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

[Tudorache, Marian]

This might help, however  I forgot to mention that I am using QGIS 2.18.
The QgsGeometry Class that you sent me is for QGIS 3.

Do you know something similar on QGIS 2.18?

Thanks,
Marian

-----Original Message-----
From: Richard Duivenvoorde <rdmailings at duif.net>
Sent: Tuesday, June 16, 2020 12:10 PM
To: Tudorache, Marian <Marian.Tudorache at navcanada.ca>; qgis-user at lists.osgeo.org
Subject: [EXT] Re: [Qgis-user] How to determine the inner surface on a sphere or ellipsoid

On 6/16/20 4:17 PM, Tudorache, Marian wrote:
> 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?

Hi Marian,

are'nt we here talking about the so called Right Hand Rule?

https://urldefense.com/v3/__https://postgis.net/docs/ST_ForceRHR.html__;!!P0kg3YazkQ!SxE0ZFgaBrgWRAqxjyrcC27wO_4FSfOZJ6YSW3RdPYb1VIj6V3RbX2nWprVt5PDgB1KFJfl8$
says:

In particular, the exterior ring is orientated in a clockwise direction and the interior rings in a counter-clockwise direction.

QGIS has this also:

https://urldefense.com/v3/__https://github.com/qgis/QGIS/blob/master/src/core/geometry/qgsgeometry.cpp*L2651__;Iw!!P0kg3YazkQ!SxE0ZFgaBrgWRAqxjyrcC27wO_4FSfOZJ6YSW3RdPYb1VIj6V3RbX2nWprVt5PDgB0hgkj92$

With pyqgis you can als use forceRHR:

https://urldefense.com/v3/__https://qgis.org/pyqgis/3.12/core/QgsGeometry.html?highlight=qgsgeometry*qgis.core.QgsGeometry.forceRHR__;Iw!!P0kg3YazkQ!SxE0ZFgaBrgWRAqxjyrcC27wO_4FSfOZJ6YSW3RdPYb1VIj6V3RbX2nWprVt5PDgBwHQeTsL$

I do know that QGIS is not so good when a geometry crosses the world-dateline though... but I do not think that is your case is it?

Or do I misunderstand you question?

Regards,

Richard Duivenvoorde







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