[GIS] Creating random points using density-QGIS

point-in-polygonqgisrandom

I'm trying to generate random points at a specific density within a polygon layer. The polygon layer has 1850 features and I'm looking to have random points within each feature at 1/meter. Going through Vector -> Research tool -> Random points -> "Use this density of points" = 1 (assuming the default is based on unit of the project) -> Save As: .shp -> click OK. The OK button darkens, then lightens to normal without producing any result.

I've also tried going through the Processing Toolbox -> QGIS geoalgo -> Vect Creation Tools -> Random points inside polygons (fixed), set density = 1, and get error:

"Algorithm Random points inside polygons (fixed) starting…

float division by zero See log for more details"

Any ideas? I think it has something to do with the density parameter, but I haven't found what the unit used for the density is. Insight welcomed.

QGIS 2.14.3-Essen

Best Answer

I've had a similar problem and I think it's due to the way that QGIS calculates the number of points when using density. It is rounding the density * polygon area for each polygon. So if that value is < 0.5 for any of your polygons, QGIS will round this to 0 and then you'll get a divide by 0 error.

For example:

Say you choose a density of 0.01. If you have a polygon with an area of 49, then 49 * 0.01 = 0.49. QGIS will then round this down to 0.

The solution for me was to remove the polygons that were smaller than this, then re-add them afterwards.

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[GIS] Creating Random Points that respect minimal distance across features / strata using ArcGIS for Desktop

I had initially posted the question on geonet. The matter being rather urgent, I posted the question here as well when I didn't recieve an answer there. In the meantime, Xander Bakker posted an excellent script that solves the problem neatly. The full answer can be view here, this is the section that solves the problem (with some slight modifications from me).

import arcpy
import random
import math

def main(path,fc_pol,fc_pnt,fc_mp,min_dist,number_of_points):
    # path = path for the input and output data
    # fc_pol = name of the input polygon shapefile
    # fc_pnt = name of the output point shapefile
    # fc_mp = name of the output point shapefile, used for visualization it and some manual checking
    arcpy.env.overwriteOutput = True

    arcpy.CreateFeatureclass_management(path, fc_pnt, "POINT")
    arcpy.CreateFeatureclass_management(path, fc_mp, "POINT")
    lst_all_pnts = []
    lst_mp = []
    cnt_pol = 0
    with arcpy.da.SearchCursor(path+fc_pol, "SHAPE@") as curs:
        for row in curs:
            cnt_pol += 1
            # print "polygon: {0}".format(cnt_pol)
            polygon = row[0]
            lst_pnts = []
            sr = polygon.spatialReference
            # create points inside polygons
            while len(lst_pnts) < number_of_points:
                print " - len(lst_pnts)={0}".format(len(lst_pnts))
                lst_pnts, bln_ok = add_random_point(lst_pnts, lst_all_pnts, polygon, min_dist)
                if bln_ok == False:
                    # saturation reached
                    print "saturation reached"
                    break
            # write points
            if len(lst_pnts) > 0:
                mp = create_multipoint(lst_pnts, sr)
                lst_mp.append(mp)
                lst_all_pnts.extend(lst_pnts)

    # create outputs
    sr = polygon.spatialReference
    lst_pntgeom = create_pointgeometry_list(lst_all_pnts, sr)
    arcpy.CopyFeatures_management(lst_pntgeom, path + fc_pnt)
    arcpy.CopyFeatures_management(lst_mp, path + fc_mp)

[GIS] How to create multiple points inside polygon in QGIS

I would guess that your polygon layer originally was not saved with the EPSG:28356 CRS but that you set it manually.

Most likely the polygon layer was orignally saved with another CRS using degrees as units.

I did a quick test using polygons with a CRS of EPSG:4326 (i.e. using degrees as units) and using your exact parameters for the Random Points Inside Polygons (Fixed) tool:

I then right-clicked the Polygons layer, chose the Save As... option, assigned a new CRS which uses meters (i.e. EPSG:28356) and again repeated the same steps. This is the result:

So try saving your polygon layer into a new shapefile with the relevant CRS and run the tool again.

Best Answer

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[GIS] Creating Random Points that respect minimal distance across features / strata using ArcGIS for Desktop

[GIS] How to create multiple points inside polygon in QGIS

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