[GRASSLIST:9515] Re: SRTM shifted 0.5 the res in X and Y axis from the r.mapcalc output

[Maciek Sieczka]

On czw, 2005-12-15 at 22:08 +1300, Hamish 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.
> ..
> > # 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 
> 
> Markus and William are correct to suspect d.zoom.
>  What happens is that
> you pick a spot on the screen with the mouse and d.zoom aligns itself to
> the nearest 0:00:03 resolution click.

Thanks Hamish, Markus and William. I think I get it now. 

>  Maybe this is a bug, maybe it is a
> feature which needs a new flag to turn off. I don't know.

IMHO d.zoom should behave in such way that it wouldn't cause this kind
of doubts as I had.

If d.zoom'ed my SRTM and reprojected this zoomed part only, my output
would become shifted - up to 3 arc sec horizontally and vertically. Same
if I wnated to do any r.mapcalc, r.to.vect, r.patch, else. This should
not take place - but it does. How are users supposed to know this
danger? Looks like quite a serious issue to me - data corruption threat.


> e.g. in UTM try setting your extent to 200075,220025 with res=50 and try
> d.zoom. It will end up on xxxx50,xxx100.
> 
> 
> > # 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
> 
> try changing the res before d.zoom and it should be ok.

I'm affraid I don't understand. Change res to what value? What for?

Cheers,
Maciek


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