I have five vector files wherein I want to determine the spatial agreements among them using intersect. Here is my illustration. (Actually these are forest shapefiles from five different forest maps.)

 A = vector_file_1.shp
 B = vector_file_2.shp
 C = vector_file_3.shp
 D = vector_file_4.shp
 E = vector_file_5.shp

So that I will have a set of spatial agreement from 2/5, 3/5, 4/5 to 5/5. For example:

1)   2/5 = A∩B, A∩C, A∩D, A∩E, B∩C, B∩D, B∩E, C∩D, C∩E
2)   3/5 = A∩B∩C, A∩B∩D, A∩B∩E, A∩C∩D, A∩D∩E, A∩C∩E, B∩C∩E, B∩C∩D, B∩D∩E, C∩D∩E
3)   4/5 = A∩B∩C∩D, A∩B∩C∩E, B∩C∩D∩E, A∩C∩D∩E
4)   5/5 = A∩B∩C∩D∩E

My question is what geoprocessing technique will I use to generate a single shapefile from each agreement above (except item # 4), supposing I've done the intersection combinations for agreement items 1-3? Will I use intersection again, union or other technique?

(My professor told me to just add a field and put a number code on that field, for each shapefile, unfortunately I forgot the next process he mentioned.)


There are two possible approaches to the problem, using Intersect or Union. First it would be helpful to understand what the Overlay options you mention actually do. Intersect only returns areas of overlap, Union returns all areas from both layers. It is further worth noting that in ArcGIS you are limited to two input layers per operation unless you have an Advanced level license.

If you use Union on all five layers at once, you end up with a single layer where everything is cut up into areas of exclusive overlap. In the attribute table there is a column for each input layer, and each shape in the output will note in those attributes which layers contributed to it. Based on some selection by attribute statements, you can export whichever layer combinations you want to individual files.

The other approach is Intersect. Since only areas of overlap are preserved, you have to do a number of intersects. With an Advanced license, since you can have more than two inputs, you basically have to run as many intersects as you have combinations (including item 4). However if you're limited to just two inputs, you have to do Item 1 first, then use those results in doing Item 2 - so it isn't int ABC, it's result AB int c. Essentially 'building' your intersections two inputs at a time, so to get ABCD would take three intersects.

Your question title currently conflicts with the body - one asks for a single file, the other asks for individual files for each combination. If you want a single file, Union is the way to go, especially if you can do all layers at once. If you want individual files, either method is probably the same amount of work.

  • just want to rephrase my question. What geoprocessing technique will I use to combine the outputs for each item agreement to form a single shapefile for each item agreement? Basically, I will use the said overlay items to extract my main data. For example, if I have a single overlay vector for 3/5, I am sure that my main data will have a 60% reliability that my data is forest. 2/5 will be 20%, 4/5 will be 80%, and 5/5 will be 100%. May 25 '15 at 23:56
  • @user35246 If that's the goal I would definitely use Union, especially if you can do all five layers at once. The reason for this is it will generate a single, non-overlapping set of shapes, and you can quickly do a field calc on a new field to count the number of inputs that cover that area. That gets one set of polys to extract with, no overlap to worry about, and a percent rating attribute. Intersect will never tell you which areas are covered by only one input without another tool like Erase since it only preserves overlap. Intersect also requires more steps, and appending/merging files.
    – Chris W
    May 26 '15 at 5:01

If i got you, your question itself is answering the question. You used A∩B i.e. Intersection symbols so use intersection analysis to separate out common areas, for A∩B run intersection between A and B, for A∩B∩C run intersection between A, B and C- mind intersection operation input can be multiple.Documentation is at here.

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