[GRASS-user] Projection errors with Aster GDEM and SHP Files

[Markus Metz]

On Thu, Jun 29, 2017 at 3:09 PM, preiaen <nils.hempel at gmx.de> wrote:
>
> YOU DID IT! Thanks for your help!
>
> Well that was a tough one and I wouldn't have figured that out on my own.

Good to hear that using the proj4 definition given in EXTENSION was the
solution!
>
> Two last questions:
>
> Why is QGIS able to deal with this problem on the fly,

Probably because QGIS uses the proj definition given in EXTENSION, whereas
GRASS via GDAL does not consider this information. A test would be to use
ogr2ogr on the shapefile for reporjection, then check if the geolocation is
correct.

> I always thought that
> GRASS is by far more powerful then QGIS?

The advantage of GRASS is that is gives you more control about how GIS
analysis is done. The advantage of QGIS is that it uses GIS processing
tools from various third-parties that are not included/available in GRASS.
QGIS is also great to have a look at spatial data without
importing/converting them first.

>
> Secondly: the manually created location had a proj parameter nadgrids:
@null
> . As long as I had this parameter in the proj file, I couldn't reproject
the
> vector map, for I always got the error "failed to load datum shift file".
So
> my solution was to delete this parameter. But what is he for and why did
he
> get a value assigned, which apperantly blocks reprojection?

the +nadgrids=@null parameter simply means that no nadgrid (North American
Datum grid shift file) is specified, therefore you can omit this parameter.

Markus M
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