arcgis-desktoparcmappointrandomraster
I need to generate random points using a raster layer, which is classified into 5 classes of TPI (terrain position index). This should be a stratified random sampling design, with an equal amount of points on each class and ideally a minimum distance between the points.
I prefer to work in ArcMap. Does a tool for this exist? I heard Hawths tools could work, but I have v.10.3 so maybe not.
You can see one of the sites (classified TPI) here:
In ArcGIS, this can be done with Create random point if you have spatial analyst or 3D analyst or Advanced(ArcInfo) licence
select each shapefile as "constraining_feature_class" and set the "minimum_allowed_distance"to 250 m.
In QGIS, from what I know, you can use "Vector ‣ Research Tools ‣ Random points" with a constraining feature class, but not for distance. Again, you can use this tool in a loop to add points one by one until you reach the required number.
On the other hand, a solution for simple random sampling with distance contraint is to recursively add your random point and erase the buffer around your random point from the feature class until all your features are completely erased. As mentioned by @Whuber, this will not optimize the number of sample points, but you can control the number of points per strata.
Finally, the optimal number of points with a distance constraint can be achieved by systematic sampling : create a grid (with fishnet) with evenly spaced points (in your case 250 m) and randomly select a rotation angle and shift value betwen 0 and 250 in X and Y direction. This will optimize the number of point samples (and you could randomly subselect some points to reach a given number inside each startum). However, note that systematic sample could be biased because of the spatial autocorrelation.
You can use Random points inside polygons (fixed) from Processing Toolbox. If you don't see Processing Toolbox panel, activate it from menu Processing --> Toolbox. For distances in meters you have to use projected CRS (e.g some UTM) for more information about CRS see QGIS Doucumentation
Point sampling tool that you mentioned, is good for retrieving values from raster layers.
Best Answer
What I do:
Create attribute for raster (Build Raster Attribute Table in ArcGIS)
Select a class/row in raster attribute table
Use raster to point tool to create a BUNCH of points for each pixel of that value
Use Subset Features Tool to create specified number of random points
Note: this is the technique I used for accuracy assessments, I compare the points against the imagery to see how valid the classification was.