[Qgis-user] How to determine the inner surface on a sphere or ellipsoid

[Tudorache, Marian]

Hi everyone,

I have a list of polygons given by a list of points.
The polygons are properly drawn on QGIS canvas by creating the geometries, the each geometry is used to create a Qgsfeature which are saved on a shapefile.
The problem appears in detecting if a point on the Earth is inside or outside the polygon.
Using pyqgis QgsGeometry intersects function returns proper value.
However when I export the polygon to intermaphics from Kongsberg Geospatial (former Gallium) sometime the point is inside other time is outside the polygon.
I tried to switch the order of the points is QGIS ,but the intersects function always give me the same result regardless of the order of the points.
In intermaphics the intersection between a polygon and a point varies with the order of the points which define a polygon.
I talked to people from gallium and they confirmed the order of the points is important.
In one direction a inner area of the polygon is the small surface and if I switch the order the inner area is the outside and it wraps the Earth on the opposite side.

Does QGIS or pyqgis has a similar mechanism to determine which is the inner part of the polygon on a sphere or ellipsoid?

Thanks,
Marian

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