[GIS] Self intersections for polygons, how to solve

arcgis-9.3arcgis-desktopgeometrypolygon

I have a question about repairing a self intersecting polygon shapefile. I have a shapefile containing Polygons (ZM) which have self intersecting features. If I use the "repair geometry" tool, it finds several self intersections and fixes them. Hurray!

But when I make an export of this file (or a selection of it), this export contains self intersections again. This happens on all the tries I've done to handle the problem, which include:

Use the tool "repair geometry", failed;
Dissolve the polygons based on all of their attributes, failed;
Dispose of the ZM properties, and then do all of above, failed;

Even when I import this shapefile into an Geodatabase using ArcCatalog, it imports correctly, but when I make an export from the database, the self intersections are there again.

So any advice on how to fix this problem would be very much appreciated. If anyone could explain why this type of behaviour exists, it would be wonderful.

I'm using ArcGIS 9.3.1 build 4000 (SP2).

Best Answer

Is it feasible to just manually edit the vertices and remove the self intersections?

I do this a lot (using QGIS because it seems to be less picky about this issue). QGIS also has a 'Check Geometry' tool that will show you where all the self intersections are.

I know you said you're running arc but this might be a helpful approach if nothing else is working.

Related Solutions

[GIS] Is it possible to have self intersecting polygons in shapefile

Here are relevant excerpts from pages 8 and 9 of the Shapefile Technical Description (emphasis mine):

A polygon consists of one or more rings. A ring is a connected sequence of four or more points that form a closed, non-self-intersecting loop.

And:

Polygons stored in a shapefile must be clean. A clean polygon is one that

Has no self-intersections. This means that a segment belonging to one ring may not intersect a segment belonging to another ring. The rings of a polygon can touch each other at vertices but not along segments. Colinear segments are considered intersecting.

So, as far as the shapefile specification is concerned, the answer is no, polygons may not intersect themselves.

This is a conceptual rule, not a limitation of the file format. It is quite possible to construct shapefiles containing self-intersecting polygons. However, since self-intersecting geometry is invalid according to the specification, applications may differ in how they handle such cases: some may attempt to clean up the features, while others may [justifiably, if less helpfully] consider the shapefile malformed. Most problematic may be cases where the software assumes a shapefile conforms to the specification (ie, contains no self-intersections) but does not validate it before performing geometric operations, which may yield wrong results.

It's unclear exactly what you want to do, but if you're generating shapefiles, I'd advise against outputting polygons with self-intersections, as it may cause difficulty for your data users. Likewise, if you're writing some sort of algorithm to process shapefiles, you do not need to support self-intersecting polygons to comply with the specification - but if possible, I'd advise you check the input files are not malformed before proceeding.

[GIS] R Spatial: Trouble with disjointed polygon that has self-intersections

I also had this same error using gContains(). I didn't realise I had anything internal in my polygon. I dissolved the internal structure like this:

spgeom1$rowNo = 1

spgeom1= unionSpatialPolygons(spgeom1, spgeom1$rowNo)

and went on to use gContains() with no problem.

Best Answer

Related Solutions

[GIS] Is it possible to have self intersecting polygons in shapefile

[GIS] R Spatial: Trouble with disjointed polygon that has self-intersections

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