[GRASS-user] (v.out.ogr .shp bug) help exporting complex vector files

Hamish hamish_nospam at yahoo.com
Wed May 2 23:08:33 EDT 2007

andrew.haywood wrote:
> 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 slope
> 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 fo 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.

I don't know about the v.out.ogr bug, but another trick instead of
r.neighbors would be "v.clean tool=rmarea" to remove areas with area
smaller than thresh=<>.


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