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

[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=sharing

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

-- 
Jonathan A. Greenberg, PhD
Assistant Professor
Global Environmental Analysis and Remote Sensing (GEARS) Laboratory
Department of Geography and Geographic Information Science
University of Illinois at Urbana-Champaign
607 South Mathews Avenue, MC 150
Urbana, IL 61801
Phone: 217-300-1924
http://www.geog.illinois.edu/~jgrn/
AIM: jgrn307, MSN: jgrn307 at hotmail.com, Gchat: jgrn307, Skype: jgrn3007