at the North Pole at least, as grass wont accept
90:02:03N as a valid region boundary!

It is now 10:30 pm and my brain is beginning to hurt with all this.
Can anyone offer clarification or suggestions?


Simon Cox

				Dr Simon Cox
         __  L				
      ,~'  L_|\            	Department of Earth Sciences       
   ,-'         \         	Monash University    
   (            \		Clayton  Vic  3168  Australia
   \    ___     /	
    L,~'   "\_x/		Phone +61 3 565 5762
              u   		Fax   +61 3 565 5062
				simon at

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