# Calculating proportional data for polygon by taking attributes from other polygon

I have a features class which has polygons in it. I also have a Block group level data for demographic variables like population. Now when I overlay the polygon on the block group level data to find out the population of that polygon.Now it would happen that the polygon boundary may not overlap exactly with the Block group boundary.It would intersect at different places with the block group data.

In such case how can you calculate the proportional population value to be considered for that polygon?

You'll want to to ensure that both of the layers are in the same Coordinate System.

1. Add a new field in the BG Data, and calculate the area, `bg_area`.

2. Run Intersect on your custom polygons and the BG data to create an `intersection` layer.

3. Ensure that you have the area of your custom polygons calculated. Arc should automatically calculate this in `Shape_Area`.

4. Create a new field in the `intersection` layer, `weighted_population`.

5. Calculate that field by taking `Shape_Area / bg_area * population = weighted_population`.

6. Dissolve your intersection layer on the Object ID's of the Custom_Polygons, and calculate a statistic field with the `SUM` of `weighted_population`.

• shouldn't it be bg_area/shape_area* population. Since many BG come under one polygon. – SNT Nov 23 '13 at 0:17
• It shouldn't matter whether there are multiple BGs inside of a polygon. In that case, the intersection function will subdivide the polygon into shapes that are the size of the bg. Then the proportion of the bg_area and the intersection shape area will be 1 to 1 and the weighted population will equal the original bg population. These then get summed up in the dissolve to polygon feature. The only thing you are concerned with is proportionally comparing the new_pop / new_area = old_pop / old_area. – willpots Nov 24 '13 at 19:52

A more automated way to accomplish this is first to run the Make Feature Layer tool on your block group data. In the Field Info section of this tool is a column called Use Ratio Policy. Check the box for any fields you want to divide proportionally, like TOTPOP. Call the output layer BlockGroup_RP or something like that. Now do your polygon overlay with BlockGroup_RP as the input layer. The overlay output will have proportional TOTPOP values from the contributing block groups. (Proportional according to the area of each block group feature that falls within the overlay polygon.)