Making terrain with vertex editing in GoldSrc is the worst thing I have ever experienced. If some psycho told me I had to make a map with terrain in it and with no vertex errors or seams, or else my family would be killed, then I would just kill my family myself to get it over with. And I love my family.
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As far as I remember, it's easy, you just divide the whole area into squares with the clipping tool and them divide the squares into triagles. After that, it's just a matter of pulling the pollygons to desired heights with the (forgot the tool name).
Yeah that's what I do lol. The problem comes from all the errors that can come from it. And invalid brushes aren't listed when you look for map problems either fsr.
pro tips : I used to divide the facing squares in triangles but it's actually not enough. You should divide in triangle in at least 2 axis. In the end, your brushes should have in total 3 triangle-shaped faces, no more, and then you should be fine ;)
There is program called Gensurf (if i not mistaken), it could help you make terrain with ease