I think but I´m not sure it´s a first thigs I try to do whit GLbasic.
If the viewport it´s Orthogonal, and you only want rotate from the camera it´s simple:
X_Camera cos(radian)*vector,high from the land,sin(radian)*vector,
0 , 0 , 0
Whit this you can trun around your from a desired Heigth from the floor.
For an FPS game, I think it´s a bit more complex, but in the examples files, concretly the Maze, I think it´s did it for Mrtatoad*, you can find the routine for a complex movement of the Camera.
*Excuseme if I´m wrong-