[Qgis-user] 'gdawarp' causing shift and other questions

[Etienne Tourigny]

1) Probably a datum issue, as shapefiles do not include datum shift
information. You could work this around by making a new shape file
with the same datum as the gtiff file.

What are the datums of your gtiff and shape files?  Please post the
contents of the .prj file.

Etienne

On Mon, Jan 30, 2012 at 9:39 AM, Tilottama Ghosh <waggymaggy at gmail.com> wrote:
> Hi,
>
> I have a huge tiff image (covering an entire country) with 3399 columns and
> 3747 rows, so a total of 12,736,053 cells. The cell size is 1000m or 1km. I
> need to extract the data per km2. Of course, I didn't think it was smart to
> work with such a huge dataset in one go. So, I followd these steps in
> Quantum gis -
>
> 1. I used the gdalwarp -crop_to_cutline in the command line to carve out a
> portion of the raster image based on the shapefile  of a state of the entire
> country. The problem is that although the gdalwarp command worked it is
> clearly causing a shift (of about 500m) during this step.
>
> 2. Then, I used the gdal2xyz tool from the command line to create the csv
> file corresponding to the 'cut out' raster image or only the raster image of
> the state.
>
> 3. Then I used the 'Create a Layer from a Delimited Text file' plugin in
> qgis to get the centroid shapefile.
>
> My problems are-
>
> 1. The shift in the image caused by the gdalward tool. Does anyone know how
> to work around that, so that no shift is caused?
>
> 2. The image is in 32 bit floating point and so does not show the attribute
> table. Even after getting the csv file I am unable to link the created csv
> file with the raster image, and thus cannot understand which pixel
> corresponds to which value in the csv file.
>
> Can anyone suggest any better way of doing this?
>
> Thanks very much for your help.
>
> Best Regards,
>
> Tilo
>
>
> --
>
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