QGIS 3 Geometry – Skipping Invalid Geometries Without Fixing Them

geometryintersectionoverlapping-featurespoint-in-polygonqgis-3

In QGIS 3.16.14 I have a shapefile with multipolygons (flood risk map of the country) and another layer of points (buildings). I need to identify which of those points are inside those polygons and which are not.

So after researching the internet I tried Data Management Tools > Join attributes by location. But it failed, because of invalid geometries.

Then I tried to fix those invalid geometries, by answers to similar issues in this forum. I tried the "Check geometries" plugin, but it stopped responding.
I tried "Simplify" under Geometry tools reducing precision to 1 meter, but the simplified result still contained geometry errors.

I tried v.clean and after working overnight, it produced a list of 11 thousand errors, but I didn't know how to proceed. Apparently, those tools struggle because there is really a lot of errors.

Since I do not need a lot of precision, is there some tool in QGIS where I could check if my points are "approximately" inside those polygons while ignoring those geometry errors (but not skipping this check)?

Alternatively, if that geometry needs to be fixed, which geometry fixing approach would be most suitable for such a large number of errors, but where I don't need super large precision. Just need to check if a building is roughly in a flood zone or not.

Best Answer

Try buffering your polygons with 0 distance, sometimes it fixes geometry errors.
If you want to ignore the invalid geometries, you can set it Options - Processing - General:

Related Solutions

[GIS] Comparing two geometries in ArcPy

The issue is most likely one of floating point precision. In your case you've already extracted the geometries using arcpy, and you've matched them with your RUID.

Happily since you've got arcpy installed you've got numpy, which makes comparing sets of numeric arrays easy. In this case I'd suggest the numpy.allclose function, which is available in numpy 1.3.0 (installed with ArcGIS 10).

From the samples you gave above

geom1geoJSON = {'type': 'Polygon', 'coordinates': [[(-122.8423481559999, 47.060497293000083), (-122.84239755599992, 47.059262423000064), (-122.84416913599989, 47.059309693000046), (-122.84416913599989, 47.060497293000083), (-122.8423481559999, 47.060497293000083)]]}
geom2geoJSON = {'type': 'Polygon', 'coordinates': [[(-122.8423481559999, 47.060497293000083), (-122.84239755599992, 47.059262423000064), (-122.84416913599989, 47.059309693000046), (-122.84416913599989, 47.060497293000083), (-122.8423481559999, 47.060497293000083)]]}

import numpy as np

close = np.allclose(np.array(geom1geoJSON["coordinates"]), np.array(geom2geoJSON["coordinates"]), atol=1e-7)
#Returns True

The atol keyword specifies the tolerance value.

Note that you shouldn't use arcpy.AsShape at all. Ever. As I noted in this question (/shameless plug) there's a known bug in ArcGIS that truncates geometries when they are created without a coordinate system (even after setting the env.XYTolerance environment variable). In arcpy.AsShape there's no way to avoid this. Fortunately geometry.__geo_interface__ does extract the correct geometries from an existing geometries (though it doesn't handle complex polygons without the fix from @JasonScheirer ).

[GIS] Invalid geometry in QGIS vs ArcGIS

The problem here is that you have two interpretations of a simple polygon and a self-tangency (intersection of parts at a vertex). ArcMap seems to accept it and the QGIS topology checker not.

I did found some word from the ESRI side here and at least for PostGis i found that it is not allowed here. But for the examples from PostGis for the OGC standard validity for figure j and p they should not be valid due to self-tangency but are written as valid.

Also as far as i understand the OGC standard "OpenGIS Implementation Standard for Geographic information - Simple feature access" the polygons should not be valid because of 6.1.2.2 (IsSimple method). But again valid because of the definition that they may touch in a finite number of points (6.1.14 b) ).

You might choose as well what you want...

Related Question

QGIS 3 – Addressing Invalid Geometries in QGIS 3 Compared to QGIS 2