[GRASSLIST:10777] Re: SRTM data transformation

[Wolfgang Zillig]

It seems that this was what I did, but I think that the height values 
did not change during r.proj. It is a bit difficult to prove, but I will 
start from the batch again that there is no error included.

Wolfgang

Maciek Sieczka schrieb:
> On Mon, 06 Mar 2006 19:42:19 +0100
> Wolfgang Zillig <wollez at gmx.net> wrote:
>
>   
>> Hi all,
>>
>> does anybody know, how I can transfer a SRTM file to an other
>> ellipsoid? My target region is within MGI-M31 (EPSG 31285)
>>     
>
>
> Setup target location, eg. using the EPSG code you mention.
>
> In source latlong location set the region properly first, using
> g.region, to avoid your output missplacement - due to SRTM cells are
> not alligned to the resolution. If you use d.zoom, use g.region
> afterwards to correct the region; see:
> https://intevation.de/rt/webrt?serial_num=3961.
>
> Now you could reproject the SRTM raster using r.proj. But an optimal
> method, which disturbs the input data less, would be:
>
> 1. Transform your srtm to vector points r.to.vect.
>
> Note. Grass vector engine is very memory consuming, so setup the region
> only for your very area of interest. Otherwise r.to.vect might use all
> your memory. System shouldn't freeze, but might get extremely slow
> when all memory and swap are gone. Give it few days then ;).
>
> 2. Reproject the vector file to your EPSG 31285 location v.proj.
>
> 3. Interploate DEM. v.surf.rst should do fine for such a regulary
> spaced input as srtm. Or use other interpolation method you prefer.
>
> Note. If you decide for v.surf.rst then in step 1. you could speed up
> the process by using r.to.vect -z. This avoids building the datable and
> stores elevation as Z coordinate. This would save you a lot of memory,
> especially if using DBF. However, if you prefer v.surf.idw you would
> need the elevation stored in datable as v.surf.idw can't use Z
> coordinate for input.
>
> Cheers,
> Maciek
>
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