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PS
it also known that the "topology checker" tool returns false positives see #13296

I guess that have you checked if is the case, right?

Thomas Nogatz wrote:

I have not experienced a false positive by the topology checker tool by now. Importing+exporting a shapefile by GRASS had solved all such topology errors (gaps, overlaps) reported by topology checker. No shapefile exported by GRASS did cause gaps or overlaps listed by topology checker in the past three years - that's what I have experienced.

I have attached a further example project with some layers created with QGIS 2.8.8 - the QGIS_2.08.08_TEST.zip I have posted 3 days ago had different KBS (layer: 4326, project: 5555):
- border_5555.shp
- biotopes_5555.shp (with added border polygon the way described 3 days ago (copied and pasted from border_5555.shp without "avoid intersection"); topology checker reports 2 gaps and 8 overlappings between border and biotopes, biotopes themself are topologically clean)
- gaps_5555.shp (created with "avoid intersection" for biotopes+border)
- biotopes_5555_grass.shp (biotopes_5555.shp imported and exported by GRASS (v.in.ogr.qgis leaving all parameters as they are / v.out.ogr) - no errors reported by topology checker now)

just imported your layer biotopes_5555.shp into a GRASS mapset: no gaps, no overlapped areas.

that's right. GRASS does not report any error during import process.

To understand what happens when GRASS imports and exports a shapefile I've just created a very simple polygon layer with a triangle. Then I imported this into GRASS and exported it again as shapefile. Now I exported the attribute tables to excel and compared the coordinates of every vertex. All vertex coordinates had tiny differences! Its about 0,0001mm.

The result of this test: GRASS changes the coordinates a tiny bit (in spite of reporting no errors).
I suppose the following: If the difference between to vertices is big enough to be reported as topology error by topology checker and tiny enough to be considered as equal by GRASS then GRASS does not report an error after shapefile import. That does not mean that there is no topological error - it's just a tiny one, tolerated by the GRASS import procedure.

Back to the project I've provided today. Try to merge the rightmost polygon of the biotopes with the border polygon - there will be a visible merging error in the upper region. So it is not possible to say there is no topological error in spite GRASS does not report one.

v.in.ogr seems to have a built-in tiny snapping tolerance. Topology errors smaller than that tolerance are not reported. But they do exist.

Thomas Nogatz wrote:

v.in.ogr seems to have a built-in tiny snapping tolerance. Topology errors smaller than that tolerance are not reported. But they do exist.

it should not

https://grass.osgeo.org/grass70/manuals/v.in.ogr.html

by default "snap" is -1 = no snap

@ Markus Neteler could you shed some light on this matter?

Thomas Nogatz wrote:

v.in.ogr seems to have a built-in tiny snapping tolerance. Topology errors smaller than that tolerance are not reported. But they do exist.

This is not the case. v.in.ogr does not snap by default.

So there should exist a little (not visualisable?) difference between both shapefiles. This would mean that the topology checker works exactly and reacts very sensitively.

where do we stand with this ticket?