Fix buffer topology problem with building merging #1704
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Building merging expands building footprints using `buffer`, simplifies them and then contracts them back. This means that a saddle point, where two buildings meet, creates two buffered out points which then collapse back onto exactly the same point when buffered negatively. This can, but doesn't usually, result in a topological error, and creates an invalid polygon. Instead of this, we can buffer out by a very, very slightly smaller value, which should tie-break the points coming back in, and mean that there's enough precision for the inside/outside or left-of/right-of tests to work properly.
rmarianski
approved these changes
Nov 14, 2018
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Would the wkt be equally opaque here?
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Pretty much. I've also had issues before where the WKT is lower precision and so doesn't exhibit the problem, but I'll go check.
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Yup, I tried a version of the test with WKT instead of WKB and it doesn't trigger the same bug. Creating simpler geometries (e.g: box(1,2,3,4)) also failed to reproduce. Unfortunately, I think we're stuck with the opaque WKB 😞
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Building merging expands building footprints using
buffer, simplifies them and then contracts them back. This means that a saddle point, where two buildings meet, creates two buffered out points which then collapse back onto exactly the same point when buffered negatively. This can, but doesn't usually, result in a topological error, and creates an invalid polygon.Instead of this, we can buffer out by a very, very slightly smaller value, which should tie-break the points coming back in, and mean that there's enough precision for the inside/outside or left-of/right-of tests to work properly.