[Qgis-user] Micro-Geometry Problems

[Mike Shand]

Steve (or any new QGIS users),

For future reference, it is maybe better practice to digitise polygon sides simply as lines and then use one of the line>polygon tools in QGIS, GRASS or SAGA.  Advantages are that you only digitise the lines once (much quicker, no double digitising) and so only require to use snapping at junctions (less change of snapping errors).  Main disadvantage is that you would have to add your polygon attribute data after the polygons have been generated rather than as you digitise them.

Regards,
Mike
GIS Cartographer
University of Glasgow

On 16/01/2020 14:48, Kirk Schmidt wrote:
Hi All:

This is a classic digitizing issue that can be corrected with GRAS  as Harrisou suggests.  If the layer does not get cleaned during the import process, you can use the grass functions v.build and v.clean to fix the digitizing slivers and incomplete intersections.

Kirk Schmidt



From: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of DelazJ
Sent: Wednesday, January 15, 2020 3:45 AM
To: dhdeangelis at comhem.se<mailto:dhdeangelis at comhem.se>
Cc: qgis-user
Subject: Re: [Qgis-user] Micro-Geometry Problems

Hi,
There's also a "snap geometries to layer" Processing algorithm in QGIS that could be worth a look.
https://docs.qgis.org/3.4/en/docs/user_manual/processing_algs/qgis/vectorgeometry.html#snap-geometries-to-layer

Regards,
Harrissou

Le mer. 15 janv. 2020 à 08:34, Hernán De Angelis <dhdeangelis at comhem.se<mailto:dhdeangelis at comhem.se>> a écrit :

Steve,

In my opinion this is where GRASS topological vector model shines. One suggestion would be to import these vectors into a GRASS location and let GRASS clean the polygons and fix the topology automatically during import. My experience with similar problems is that after the import and cleaning there would be few if any errors of the sort you mention. In the worst case any surviving error is found it could in principle be solved by deleting the stubborn surface and digitizing it again. It will definitely not take two weeks of work!

Good luck!

H.


On 2020-01-14 22:51, smiller192 at cfl.rr.com<mailto:smiller192 at cfl.rr.com> wrote:
All-

I have digitized a land use/land cover layer as individual polygons.  The snap options were used and, for most vertices, the snap was precise.  A vertex in polygon A has a precise match to a vertex in neighboring polygon B.

But in 582 out of 2799 polygons, there are mis-matches in adjoining vertices that are only evident if you zoom into the layer at a magnification (i.e., a scale) of from 20000:1 to 40000 to 1.  I have spent a couple of weeks trying to remedy these by hand using the Vertex Tool and other means.  My question is this: is there an automated way in QIS to detect and remedy these situations?

Thanks,
Steve Miler


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