Simplify network lead to a bug in GADM shapes

[pz-max]

Checklist

• I am using the current main branch or the latest release. Please indicate.
• I am running on an up-to-date pypsa-africa environment. Update via conda env update -f envs/environment.yaml.

Describe the Bug

Summary of bugs:

• Administrative zone is removed in gadm_regions to onshore_regions conversion
• Multiple administrative zones are associated to one shape after simplify_network.

I am running PyPSA-Africa for Nigeria with the GADM clustering option which retrieves GADM shapes also known as administrative zones. See the automatic download:

Creating the regions_onshore looks still fine. I am not sure yet what the colour change means. Maybe it is indicating an error? One bug is that black hole inside Africa!

Nevertheless, the real bug appears when running simplify_network.py which then creates networks/elec_s{simpl}.nc. As you can see, hovering over one shape, it seems that some shapes are combined.

One needs to debug simplify_network and investigate the step where this occurs. We need to find a solution to this issue.

[jome1]

I think I know now why some shapes are weirdly combined when hovering over them and why the there is the color change. (or at least it is a puzzle piece :)

It is because the single polygons do not conterminous properly anymore. To see this, you have to zoom in a lot (maybe use QGIS to better analyse).
In the script build_shapes.py the GADM zones are downloaded and then simplified. During this simplification holes and overlaps between the polygons can occur. (See #1051)

The simplification in the script build_shapes.py is done with shapely.simplify

To solve this problem, shared boarders of conterminous (neighbouring) polygons should not be simplified or the neighbouring polygons have to be simplified to the same line.

I did a little internet search.
Maybe shapely.snap could help to solve this problem.
Here is also a helpful mini thread about this phenomenon