[GRASS-dev] import/export projection error

Hello,

I just encountered a very peculiar error.

import of latlong/WGS84 geotif
import of latlong/WGS84 shp

border fits well

export of latlong/WGS84 asc/img etc. via r.out.gdal

load GRASS export asc into QGIS plus shp layer:

upper left: border fits good
middle left: border is shifted slightly
lower right: border does not fit at all

load original geotif into QGIS, overlay with shp layer: border fits

it is a continental raster/shp - perhaps only this extent makes the problem
visible.

I attached the original raster + shp and the GRASS exported raster + shp in
QGIS

if somebody wants to have the original files for testing, please feel free to
contact me.

cheers, Martin

--
Martin Wegmann

DLR - German Aerospace Center
German Remote Sensing Data Center
@
Dept.of Geography
Remote Sensing and Biodiversity Unit
&&
Dept. of Animal Ecology and Tropical Biology
University of Wuerzburg
Am Hubland
97074 Würzburg

phone: +49-(0)931 - 888 4797
mobile: +49-(0)175 2091725
fax: +49-(0)931 - 888 4961
http://www.biota-africa.org
http://www.biogis.de

(attachments)

grass_export_raster.jpg
original_raster.jpg

the shift only happens for r.out.gdal export of AAIGrid not for geotiff or
hfa.
So far it seems that the 3 ( and upward) decimal place of the pixel size
(AAIGrid exports only) differs from the original (and GRASS imported) file
what causes the error on a very large scale.

regards, Martin

On Friday 05 May 2006 18:43, Martin Wegmann wrote:

Hello,

I just encountered a very peculiar error.

import of latlong/WGS84 geotif
import of latlong/WGS84 shp

border fits well

export of latlong/WGS84 asc/img etc. via r.out.gdal

load GRASS export asc into QGIS plus shp layer:

upper left: border fits good
middle left: border is shifted slightly
lower right: border does not fit at all

load original geotif into QGIS, overlay with shp layer: border fits

it is a continental raster/shp - perhaps only this extent makes the problem
visible.

I attached the original raster + shp and the GRASS exported raster + shp in
QGIS

if somebody wants to have the original files for testing, please feel free
to contact me.

cheers, Martin

--
Martin Wegmann

DLR - German Aerospace Center
German Remote Sensing Data Center
@
Dept.of Geography
Remote Sensing and Biodiversity Unit
&&
Dept. of Animal Ecology and Tropical Biology
University of Wuerzburg
Am Hubland
97074 Würzburg

phone: +49-(0)931 - 888 4797
mobile: +49-(0)175 2091725
fax: +49-(0)931 - 888 4961
http://www.biota-africa.org
http://www.biogis.de

well, as far as I understand AAIGrid now, it stores one value for raster file
resolution only. If the values (nsres/ewres) differs slightly the single
resolution value in the AAIGrid export file holds a wrong resolution for ns
or ew which is only visible if number of columns/rows is high (e.g. high
resolution imagery or continental scale raster file).
It can be solved by setting g.region to the same nsres/ewsres and resample the
respective raster files before exporting the raster.

If my error detection result is correct then I would suggest that r.out.gdal
prompts a warning for AAIgrid if g.region nsres/ewres resolution differs.

Martin

On Monday 08 May 2006 14:31, Martin Wegmann wrote:

the shift only happens for r.out.gdal export of AAIGrid not for geotiff or
hfa.
So far it seems that the 3 ( and upward) decimal place of the pixel size
(AAIGrid exports only) differs from the original (and GRASS imported) file
what causes the error on a very large scale.

regards, Martin

On Friday 05 May 2006 18:43, Martin Wegmann wrote:
> Hello,
>
> I just encountered a very peculiar error.
>
> import of latlong/WGS84 geotif
> import of latlong/WGS84 shp
>
> border fits well
>
> export of latlong/WGS84 asc/img etc. via r.out.gdal
>
> load GRASS export asc into QGIS plus shp layer:
>
> upper left: border fits good
> middle left: border is shifted slightly
> lower right: border does not fit at all
>
> load original geotif into QGIS, overlay with shp layer: border fits
>
> it is a continental raster/shp - perhaps only this extent makes the
> problem visible.
>
> I attached the original raster + shp and the GRASS exported raster + shp
> in QGIS
>
> if somebody wants to have the original files for testing, please feel
> free to contact me.
>
> cheers, Martin

--
Martin Wegmann

DLR - German Aerospace Center
German Remote Sensing Data Center
@
Dept.of Geography
Remote Sensing and Biodiversity Unit
&&
Dept. of Animal Ecology and Tropical Biology
University of Wuerzburg
Am Hubland
97074 Würzburg

phone: +49-(0)931 - 888 4797
mobile: +49-(0)175 2091725
fax: +49-(0)931 - 888 4961
http://www.biota-africa.org
http://www.biogis.de