[Qgis-user] Stratified sampling based on polygon ID (vs. individual polyons)

[Robert Nuske]

Hi Jonathan,

the process you described works for me in QGIS 1.8.

After fiddling with your dataset I  reverted to a minimal example of a dozen 
polygons and three groups. After converting the singleparts to multiparts I  
asked for 5 "random points" in "stratified sampling design".  Each multipart 
feature got 5 points and in every multipart the points where spread out across 
all polygons belonging to that multipart.

So I guess its a matter of size.

cheers
  robert


Am Donnerstag, 5. September 2013, 16:09:53 schrieb Jonathan Greenberg:
> QGISers:
> 
> I'm trying to realize stratified random sampling using a
> classification raster that I polygonized.  It seems like the random
> points tool will create random points within each polygon, but in my
> case the strata are the classes, which can be scattered across a
> landscape -- in other words, multiple polygons make up a single strata
> (I'd argue this is a more valid definition of stratified random
> sampling).  I'd like to choose, say, 10 random points falling in each
> group of polygons with a given shared (classification) attribute.  Is
> there any way to realize this in QGIS?  I tried pre-converting the
> polygonized classification file to a multi-part polygon, but the
> random sampling tool froze up when I used this as an input.
> 
> I dropped the polygon layer (derived from the classification raster) in:
> https://drive.google.com/folderview?id=0B8Kij67bij_AMmNPakJmT1p4OWc&usp=shar
> ing
> 
> The "DN" attribute is the strata I want to use (notice multiple
> polygons have the same DN).
> 
> To be clear: I don't want 10 points per polygon, I want 10 points per "DN".
> 
> Thanks!
> 
> --jonathan