How to keep flying on the ground?

How to keep flying on the ground?

I need help because for some reason if I give a unit Movement type Fly and height 0 it keeps moving up because of trees or units, how can i fix this?

This is my problem I talked a about in the other fly topic. If a flying unit is fairly close to anything that would change it's height it will have it's height changed. I mean, it dean't have to be excatly abouve a higher thing just close. Even more annying is that if you let the game display the unit's height in this situatuions, it will display it's true height that it should have even if one can see that it is not true.