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What is wrong with my Union setup?

Question asked by henry72 on Jul 23, 2019



Hi all,


I'm trying to use Union to find each small polygon created by overlapping three different layer polygons together. I am certainly a beginner at understanding ArcMap. The situation right now is I have 1 layer merged together from 2 different layers representing grass surfaces and impervious surfaces respectively in a shopping center site.The other layer I have loaded is a combination of the one drainage area and several soil types at the site.


Attached is an image of what the site looks like in AutoCAD with each layer shown. Pink is the curb/road boundaries, white is the drainage area, and red and blue tones represent soil types. I have used Union in smaller test cases to find overlaps and produce useful data (sets of 3 polygons, one for each overlapping layer). However, I cannot seem to find a Union of this more complex setup. All of my data has been imported as a .DWG from AutoCAD into ArcMap. Attached is a screenshot of my ArcMap window.



To find what I have now, I used Hatches in AutoCAD to create a boundaries of the grass and impervious areas (pink & green hatches). I found the union of the green hatches and the pink outline (Union 43). Then, using select by location, I deleted the parts of Union 43 where it intersected the original green hatches. This created the areas of yellow-green, indicating impervious surfaces, in the screenshot. I also used the union tool on the green hatches to create Union 44, the area of grass.


I have tried several combinations of merging and union to find the desired combinations I am looking for. However, the only Union that occurs is between the different soil polygons and the one drainage area. The end result in the attribute table is the original list of green "grass" areas, one large impervious area, and the chopped up pieces of the drainage area/soil type union. The latter is what I want, but applied to the whole set to create probably a 100-200 polygons. What should I do? I'm open to any suggestions. Thank you!