Data from an external source can have issues in the table structure as well as in the topology, endemic to the geospatial data itself. Take, for example, the problem of data with overlapping polygons. If our dataset has polygons that overlap with internal overlays, then queries for area, perimeter, and other metrics may not produce predictable or consistent results.
There are a few approaches that can solve the problem of polygon datasets with internal overlays. The general approach presented here was originally proposed by Kevin Neufeld of Refractions Research.
Over the course of writing our query, we will also produce a solution for converting polygons to linestrings.
