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Symmetrical Difference not capturing all changes

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06-12-2024 09:42 PM
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AMcc
by
New Contributor

Hello,

I have two polygon datasets representing vegetation boundaries. They are both in the same location, one from 2018, one from 2023. I am trying to represent the change in boundaries over time, ie plotting polygons everywhere the boundaries are different. 

I have attempted to use the Symmetrical Difference tool but it has failed to pick up a number of differences. I attempted to change the settings on the tool but there was no difference in the output. There is also not much consistency - external polygons are captured, sometimes they're not, contained polygons are sometimes captured, sometimes not.  I have attached pictures below for reference.

I have also tried looking at doing some more convoluted steps to achieve the desired ouput, with dissolves, merges, clips, overlaps, etc. but nothing is quite working. (Some boundaries have expanded, some have contracted, there are some completely new polygons, there are some polygons which have been deleted)

If someone could advise me on how to get Symmetrical Difference working to capture all of the differences, or at least some steps which would result in getting the desired outcome, I'd be very grateful

Red = 2018 vegetation boundaries, Green = 2023 boundaries, Blue = Symmetrical Difference output. (Green is layered on top, with semi-transparent fill, solid outline. The two polygon layers are mostly overlapping)

AMcc_1-1718253727143.png

 

AMcc_0-1718253703967.png

 

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2 Replies
DanPatterson
MVP Esteemed Contributor

try 2 Erase 's switching the order between runs.  The combination of the results should show the boundary differences where they shrank or expanded.  Keep that in mind when interpreting the results


... sort of retired...
AMcc
by
New Contributor

Great, thanks for the tip. I'll give it a go and see what happens

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