[GIS] QGIS remove useless vertex to simplify polygon

Question

I received a shapefile which is the result of a combination of several polygon. I want to clean and simplify it as there are many useless vertex as you can see on the picture. This is only one feature so there is no need for all the lines in the middle. Just one simple polygon would be just fine.
I tried the function "simplify" but it doesn't change anything. Merge and dissolve functions won't do the trick neither. So I'm not sure how to proceed.

Answer 1

This polygon has invalid geometry, because the edges intersect. Standard geometry correction methods are unlikely to work.

Fortunately, you only have one feature, so you don't need an automated method. Instead, you can digitize a new polygon by tracing the outline of the invalid one.

1. Turn on snapping (snapping toolbar in version 3.0 and later, Settings menu > options in 2.18 and older). Snapping settings:
   • Active layer
   • Snap to vertex
   • 10 pixels
   • Enable tracing
2. Put the layer in editing mode. Add a new polygon to this layer by tracing around the outside of the existing polygon.
   • Note that you can zoom in and out while digitizing by using the scroll button on your mouse.
   • You can pan the map while digitizing by holding the space bar and moving the mouse.
3. Optional: copy attributes. Select both polygons and use the merge attributes tool (advanced digitizing toolbar) to copy attributes from the old polygon to the new one.
4. Delete the old polygon.
5. Save the edits.