Snapping function doesn't work with complex polygons in QGIS?

Question

I need to snap out parts from a polygon. I am using the "split part" to mark the area I need to erase:

In order of snapping an area, I drew a line inside the polygon. Sometimes this method does not work and I receive an error message like this:

I tried to change the snap settings, but it did not help:

Can anyone help me?

Answer 1

With reference to the error you get, it seems that some geometries are invalid (in this case, the lake layer). You may check it by running several tools, for example the Check validity algorithm from Processing Toolbox. It will return three outputs:

• Valid output (i.e. the layer or the features eventually valid);
• Invalid output (i.e. the layer or the features eventually not valid);
• Error output (i.e. a point vector layer where each point indicates a geometry issue, for example a self-intersection).

The last output is useful for quickly identifying the issues, so you can repair the geometry where needed. Once you have done this operation, the splitting operation should work.

Answer 2

I have tried a lot of things from an answers in other posts. It's look like there is a beg in the last version of Qgis. If this error is happening I just need to do the same thing other again and after some do over it will work.

However, it seems that if the snapping is not working the best way is to make the next one in a slightly different area in order to "fix" the problem.