I have a SewerGEMS network of a sewerage scheme for a town. The town is divided into 17 wards having different population densities in (Population/Hectare). There is an AutoCAD drawing showing the ward boundaries with the number for each ward. I also have a separate Excel sheet containing 2 columns specifying ward numbers and population density for each ward (Population/Hectare). I have generated the Thiessen polygons for the 1476 manholes in the network specifying a buffering percentage only (i.e. no separate ward boundary used).

Now the issue is that many manholes will have areas of influence in 2 or more wards with different population densities and thereby different flows. How do I calculate the corresponding areas served by each manhole in different wards?

Once I get these bifurcated areas I can multiply it by the corresponding population density for the corresponding ward / wards (If the area of influence is in 2 or more Wards) to get the population count for each manhole.

I want to know the detailed process for getting these areas of influence of each manhole in separate wards i.e. the bifurcated Thiessen polygon areas for each manhole having areas of influence in 1 or 2 or more wards having different population densities.


You need to intersect your Thiessen polygons and your ward polygons.

  1. Import your CAD file into ArcGIS
  2. Make sure that the extent of your Theissen polygons equals or exceeds the total extent of your wards. If it doesn't you can remake the Thiessen polygons with the exact extent by using Tool Environment Settings > Extent in the Create Thiessen Polygons tool
  3. Use Spatial Join with the "Target features Completely Contain the Join Features" to join the attributes of your manholes to your polygon. That way you will know which polygon goes to which manhole.
  4. Use Intersect to combine your ward polygons and your newly created/spatially joined Thiessen polygons. This will create a new combined layer of polygons with manhole information attached.
  5. The Shape_STArea field is already calculated for you. You can also use Calculate Geometry if you want an easy way to get the areas in another unit within ArcMap

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