[GRASSLIST:2358] Correct hdr file and Location for SRTM-3 DEM?

[paallen at attglobal.net]

Hi,


I have just downloaded some of the SRTM-3 hgd DEM
files from
ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/South_America/.

I have re-named the files to *.bil and am creating
the text header files (*.hdr).  

My question is about the ULXMAP / ULYMAP header
coordinate and the coordinates for the default
region when I setup the LOCATION/MAPSET in grass5.

The readme files on the ftp site state that the
coordinates are the “geometric center of the pixel”.  

So should I subtract/add 0.000833333333333/2 from
the coordinates or just use integers in the header
files?  Also when I setup my Location/Mapset
should I open it with integers or subtract/add the
0.000833333333333/2

Is grass using the pixel center or the corner of
the pixel?

Example: downloaded S07W080.hgd -> renamed to
“S07W080.bil”
Header file “S07W080.hdr” reads as follows:
BYTEORDER M
LAYOUT BIL
NROWS 1201
NCOLS 1201
NBANDS 1
NBITS 16
BANDROWBYTES 2402
TOTALROWBYTES 2402
BANDGAPBYTES 0
NODATA -32768
ULXMAP -80.00000000000000
ULYMAP -6.0000000000000
XDIM 0.000833333333333
YDIM 0.000833333333333


And a LOCATION default region to hold just this image:
e=-79.00000
w=-80.00000
n=-6.0000
s=-7.0000


I plan on using these SRTM DEMs to calculate
watershed basins for my stream sediment samples. 
All of the stream sediment samples were located
with gps. I am going to create a stream raster
layer from r.watershed command and then move all
my gps points to exactly fall on the r.watershed
streams.  Since I have approx. 10,000 samples I
don’t want to have to do it twice due to incorrect
georeferencing of the DEM.

Thanks,

Phillip J. Allen
Chief Geochemist South America
Anglogold South America
e-mail: pallen at attglobal.net
            paallen at anglogold.com.pe