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I'm trying to find the actual definition of the curve types supported by PostGIS in WKT.

PostGIS's documentation refers to the " SQL Multimedia Applications Spatial specification":

The SQL Multimedia Applications Spatial specification extends the simple features for SQL spec by defining a number of circularly interpolated curves.

That paper, SQL/MM Spatial - The Standard to Manage Spatial Data in a Relational Database System, defines database functions for interacting with curves (eg, ST_CompoundCurve) but does not define any WKT data types, or give very precise definitions.

The OGC's "Simple Feature Access - Part 1: Common Architecture" standard" provides names and WKB codes for these data types, but doesn't define them further.

So, um, where are the circular curve types in WKT (or is it EWKT? or SQL MM?) actually defined?

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The core types and well-known architecture is defined and standardized in the proprietary

commonly abbreviated as SQL/MM and mentioned in the PostGIS docs (including some examples).


A generic (textual) technical documentation of the supported WKT components and their construction can be found as part of the PostGIS repository docs here.

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    Huh, $220 just to read the standard. Anyway, thanks, super helpful. Jul 30 at 12:15
  • I think you mean commercial (or expensive) not proprietary as that would imply it was owned by one company.
    – Ian Turton
    Jul 30 at 16:47
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    @IanTurton oh I surely should have added 'expensive' ,). Well, somewhere I read that the ISO "[...] develops and publishes a wide range of proprietary, industrial, and commercial standards [...]", but this specification may be any, or all of the mentioned. Though I have to agree that it likely doesn't count as competitive property, even if you have to get a license to implement AFAIK.
    – geozelot
    Jul 30 at 17:17

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