Possible Duplicate:
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.


I suspect, have not had a chance to test, that this may be the relevant part of Union help to read:

"Attribute values from the input feature classes will be copied to the output feature class. However, if the input is a layer or layers created by the Make Feature Layer tool and a field's Use Ratio Policy is checked, then a ratio of the input attribute value is calculated for the output attribute value. When Use Ratio Policy is enabled, whenever a feature in an overlay operation is split, the attributes of the resulting features are a ratio of the attribute value of the input feature. The output value is based on the ratio in which the input feature geometry was divided. For example, If the input geometry was divided equally, each new feature's attribute value is assigned one-half of the value of the input feature's attribute value. Use Ratio Policy only applies to numeric field types."

  • I'm just having a hard time figuring out how to implement this. Is is the Shapre_Area variable that will be helpful? Line? How do I get the percentages in the end? – user1690130 Oct 8 '12 at 5:12
  • I'll try to flesh out the steps tonight to ensure that this will work. – PolyGeo Oct 8 '12 at 6:40
  • Started upgrading my Answer to one similar to gis.stackexchange.com/questions/30675/… (before I saw it) - I recommend using that rather than the approach I suggested here. – PolyGeo Oct 8 '12 at 22:13

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