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

[Nicolas Cadieux]

Hi,

To my knowledge no.  The second option would be a polygone that covers the earth but that has a hole in it.  That is the Generally accepted topology in GIS.  

Nicolas Cadieux
Ça va bien aller!

> Le 16 juin 2020 à 10:28, Tudorache, Marian <Marian.Tudorache at navcanada.ca> a écrit :
> 
> 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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