GIS: Converting multi polygons to multiple features

Question

I am involved in a GIS project. I have a base map file (shape file) that contains the road layer for a large portion of a town. The problem is that the shape file contains only two features each containing around 500000 points each. The features are multipolygons containing a large no of polygons inside. I wish to convert it to numerous features each containing not more than one polygon. Is it possible? If yes, how?

Answer 1

Seems like what you have here is a multi-part feature. If you are using ArcGIS, you need to add the advance editor toolbar in your arcmap. Start an editing session and use the explode multi-part feature tool and then you will have one geometry for each record.

Answer 2

If you have connectivity information (e.g. you have polygons and not just points) it's not too tough to do a decent job of polygon reduction.

What I've done in the past consisted of two steps.

1. Any vertex that is surrounded by polygons, all of which are coplanar, can be removed. I did this by "sliding" the vertex to a neighbor vertex, that neighbor getting all of the test vertex's neighbors and any triangles that become degenerate (e.g. any triangles shared between the two vertices) were removed.

2. Any vertex which has two edges leaving opposite one another, where the polygons on either side are either completely nonexistent or are coplanar can also be similarly collapsed into a neighbor vertex, but obviously only one that is along one of the parallel edges.

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note-
Two polygons are coplanar if they share at least one point in common and if they have the same normal. Since the candidate polygons are always attached to the candidate vertex, you just need to compare polygon normals. The normal can be computed by taking the cross product of two of the edges of the polygon.