overlay county shapefiles for 2 different years
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I have county-level shapefile for South Caroline in 1880 and one for 1940. I would like to overlay these two files so that they do the union and indicate the percentage of the land in 1880 that went to form counties in 1940.
For example, McCormick county formed in 1914. Thus, it only appears in the 1940 shapefile. It formed from Edgefield and Greenwood. I want to know the percent of the land from Edgefield and Greenwood that went into forming McCormick. For example, is it 20%, 26.4 percent or some other percent of Edgefield County land that was lost to form McCormick?
I think I should be using overlay, and then within overlay using union.
However, I do not know how to pick the tolerance. How does that work? In addition, I cannot at all interpret the results. I cannot understand the columns being added and what they mean.
I am using ArcMap 10.1 within ArcGis.
Thank you very much for any help that you can provide.