Hi,
Am 08.02.2016, 20:26 Uhr, schrieb Springfield Harrison
<stellar...@gmail.com>:
Hello Group,
I have been manually creating a few simple polygons, always
intending to have no gaps, slivers or overlaps. To do this I >use
snapping and occasionally the Autotrace feature.
However, when I run the topology checker, it often flags a
variety of errors although I am careful to create polygons by >only
snapping to existing vertices of adjacent polygons. It does a great job
of finding the errors, but I can find no >instructions on methodology
for repairing them.
In the list of things you do while digitizing, I miss the option to tick
"Avoid intersections"-checkbox in the snapping option for your polygons
layer. Actually, I never snap to existing vertices (maybe only the first
and the last of adjacent polygons), but simply click into the adjacent
polygons and then out again. Ticking "Avoid intersections" will
automatically create the common border of the new polygon with the new
one. So there is generally no need for any autotrace or carefully
digitizing on existing vertices
Maybe you just forgot to mention this, but having many errors looks to me
that this option might be missing in your workflow.
Any suggesting on repairing topology errors easily?
Cleaning up topology errors is best done with GRASS v.clean through the
processing toolbox.
I normally use the bpol option, but more important is to use the advanced
options there and setting the thresholds for snapping and the minimum area
for resulting polygons. You might have to experiment with those settings
and check the results.
If you still have slivers cause the snapping can't connect enough existing
vertices, try "Densify geometries given an interval" to add more vertices
before the cleaning. You can remove those extra vertices later again with
"Simplify geometries" from the tool box.
That's my usual workflow, maybe other have better tools.
I never bother to use the topology checker, am simply too lazy for that
and doing this stuff manually.
Thank you, Cheers . . . . . . . . Spring Harrison
Good luck
Bernd
At 03-02-2016 07:51 Wednesday, Carlos Cerdán wrote:
Hi Springfield:
I've had similar trouble some time ago, as a QGIS beginner user.
Previous given advices are for not too-beginner users, >>but I think my
experience can be useful for the list (or even for you?):
That time my error was my Snapping options: I had selected two or three
layers and it was selected the "avoid >>intersections" option, then,
when I added new digitized polygons, they were splitted or deleted (as
you tell) because >>new polygons were intersecting marked layers in
snapping options. It fixed when new digitizing shape was the one marked
>>(of course to avoid intersections between her elements) .
Yes it was something simple, my foul. Is it not same trouble?. If so,
Check it in "Configuration --> Snapping options >>--> Change the
Self-assembling mode to "advanced" and be sure that "avoid
intersections" is marked in your target layer >>only.
Cheers,
Carlos Cerdán
2016-02-02 20:57 GMT-05:00 Springfield Harrison <stellar...@gmail.com >:
Hello Group,
        Am using QGIS 2.10.1 and have done so without
significant problems for many months. I am >>attempting to create
simple polygons in a shapefile by direct vertex entry or copying
polygons from another >>shapefile layer and pasting them into the
active layer.
        In almost every case, these features disappear. There
is an entity entry in the attribute table >>but there are no polygons
created. In one case a polygon was created but it was not the shape
that I digitized; >>no more polygons could be created although the
process for doing so appears to be working. No error messages >>were
created. The CRS for the project and the shapefile are the same.
        The shapefile can be opened in another GIS but has no
content.
       Â        Here are some product details: QGIS
version 2.10.1-Pisa QGIS code revision d20c5b7
        Thanks very much for any help you can render, Cheers .
. . . . . . . Springfield Harrison
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