[Qgis-user] Geoprocessing Tools give wierd answerd

[Hillers Heather]

Hi,

I have a Multiline layer A with about 900 Features loaded (only a few of those are actually multi part geometries)  in QGIS 2.14 and I try to do Vector  -> Geoprocessing Tools -> Difference with Layer A as the input vector layer and Layer A as the difference layer, expecting to get as a result an empty layer.  But, the Output Shapefile has ~150 Features in it.  Topology Checker says, there are no invalid geometries, but there are about 90 pseudo nodes in the layer.  I started snooping around and found other line shapes that behave this way.  Have I fundamentally misunderstood what a difference is, or is there some specific geometry problem that causes difference to behave oddly?  The use case is silly, but I started with comparing an exported shape to its source, got weird results, and then tried to compare the source to itself as a sanity check.  Now I feel I might have lost sanity.  Help?

Thanks,

Heather
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