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My current work involves extensive digitising of historic geology data - essentially the creation of new polygons and polylines based on overlain raster and vector layers. This is all done manually using QGIS 2.14.3. I was hoping to get some tips from the community to make this process more efficient and to attain a better outcome.

As it stands now, I am simply tracing features and creating polygons and lines by hand. I've found this quite a frustrating process in QGIS. How do I create perfectly snapped adjacent polygons? Is the only way to play with snapping options (tolerances)? I've tried this but it still seems a bit clunky when you have a large density of vertices, and I still seem to miss some, and/or snap to polygons close by that I want to keep separate. In MapInfo, I could roughly digitise the boundary of the neighbouring polygon (making sure it overlaps) and then erase this overlap with a click of the mouse.

Similarly, I cannot find a tool that will allow me to easily edit the shape of a polygon. In MapInfo, I could simply draw another overlapping (overlaying) polygon, and use the erase tool to erase its extent from the underlying polygon. In QGIS, all I've found that comes close is the add ring feature, however it must be totally enclosed in the polygon being edited. As a result, when I want to edit the shape of a polygon, I find myself manually selecting, moving and adding nodes. This takes forever in some instances, and you have to add and move many many nodes in order to get a smooth result. I probably could draw an overlapping polygon in another layer and then use the layer based geoprocessing tools (Difference?) but I'm hoping there is a more simple solution.

Can anyone help me? I've conducted a fairly extensive search and drawn mostly blanks. I found this http://confound.me.uk/maps/ppv4.pdf which looks like a good technique, but I'm unsure how it would work with multiple overlapping enclosed lines (areas).

  • Have you seen my answer to this similar question, gis.stackexchange.com/questions/12331/… – artwork21 Jul 16 '16 at 2:34
  • Thanks for the link. I've played with the snapping tolerances but have found it's still easy to miss some vertices. Maybe I need more patience and practice. Bumping the tolerance up higher helps, but then it snaps to other polygons close by that I want to keep separate. I guess I was hoping there some technique like MapInfo's where I don't have to worry about tracing a boundary. – rhys Jul 16 '16 at 2:48
  • From what I see auto complete polygon in qgis is very poor. You might try to digitize polylines / arcs, I.e. Boundary between neighbouring polygons and built polygons later – FelixIP Jul 16 '16 at 3:29
  • There's also the traceDigitize plugin which could be helpful. You can download this from the menubar: Plugins > Manage and Install Plugins... – Joseph Jul 26 '16 at 9:26
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Oh my god!!!. Those are the kind of problems that we the geologists have. I tried digitize geological maps with OpenJump. It has much more digitizing tools than QGIS but is difficult. I think digitizing polygons that share boundaries is still a debt for FOSS4G.

At present my solution was to work like the old Arc-Info fashion way.

  1. Digitize polygon boundaries as linestrings
  2. Digitize polygons centers as labels. You will use them to identify your future polygons
  3. Convert closed linestrings to polygons
  4. Copy the label atribute table to the new polygons.

This what I did about two years ago with good results. But I have to confess is not the ideal.

Cheers

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