topic Re: Selection Based on % Crossing Layer Border in ArcGIS Spatial Analyst Questions

[SOLVED]Selection Based on % Crossing Layer Border

JoelMadero — Fri, 13 Apr 2012 15:04:24 GMT

Hi All,
First post, thanks in advance. Wasn't positive where to put this thread so I apologize in advance if this is the wrong place.

My situation is that I have two polygon layers in my project. Layer A has large polygons while Layer B has much smaller polygons which for the most part are completely contained within individual polygons of Layer A. There are some (<10%) of Layer B polygons that cross over a border of a Layer A polygon. In 99% of these cases the crossover is minuscule and doesn't matter for my purposes. What I need to select are only polygons from Layer B that have at least some % OR some # of acres in two or more polygons from Layer A. If I do a selection just based on crossing over Layer A's border I get all of the ones that the crossover is much too small for me to care about them so I get a huge false positive (in terms of what I need). Because there are 1.2 million polygons in Layer B, even a false positive for 10% of the polygons is too much to manually check.

If I'm not being clear I can rephrase, I appreciate any help as automating this process will make my life a lot easier

Re: Selection Based on % Crossing Layer Border

DarrenWiens2 — Fri, 13 Apr 2012 15:58:10 GMT

1.) Calculate a new field to hold the starting area of each polygon in Layer B
2.) Use the Identity tool (requires ArcInfo) to split the polygons in Layer B by the borders in Layer A
3.) Calculate a new field to get the percentage of each divided polygon
4.) Select based on the percentages, then Export to a new layer and Dissolve the pieces back together

Re: Selection Based on % Crossing Layer Border

JoelMadero — Fri, 13 Apr 2012 16:00:34 GMT

1.) Calculate a new field to hold the starting area of each polygon in Layer B
2.) Use the Identity tool (requires ArcInfo) to split the polygons in Layer B by the borders in Layer A
3.) Calculate a new field to get the percentage of each divided polygon
4.) Select based on the percentages, then Export to a new layer and Dissolve the pieces back together

Unfortunately no ArcInfo, any clever ways to do it without it ?

Re: Selection Based on % Crossing Layer Border

DarrenWiens2 — Fri, 13 Apr 2012 16:05:45 GMT

Possibly Intersect...? It will split Layer B by Layer A, although you'll lose areas in Layer B that don't overlap with Layer A.

Re: Selection Based on % Crossing Layer Border

JoelMadero — Fri, 13 Apr 2012 16:09:39 GMT

Possibly Intersect...? It will split Layer B by Layer A, although you'll lose areas in Layer B that don't overlap with Layer A.

I'll give it a try. I don't care about the polygons that aren't split so I don't see that being an issue. Basically I just need a complete list of only split polygons that are legitimately split and not just minor data entry problems that resulted in <1% of polygon being split by the layer A. I'll report back shortly to see if I make progress. Thanks again

Re: Selection Based on % Crossing Layer Border

JoelMadero — Fri, 13 Apr 2012 16:57:37 GMT

No luck, select by intersection selects even the ones that only go over the Layer A polygon borders by a tiny amount

Re: Selection Based on % Crossing Layer Border

JoelMadero — Fri, 13 Apr 2012 18:41:59 GMT

That did the trick. A bit of manipulation in GIS, then exported to OpenOffice so that I could play around with the data a bit more. THank you!