Robert: Thanks! Yeah, I ended up being a bit more patient (after your response) and realized it WAS working, it just took a REALLY long time. I found this bug report from 2 years ago:
http://hub.qgis.org/issues/3371 (Random points is terribly slow with complex features) which seems to still be unresolved. I'll check out the internal code at some point, but I'm not a QGIS developer -- is there anyone out there that might like to "collaborate" on speeding this up (if I come up with some algorithm modifications, could someone actually implement them?) By the way: this is a non-trivial issue, in my opinion, if someone wants to do "true" stratified sampling (per-polygon sampling isn't really stratified in most cases), this tool, applied to a multi-part polygon, is the only tool I've found that can do this (ArcGIS and plugins don't seem to work). --j On Fri, Sep 6, 2013 at 6:32 AM, Robert Nuske <[email protected]> wrote: > 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 > _______________________________________________ > Qgis-user mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/qgis-user -- 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: [email protected], Gchat: jgrn307, Skype: jgrn3007 _______________________________________________ Qgis-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-user
