Boolean modifier not working for no reason

I’m making this landscape from a game Im making in blender, and it needs to be in a certain shape. So I did some testing first, I of course added the Boolean modifier to the terrain, used the intersect operation, use an extruded circle as the second object, and it worked. Then I tried with a plane, made a random shape, did the same thing, worked perfectly again. SO then I began making my actual mold, which is the shape of the UK. So anyway, I made this, did the exact same thing, but got ‘Cannot execute Boolean operation’. I have no idea why. Anyone have any ideas, and if so, how to fix it?

Heres some pictures of the process. I used the exact same terrain for the tests.,GBcCLtp,uNvSIlB#0,GBcCLtp,uNvSIlB#2,GBcCLtp,uNvSIlB#1

Thanks so much guys, I worked for hours on that mold and its really annoying me that it wont work

Boolean requires that the meshes are manifold (like I mentioned in the thread I linked in your other thread).
Apply object scales with ctrl+A -> scale in object mode.
Edit mode, select all (A), vertex select mode (ctrl+tab), W -> remove doubles, ctrl+N to make normals consistent (and pointing out), select none (A), ctrl+shift+alt+M to select non-manifold and if something gets selected, you have to manually fix those parts.

Boolean should be avoided at all costs. I don’t understand why you are even using it. Why not just use curves? Boolean creates terrible mesh topology which leads to terrible shading and precludes the use of subsurf in most cases. I’ve only been able to get boolean to work well with ultra high poly models. But then again I don’t use it very often. I see it as a lazy mans tool that almost never works.

When I do that to select the non manifold parts, the entire mesh is selected

Because I dont know any way of using curves to make the shape of my UK mesh out of my ready made terrain. Boolean was the only way I could find

It’s bad then. Check if those are just unconnected vertices. Select all, W -> remove doubles removes double vertices within merge distance, which you can increase from the operator panel (bottom of the tool shelf or F6 afterwards).

It said there were no vertices to remove and all that, I tried increasing and decreasing the operator panel thingy, still it selects the whole mesh for the manifold.
Hmm, this is a tricky one. All I did was add a plane, add a background picture, delete 3 of the 4 vertices on the plane, and extrude the other one around the background picture. And then on the last two points, I connected them with alt M. That’s how I made this mold.

Your objects must have thickness and closed all around for boolean. If you have mere outlines then of course it selects everything when selecting non-manifold. Can’t boolean a cube and a plane for example. Plane primitive is non-manifold.

Select all vertices in the outline object, alt+F to fill (could also try F to fill with ngon), still everything selected E to extrude and make it higher than the object you’re cutting, ctrl+N.

If you want to use the outline to make cuts, try knife project tool. It works like a cookie cutter of sorts.

Curves take a bit of getting used to. You just have to play with them. Stay on one orth view and in a couple minutes you’ll have the hang of it. They will take more work than a boolean, but the finished result will be 100% usable as opposed to the shot in the dark boolean method. Srsly though, Booleans are shit. They have a very narrow use and what you are trying to do does not fall into that category. imo.

The issue with the method you told me about is that it cuts the shape of the mold out of the terrain, so its the whole terrain with a hole in the shape of the mold. I wanted to do it the opposite way around, so all of it is deleted except the section of the terrain in the mold (it doesnt have to be deleted, I just want a terrain shaped like the mold).
However, whats this knife project tool?