[gdal-dev] Clipping shapefile with another produces invalid shapes

[jratike80]

Even Rouault-2 wrote
> You could possibly use the ST_ForceLHR() function of spatialite (through
> the SQL SQLite 
> dialect) to post process your result:
> http://www.gaia-gis.it/gaia-sins/spatialite-sql-4.2.0.html

Interesting, that ST_ForceLHR in Spatialite gives the same result than
ST_ForceRHR in PostGIS.

Description for Spatialite:
Any Polygon Ring will be oriented accordingly to Left Hand Rule (Exterior
Ring will be clockwise oriented, and Interior Rings will be
counter-clockwise oriented).

Description for PostGIS:
Forces the orientation of the vertices in a polygon to follow a
Right-Hand-Rule, in which the area that is bounded by the polygon is to the
right of the boundary. In particular, the exterior ring is orientated in a
clockwise direction and the interior rings in a counter-clockwise direction.
This function is a synonym for ST_ForcePolygonCW 

Perhaps the rule of thumb would be not to talk just about right hand/left
hand rules in GIS because they are ambiguous, but mention also what it
really means like in the descriptions above, or in the GeoJSON
specification:

A linear ring MUST follow the right-hand rule with respect to the
area it bounds, i.e., exterior rings are counterclockwise, and
holes are clockwise.

-Jukka Rahkonen-




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