[GIS] How to set tolerance value of Geoprocessing tool in QGIS

geoprocessingqgis

Is it possible to set tolerance value of geoprocessing tool on QGIS.

History is below.
There are two polygon which have to be the same position.

so, I set topological rule "A must covered by B" on ArcInfo with setting 0.0001 as tolerance value.
There was no error.

On QGIS, I implemented "Difference" from geoprocessing tool for two polygons.

I found hundred errors which has differences less than 0.001m.
As default, there is no tolerance value on QGIS.

Please tell me how to set tolerance value on QGIS if possible.

Best Answer

You speak of topological rule, but QGIS is not yet a full topological GIS. You can examine all of the options in 2. Lesson: Feature Topology or How to check Topology in QGIS?

If you want a full topological GIS, use GRASS GIS

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[GIS] How to select and copy a group of polygons in QGIS

The select tool has a "Select by Rectangle" option. (There are also freehand/polygon options if that would be more appropriate.)

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The node tool is for digitizing. I don't think that's what you want to do here.

Then, right-click on the layer in layer list and use "Save selection as ...". This will create a new file with the features you selected. (You can chose from many different formats.) Afterwards, load the file into the same old or a new project.

Dissolve Polygons with Tolerance in QGIS

In this case, it looks like you need to help the dissolve tool along by fixing the topology first. Here's how you can do this in GRASS.

When you load the shapefile, this is how GRASS sees it:

Each cross marks an area. You can see that there are some crosses on the boundary lines: These are the sliver polygons caused by the slight offset between the boundary lines.

Remove the sliver polygons formed by the gaps between the real polygons:
```
v.clean input=sample output=tempA --o tool=rmarea thresh=0.001
```

Snap the boundary lines:

v.clean input=tempA output=tempB --o tool=snap thresh=0.001

Break the boundary lines at the new intersections:

v.clean input=tempB output=tempA --o tool=break thresh=0.001

Remove the now duplicate boundary lines:

v.clean input=tempA output=tempB --o tool=rmdupl thresh=0.001

Dissolve the polygons

v.dissolve input=tempB output=cleaned --o column=ID

Et voilà:

Best Answer

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