I use both the top down view and preview window to move the camera around and find it very useful. The only thing I miss is not being able to rotate/tilt the camera in the heightfield view. (maybe a right click on the axis to rotate around the axis??)
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For loaded TER files, it doesn't link properly unless the lower left of the TER is set to 0,0 (ie don't centre on 0,0)
If you change the camera position/orientation in the preview window, after you click on "set camera", just click on the hieghtfield view to correct the camera position (camera also becomes unselected).
It would also be really helpful if the y-axis could be extended to the point where it intersects with the terrain. This would make positioning the camera a lot easier in oblique views, providing both a ground reference and an indication of altitude.