importing global data

GRASS GRASS user
1

From grass-lists-owner@max.cecer.army.mil Wed Aug 4 05:36:35 1993
Date: Wed, 4 Aug 93 22:22:59 EST
From: simon@cerberus.earth.monash.edu.au (Simon Cox)
Sender: grass-lists-owner@max.cecer.army.mil
Reply-To: grassu-list@max.cecer.army.mil
To: grassu-list@amber.cecer.army.mil
Subject: importing global data
Content-Length: 1869

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 r.support to
make the rest. This worked fine except

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

We had a similar problem with our elevation data, except they had wrapped
at 32768 (=2^15) rather than 2^16. We did a g.mapcalc, ex:

  elevation=if(oldelevation>65500,oldelevation-65536,oldelevation)

(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?

Center of cell

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!

I'm not sure I understand what you tried to do here, but it sounds like
rather than changing your region, you might want to fiddle with the cellhd
north, south, east and west.

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

Thanks

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@cerberus.earth.monash.edu.au
__________________________________________________________________