Intersection reports invalid geometries

Question

I want to intersect two layers (shapefiles) with QGIS 2.18. For both layers, I tested validity with

• QGIS Vector / Vector Tools / Check validity and
• GRASS v.build.check (with default parameters)

both tests report no errors for both layers, but QGIS Intersection reports invalid geometry for one or more input features.

The two layers may have some boundaries, that should be equal, but may actually have slight differences (unprecise manual digitalisation of one layer) possibly resulting in several very small slices.

What do you recommend me to try?

Answer 1

SAGA - vector polygon tools - intersection is my friend. It creates a new Layer without stopping because of errors. Now, I have to delete features with small areas from the new layer.

Answer 2

Maybe this works (Fixing invalid polygon ..):

https://anitagraser.com/2017/08/29/fixing-invalid-polygon-geometries/

Answer 3

Or another problem could be that, you have multipolygons, therefore you have to explode them before intersecting:

Vector / Geometry Tools / Multipart to Singleparts

Answer 4

Saga gis intersect works, from my experience, as long as intersect does not include any of the faulty polygons. For me worked Fix geometry tool in QGIS3. I suspect in my case the problem was self intersecting poygons. Similar issue: Intersection error QGis - invalid geometries that are actually valid.