[Qgis-user] problems making grass training map via qgis

[Samantha Goodchild-Brown]

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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