[postgis-devel] Big New Functionality

Paul Ramsey pramsey at cleverelephant.ca
Mon Jan 8 06:02:42 PST 2018

So, I never expected anyone would show up with an overlay
implementation for geography, but there it is.


The trouble is, overlay is difficult and geography is even worse,
insofar as segment intersection is a lot less stable in geography
space than in planar space, so there's going to be a couple issues
with this implementation for sure:

* it'll have failure modes for inputs that geometry handles fine
* I could be wrong, but eyeballing it I'd expect O(n^2) performance,
so for large things it could be quite slow, even by the standards of
geometry, since the intersection calculations are also much slower in
geodetic space.

So, putting on my old fuddy duddy hat, I admit that Something is
always in principle better than Nothing, while being concerned that
we'll have a lot of support to do on this block of code.

In my heart of hearts, I guess I had always hoped/thought that
geodetic support would come from a generalization of the 'edge'
concept in GEOS, so that the same graph structure currently used for
relate/union/intersect could be used for geodetic operations.
Topologically, geodetic objects have the same structure as geometric
ones, it's just the calculations of intersections and stuff are
different, so it made sense that it should be possible to get a lot of
GEOS functionality for "free" once the edge handling was generalized.
Part of that work would be a lot of experimenting and so on with
making geodetic edge intersection more robust. (quad-double?)

Anyways, now there's a different avenue, and I'm wondering what you
other fuddy duddies are thinking?


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