Find the Proportion Of A Polygon From an Overlaid Buffer

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09-13-2021 08:57 PM
GarrettRSmith
Occasional Contributor II

Hello

I am working on a project where I am running a 1-mile buffer from a point. The buffer is then overlaid on census tract data. I was wondering if there is a way to find out what proportion of a census tract is contained with the buffer?

I am trying to come up with a way to calculate the population of that census tract that is not fully contained within the buffer. For example, if a tract has 100 people and that tract is only crossed by 10 percent of the buffer, I would want the population to be 10 and not 100. 

It is in an urban area so there are a number of tracts that are like this within the various buffers.

Thanks

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GarrettRSmith
Occasional Contributor II

Hello

I found a way to do it all through Fields.

  1. Create and calculate an area field for the unclipped census tracts
  2. Clip the census tracts to the one-mile buffer 
  3. Create and calculate an area field for the clipped census tracts
  4. Create a percentage field dividing Step 1 from Step 3
  5. Create a Clip field for each count demographic and multiply the unclipped demographic value by the percentage

This is by no means elegant but it works.

Thanks for all your help.

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DanPatterson
MVP Esteemed Contributor

Tabulate Intersection (Analysis)—ArcGIS Pro | Documentation comes to mind.  Or have a look at the other overlay tools in the An overview of the Overlay toolset—ArcGIS Pro | Documentation for other options


... sort of retired...
GarrettRSmith
Occasional Contributor II

Hi Dan.

Thanks for the suggestions. Tabulate intersection gives the percentage of the polygons that make up the buffer and none of the other Overlay tools are what I am looking for.

I am including a picture of the project so you can see what I am talking about.

I have a study site (red point) and a one mile buffer overlaid on census tracts. As you can see, some of the tracts are fully contained within the buffer so I can assume that 100% of a value, say total population, would be appropriate for the analysis. The tracts on the outer edges of the buffer however are not 100% within the buffer so I cannot assume that 100% of a value, population again, would be appropriate for the analysis.

I am most interested in finding out what percentage of the outlying tracts fall within the buffer. I can do the calculations outside of Arc if that is needed. I would imagine with the strange nature of census data and other administrative boundaries that someone would have come up with a way to so this. 

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DanPatterson
MVP Esteemed Contributor

If you have the total area and the areas from tabulate intersection, then you can use a simple field calculator expression to get the percentage


... sort of retired...
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GarrettRSmith
Occasional Contributor II

Hello

I found a way to do it all through Fields.

  1. Create and calculate an area field for the unclipped census tracts
  2. Clip the census tracts to the one-mile buffer 
  3. Create and calculate an area field for the clipped census tracts
  4. Create a percentage field dividing Step 1 from Step 3
  5. Create a Clip field for each count demographic and multiply the unclipped demographic value by the percentage

This is by no means elegant but it works.

Thanks for all your help.

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