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I have a problem with the Network Analysis function v.net.iso in GRASS. I tried it so many times but the function doesn't work correctly.

I tried to calculate isolines via the function v.net.iso, based on a street shapefile (OpenStreetMap, Geofabrik) and a point shapefile with grocery stores (My own). I created the network via v.net and used the v.net.iso function for isolines (here in the picture 50, 100, 150 meters and so on). The result can be seen in the picture. It seems that only some of the street lines are used for the calculation (the ones with colors like red, yellow,...) and others are not (the black ones).

It's not a problem of colors. Important note: The streets are completely connected with each other! There are no minimal gaps between them.

Does anyone have any idea?

Screenshot GRASS Network Analysis v.net.iso