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

Tilottama Ghosh waggymaggy at gmail.com
Mon Jan 30 06:39:55 EST 2012


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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