I have a polygon in QGIS 2.14 and want to generate a specified number of coordinate points within that polygon, with a minimum distance apart. The fTools Random Points tool doesn't give this option, and neither does the "Point sampling" analysis tool in the Point Sampling plugin (or maybe I don't know how to use it properly). Is there a way to do this in QGIS? I know it's quite straightforward in the Random Points tool in ArcGIS.
[GIS] Generate random points within a polygon with minimum distances in QGIS 2.14
qgisrandomsampling
Related Solutions
fTools is still there under the Vector menu. The sampling tool you need is Vector|Research Tools|Random Points.
You need to create a new attribute (like 'points') for your polygon layer; for each stratum, you enter the number of points you need to be generated for that stratum. In the dialog you select that new attribute in the 'value from input field' dropdown.
Update: This only seems to be working in QGIS 1.9, not 1.7.4.
Update 2: If you're using 1.7, this is a known bug but there is a workaround:
- Make sure you have another polygon layer loaded;
- Open the Random Points tool again, and first select a different layer to the one you want to use as the strata boundaries;
- Select the strata boundaries layer, and then you can select the proper field name.
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.
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Best Answer
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 DoucumentationPoint sampling tool that you mentioned, is good for retrieving values from raster layers.