On 2012/08/15 12:40, Micha Silver wrote:
On 15/08/2012 11:17, Samantha Goodchild-Brown wrote:
Can anyone tell me how to make a training map for supervised
classification using QGIS Grass (1.8).
I have successfully made the polygons with label attributes (as a
grass vector layer - attribute table now looks the same as the sample
datasets from installation) but these attributes are lost when
converting to raster. Hence my raster comes out as one cell (one
value to region extents).
You should be getting a raster with different cat values for each of
the input vector categories.
Yes
In the GRASS toolbox is the module v.to.rast.attr which takes a
numeric column in the vector attrib table to be used as the new raster
categories.
yes
The full options for v.to.rast (not available in the QGIS GRASS
plugin, only in GRASS itslef)
Ah, confirmation! I was thinking to go to linux - any comments on which
is the better to use?
include a parameter "labelcol=" to give the resulting raster
categories a text label in addition to the numeric cat value.
Also be sure to run g.region vect=<your training vector map> before
converting.
This is new, thanks. This could be a missing link.....
If I use Grass GUI (6.4.2) to convert a vector polygon to a raster,
georeferencing gets lost.
That should certainly not be happening. What do you get from v.info on
the input vector, and r.info on that output raster?
In the process (with this interface) it does complain about a colours
column that it needs extra - have tried a few different methods there.
Indicates it made it sucessfully but 'couldn't resolve the colour table
issue' (with everything I have tried) - then when you zoom to that layer
you are at the CM's and of course - no colours. But I didn't know about
g.region.vect.....
Are these known bugs? can someone give me a workaround? Or fix my
methodology if I am making common dingbat errors.
Thanks
Sam
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Micha Silver
052-3665918
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