Landscape texturing - artifacts even with a very simple material


I am having problems trying to texture a landscape with layers and am hoping someone can point out what I am doing wrong :-S

I have imported a 2017 x 2017 landscape from world machine. The landscape has a size of 1 km x 1 km, and I’m using 63 quads/sections, 4 sections per component as suggested in the documentation. I understand 1 km x 1 km is quite large for a map, so following advice I saw on another thread I’ve tried to disable static lighting so the project’s lighting is dynamic only (by unticking the box to allow static lighting in project settings -> rendering). The landscape has 4 layers. Even with a simple material (each layer corresponds to a single color, no use of textures) I’m getting artifacts (highlighted by the red circles):

There are straight edges at the sides of some of the components. Can anyone tell me the reason for this? Is 4 layers too many to use?

Any advice would be much appreciated!