Unity breaks a smooth ground on my .dae model and doesn't light it up properly

A picture is worth a thousand words.

The first 3 pictures show how Unity lights up the model. Only the light is moved on the X axis. The darker places remain darker, no matter at what height the light is; they are not darker because they are “over a hill” (shadows aren’t on anyway)

The 4th picture shows how Unity thinks the ground looks like. The screenshot is taken when the map is selected and there is no light on the scene. The lines starting from the top left corner are exactly how the lighting disperses.

The 5th picture shows how Sketchup thinks the ground looks, which is a smooth surface, which is what i meant it to be. I tried having it a 2-dimensional plane, or a cube with some height, it didn’t change in Unity. I tried importing it to Blender, it looked the same as it did in Sketchup (no lines, just a smooth surface) and then exporting to .dae from Blender and again into Unity, it didn’t change in Unity.

The houses and any other models that i had on the map that “pop off the ground” were lit up properly. Only the actual ground has this problem.

Why does this happen?

Looks like an issue with normals being smoothed (which they shouldn’t be on the ground). I’d say just don’t use sketchup at all, it has a history of creating models with lots of issues when used in Unity.

I had this issue too for quite some time. My way around it makes no sense, but it works.

Select your grass material and change the shader type to Bumped Diffuse, than it will work, even if you switch it back to Diffuse.