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I have shapefiles for school attendance zones (SAZs) and Census Tracts (CTs), where the red lines below are CTs and the lighter lines are SAZs. I would like to calculate the proportion (%) of each CT covered by each SAZ. For example, if one CT was covered by two SAZs (SAZ1 and SAZ2), I want to be able to estimate that 28% of the CT is covered by SAZ1 and 72% of it is covered by SAZ2.

I have not tried anything yet, since I am fairly new at this and cannot think of a solution. I think I would start out by calculating the areas of each polygon in sq. miles, ft, meters, etc., then do a join of some sort (union? intersect?), but I am stuck.

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    Welcome. Please take the community tour here: gis.stackexchange.com/tour and edit your question to include additional details, such as what you have tried, text of any errors you have received, etc. – MaryBeth Aug 9 '17 at 18:31
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You are right.

In a simple example, if you have 3 different polygons:

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You can make a union or an intersect, in this case, a union:

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After this, calculate area with Field calculator. The output unit, in this case, is in meters, because I'm working with UTM projection:

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You can make dynamic tables with GroupStats plugin:

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Or, you can save shapefile as .csv file or copy/paste observations to a spreadsheet and create a pivot table to determine percent of one area over other:

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If "layer_1" is your objective layer, you can save pivot table as .csv file, open it in QGis and join it to the layer in Properties window / Joins.

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  • Thanks! I will try to do this and let you know how successful I am at it! – echan170 Aug 9 '17 at 21:31

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