[Help] Cinemachine - How to keep camera higher than terrain?

Hello, I am using Cinemachine to control my mainCamera, and there is a problem that bothers me a lot. Can I change main Camera's position manually or use custom interpolation during the transition between two virtual camera? In most case, Cinemachine do the interpolation automatically and perfectly, but sometimes the camera will cross the terrain and go below the terrain, so we can see the other side of terrain. I can get the y position of the ground so I can avoid the camera entering the terrain if i can change the position of camera manually. However simply change transform,position of camera seems not working during the transition(or with Follow). What I want to do is just to keep camera's y higher than ground in our game. I have tried use collider but it seems to only push camera forward rather than rise the camera higher. Can you give me any hints? Thanks.

 

Thank you for replying. I read some source code and finally realized I need to make sure my code which push the camera out of the ground need to be executed after CinemachineBrain.
Actually, another reason why I try to customize the vcam transition is because sometimes it behaves so weirdly. For example, when two vcams are placed in opposite direction, brain will try to approach the second vcam, and suddenly turn around. But I think the better way is to use Bezier or other curve to avoid suddenly turning around, because it might result in strange change in light and shadow or other effect. Is there any way to avoid camera suddenly turn around?