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polygon winding direction #2

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@pdowler

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@pdowler

The current spoly in pgsphere is allowed to be clockwise or counter clockwise and the "inside" is the smaller side of the boundary.

In the IVOA community, where postgresql+phsphere is used inn many places, the DALI standard specifies polygon as always counter clockwise with the "inside" on the left (https://www.ivoa.net/documents/DALI/20170517/REC-DALI-1.1.html#tth_sEc3.3.7). The reasons for that choice were well justified by use cases identified by the authors (me and others); I will add that material or a link in a subsequent post on this issue if needed.

The issue is that the pgsphere behaviour trips many implementers up because it is subtle and basic tests/trials work fine and the discrepancy show sup after they'd ingested a lot of data. I don't know what the solution is for this, but I'd like to start a discussion on this issue and try to find one that works. Basically, your users are often implementing IVOA standards and I think we both want that to be as easy and straightforward as possible.

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