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I am a CS major with little experience in GIS. I am working on vector polygon overlay. Looking at GML and shapefile data, I realized that a map is made up of a large number of polygons. Does a polygon in a map intersect with another polygon in the same map? In other words, is it correct to assume that no two polygons in one map intersect with one another? It is very difficult to verify this by looking at the vertices of the polygon because of the sheer size of the files.

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closed as not a real question by blah238, iant, PolyGeo, Get Spatial, djq Oct 28 '12 at 23:04

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

    
I sense a bit of a language issue here. Are you asking how to find out whether two polygons intersect? –  blah238 Oct 22 '12 at 1:05
    
@blah238 i have modified to clarify my question. –  justin waugh Oct 22 '12 at 1:44
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It is certainly possible and indeed often necessary for polygons to overlap depending on the nature and purpose of the data. I still am not seeing a real question here. –  blah238 Oct 22 '12 at 1:50
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Your example kind of proves my point -- it's all up to how the data was designed and entered. So what are you getting at? Is there an actual problem you are trying to solve? –  blah238 Oct 22 '12 at 4:52
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Justin, there are many distinct senses in which polygons can "intersect". To help clarify your question, you might be interested in learning more about relationships among spatial features or perhaps in how ArcGIS views intersection. –  whuber Oct 22 '12 at 13:08

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I think what you are describing is known as coplanar polygons - shapefiles are more likely to keep two polygons in separate "planes", i.e. no intersections created, but the Union tool can be used to planarize them.

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Unless you are talking about PolygonZM's with different Z values all polygons in a projected coordinate system are coplanar. –  blah238 Oct 22 '12 at 4:19
    
I don't know what else you would call that other than "intersecting" polygons, BTW. –  blah238 Oct 22 '12 at 4:21
    
Coplanar polygons are ARC/INFO (ArcInfo Workstation) speak for the way polygons are stored in coverage format where there was no support for loop topology. It led to overlapping polygons being modelled using region topology. All came long before Zs and Ms got stored with X,Y. Another relevant term may planar enforcement which coverages required but shapefiles do not. –  PolyGeo Oct 22 '12 at 4:26
    
Ah, you are talking about this then: Overlapping polygons in a coverage: Regions –  blah238 Oct 22 '12 at 4:30
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I'm not sure if you were referring to me or @PolyGeo, but as it stands this is not a yes/no question. As such you get an "it depends" answer. –  blah238 Oct 22 '12 at 5:05

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