[GRASS-user] help exporting complex vector files

andrew.haywood at poyry.com andrew.haywood at poyry.com
Wed May 2 20:49:50 EDT 2007

Dear List,

I am looking for some advice on the best way to polygonise large raster 
layers into shapefile format
The context for this question is the need to creating a slope-class vector 
layer for land-use classification. 

The following steps are what I am currently doing

import SRTM data for the area I am interested in and then run r.fillnulls 
to fill the gaps
run r.slope.aspect to generate a raster of slope (in percent)
run r.reclass to classify the slope classes into 0-10%, 11-20%, 21-30% 
.... 81-90%, 90-100% 100+%
then run a r.neighbour with type=mode on a 3 by 3 window to smooth some of 
the salt and pepper effect away.
then run r.to vect with the area flag to create a vector representation of 
This usually creates a vector with tens of thousands of polygons 
Then I run a v.out.ogr to create a shapefile

The last step is when I run into problems with 50 000 or more polygons. 
The shapefile tends to be corrupted with empty values in the dbf for slope 
and doesn't reflect the vector file in the grass format.

The current work around is export the slope raster file as a tiff file. 
Import into arcgis then convert to shapefile. This approach im not happy 
with!! So any solutions would be greatly appreciated.

Kind regards


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