![]() In the figure, point A overlays two districts and gets counted twice, once in district 5 and once in district 6. When a point falls exactly on a polygon boundary, it gets overlaid with each polygon that shares the boundary. More interesting are the blue asterisks that show points that fall exactly on a polygon boundary. The red triangles show points that fall outside the coverage of the polygons and do not overlay any of the polygons. The issue is that some points are ambiguous some fall outside the coverage of the polygons and some fall exactly on polygon boundaries, as shown on the map to the left. The HHSize sums are off as well instead of 1670, there’s 1621. Huh? Out of the original 340 points, only 327 were found to intersect the polygons. There are 340 points with a HHSize total of 1670. Now let’s go back to the Households table and get some statistics. These statistics tell us that 327 points were overlaid and the sum of HHSize for all polygons is 1621. The figure below shows the output of Spatial Join, the Districts_SpatialJoin feature class, symbolized by the sum of HHSize, and its table with statistics about the Join_Count field (the number of points found in the polygon) and the HHSize field (the sum of all HHSize values for the polygon). Since I want the sum of HHSize, I right-click the HHSize field in the Field Map and choose Sum as the Merge Rule. The Target Featuresare the polygons (Districts) and the Join Features are the points (Households). For now, let’s look at what I would describe as the “standard method”: Spatial Join, shown to the right. There are lots of ways to perform the overlay to get the number of households and the sum of household size. The map to the left shows the household points with graduated symbols based on HHSIZE. I want to count the number of households and the total size of the household in each polygon.Each household has an attribute HHSize which is the number of people in the household. And I have some points representing households.I have polygons representing some administrative districts.But unless you’re aware of what happens when points fall on polygon boundaries, or when points fall just outside the coverage of your polygons, you may not be getting the results you expect. Counting the number of points in a polygon is a common overlay operation. ![]()
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