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I georeferenced a map and then traced a vector polygon shapefile over it, but when I dissolve it I am left with an enormous amount of slivers. All the vertices are snapped as far as I'm aware. What else could be causing all the slivers? I work in QGIS

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  • why bother digitising all the polygons if you then union them? you probably need to increase your snapping distance to make the polygons match more closely.
    – Ian Turton ♦
    Commented Mar 9 at 12:12
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    The polygons in shapefiles are independent geometries without topology rules that would guarantee that the common boundaries have exactly same vertices. I guess that your assumption "All the vertices are snapped as far as I'm aware" is wrong. See for example gis.stackexchange.com/questions/277334/…
    – user30184
    Commented Mar 9 at 17:29
  • You can fill them in like this
    – Bera
    Commented Mar 9 at 18:10
  • You can enable topological editing to keep from creating overlapping polygons, but you could still produce gaps. The trace tool can skip vertices, and to avoid gaps every vertice in one polygon must be matched by one in the adjoining. At this point I'd run a v.clean with a tolerance similar to the one you were using for snapping and see how that doesbefore increasing it. You could also try the snapping tool with the add vertices between nodes option but I always thought v.clean was better. If you do have overlapping polys too you should run delete duplicate geometries after the v.clean.
    – John
    Commented Mar 9 at 18:47

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