[GRASSLIST:9483] Re: SRTM shifted 0.5 the res in X and Y axis from the r.mapcalc output

[Markus Neteler]

Maciek,

since we have spent significant time on GDAL vs SRTM
vs BIL sv GRASS cell positioning, I wonder that
SRTM in GRASS is shifted. It's at least unlikely.
See the r.in.srtm script for comments.

Probably you are facing a nearest-neighbor problem?
I don't trust d.zoom to much...

Markus

On Wed, Dec 14, 2005 at 08:28:51PM +0100, Maciek Sieczka wrote:
> This might be connected with my [GRASSLIST:9409].
> 
> SRTM tiles imported with r.in.srtm are shifted 0.5 the resolution in X
> and Y axis from the native Grass rasters cretaed with r.mapcalc in the
> same location.
> 
> See the attached picture and notes below. I'm not doing anything
> unusuall. Why are these two rasters shifted? I don't think they should
> be. I guess that when importing data like SRTM (BIL), where the
> coordinates are of the cell centre, into Grass, where coordinates are of
> the cell ll corner, shuch data should shifted to conform to Grass grid.
> Correct me please if this is rubbish what I suppose, but I'm puzzled
> that I have two rasters of the same res in Grass and they don't fit each
> other.
> 
> 
> 
> # 1. import a 3" SRTM tile
> r.in.srtm input=/home/maciek/gis/dane/srtm2/N49E023.hgt.zip
> output=N49E023
> 
> g.region rast=N49E023 -p projection: 3 (Latitude-Longitude)
> zone:       0
> datum:      wgs84
> ellipsoid:  wgs84
> north:      50:00:01.5N
> south:      48:59:58.5N
> west:       22:59:58.5E
> east:       24:00:01.5E
> nsres:      0:00:03
> ewres:      0:00:03
> rows:       1201
> cols:       1201
> 
> # colour it and zoom to the top-right corner of the tile to see single
> # cells
> r.colors N49E023 col=grey.eq
> d.rast N49E023
> d.zoom 
> 
> # create a GRASS raster there, in the same resolution
> r.mapcalc 'test=rand(0,1000)'
> 
> # increase the resolution so that any small shift is visible
> g.region res=0:0:0.1 -p
> projection: 3 (Latitude-Longitude)
> zone:       0
> datum:      wgs84
> ellipsoid:  wgs84
> north:      50:00:09N
> south:      49:59:48N
> west:       23:59:54E
> east:       24:00:12E
> nsres:      0:00:00.1
> ewres:      0:00:00.1
> rows:       210
> cols:       180
> 
> # display the Grass raster and overlay an SRTM
> d.erase; d.rast test; d.rast N49E023 -o
> 
> # here's the shift
> 
> See the attachment - grey is the SRTM, r.mapcalc ouput in random colors.
> 
> Maciek
> 
> 
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-- 
Markus Neteler     <neteler itc it>       http://mpa.itc.it
ITC-irst -  Centro per la Ricerca Scientifica e Tecnologica
MPBA - Predictive Models for Biol. & Environ. Data Analysis
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