Решено.
float cdist=100; // cam clip plane
Vector3 p1=Camera.main.ViewportToWorldPoint(new Vector3 (0.5f, 0f, cdist));
Vector3 p2=Camera.main.ViewportToWorldPoint(new Vector3 (0.5f, 1f, cdist));
float deltaz = (p1.z - p2.z);
float groundoffset = 0.6f;
float h = 1.0f/deltaz*(p1.z-groundoffset);
Debug.Log("p1z="+p1.z+" p2z="+p2.z+" h="+h);