How to make impassable tiles visible?

Seems like a funny question but... where exactly is the coast of that island I'm on? What is a mountain and what is not?

I spend entirely too much time fully zoomed in just so I can more or less see which tiles have the faint grid outline - and are therefore passable - and which do not.

How can I make this easily visible both in 3D and especially on the cloth map. The cloth map is even worse. It doesn't even have the grid so it's all guesswork.

I haven't even started to look into modding so I don't know about the graphics/data/map structure this obviously depends on...