Polygon CCW winding check request

Marco Molinaro marco.molinaro at inaf.it
Tue Jun 12 17:45:12 CEST 2018

Dear DAL members,
at the recent IVOA Interop in Victoria it was pointed out
by Alberto


that not all the data providers follow the STC specification
about the winding direction of polygons stored in their

STC states that "The inside of the region is defined as
that part of coordinate space that is encircled by the
polygon in a counter-clockwise sense".
And this is to be considered when looking at the
celestial sphere from the inside.

The different behaviour of the polygons stored at
different sites creates an interoperability issue.

This email is a request on data providers to check
on their data and implementations (when dealing
with polygons) to solve the presented issue.

James also put together a python code snippet
implementing a simple test, it is available here:


Thank you in advance!
       Marco & James
      (your DAL chair & vice)
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