I'm trying to do some terrain-blends, but it's taking up a lot of time... Ever since i copied most of my map to a new map file, because i wanted to start over i'm having the problem that whenever i copy a terrain patch it has a tiny offset that i cant correct because it's smaller than the grid and barely visible. But it makes the textures glitch because the vertex's dont match up. I have to weld every vertex by hand before i can continue. Very annoying and time consuming.. Does anybody know a sollution for this issue? or perhaps a way to weld the patches faster?

Since you didnt always have snap to grid enabled there is really nothing you can do besides starting over. terrain is always going to be time consuming anyways, welding individual verts is just something you have to deal with. you can always just use the W hotkey to merge patches together to form one patch but that restricts some functionality depending on what you are doing. You have to have the exact same sized edges for the two patches in order to do this anyways so in your case with your grid being off it wont work. I would personally re-due my terrain no matter how terrible that sounds but it will save a headache in the end. Almost never have snap to grid off there is only really a few applications for it but nothing really useful. I know mine tends to become disabled once in a great while but it is really noticeable right when it happens anyways. It is probably because you copied your map over when it was disabled so if you still have the old map on grip just copy it into a new map file again and it should be fine.

Crtl g  - snap to grid