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Related to #14494.

The processing tool calls "geom.intersection(tmpGeom)", which only works as expected if the intersecting lines share a vertex on intersection.

The problems trace back to geos::intersection, which only "Returns a Geometry representing the points shared by this Geometry and other."[1] If the intersection is not at a vertex, nothing is returned.

I think there needs to be a API function "compare two line segments and if these intersect, return both as lines with a vertex on the intersection"

[1]http:// geos.refractions.net/ro/doxygen_docs/html/classgeos_1_1geom_1_1Geometry.html#a44

Geos has such a function, geos::algorithm::LineIntersector:computeIntersection.

https://github.com/qgis/QGIS/blob/fab8dc21762b8ca0a9a48ffffe343788fddabc24/src/core/geometry/qgsgeometry.cpp#L1381
should check if there is a geos:geometry:intersection, and if not, try geos:algorithm:LineIntersector:computeIntersection. Other functions, like makeDifference (in the same file) may profit from this check, too.

This is a bit of a design issue: Do two overlapping lines intersect? I can think of situations without intersection (bridges etc.), but for every day usage, most lines DO intersect. Maybe some switch ("only intersect if there is a vertex on intersection") could be added?

Maximilian Krambach wrote:

Geos has such a function, geos::algorithm::LineIntersector:computeIntersection.

is it exposed in the C-API?

Sorry, I was wrong. Upon further investigation, Intersection seems not to be the culprit.
However, only polygons1 that touch the outer line of the intersection polygon2 are affected, (but not all of them). I zoomed all the way in, and they still touch.

I looked further on a failing geometry1 (FID 2883 in shrublands). This geometry1 has no vertices in common with the geometry2 it intersects, and returns NoGeometry on intersection, even if it is the only feature in a shapefile.

If I snap two vertices1 to their respective nearest vertex2, the intersection returns a multipolygon.

Hello Maximilian and Jürgen

Thank's for your input into this matter.

I think the problem originates from the fact that I first made a clip of the land cover layer with the same layer I later used for the intersection (NationalBoundaries). If I skip the clipping and just do the intersection of the original land cover layer the result is as it should - all land cover units that intersect the layer NationalBoundaries are obtained.

Back to the results of using the clipped land cover layer in the intersection. I find it strange that some but not all of the land cover polygons where a part of their boundary is identical with polygon boundaries in the intersection layer are lost in the intersection. It seems that this identity is the problem, but it does not appear to be general.

I looked into Maximilians comment that the polygon with FID 2883 does not have any vertices in common with the polygon it intersects in the intersection layer. I do not understand what you mean here - since the land cover layer (prior to intersection) is clipped with the NationalBoundaries layer it results in identical outer boundaries (i.e. identical vertices) in the two layers used in the intersection.

I do not know how the intersection algorithm works but I wonder if partly identical boundaries is a special case that the algorithm does not manage to handle, and if so, whether it is important to fix it. It is probably rare to first clip and then intersect with the same layer as you can do the intersection directly. But as I have done it this way it must mean that other users may do it too...

/Mattias

When I use the files clip.py and intersection.py from the processing 2.12
(Https://plugins.qgis.org/plugins/processing/version/2.12.2/)
in the processing 2.14 everything works well. (copy in the folder */python/plugins/processing/algs/qgis).

Claas

About "Returns a Geometry representing the points shared by this Geometry and other." -- the GEOS statement has nothing to do with vertexes. Rather "points" is meant in the point-set theory.

Example:

strk=# select ST_AsText(ST_Intersection('POLYGON((0 5, 10 0, 10 10, 0 5))'::geometry, 'POLYGON((10 5, 20 10, 20 5, 10 5))'::geometry));
  st_astext  
-------------
 POINT(10 5)
(1 row)

Are you sure your expectancies are correct ?