declination
azimuth
dip
bearing.
bench mark
datum point
reduced level
reference point.
in the objective glass
at the centre of the telescope
at the optical centre of the eye piece
in front of the eye piece.
10 m and 20 m length
15 m and 20 m length
20 m and 30 m length
25 m and 100 m length
one dimension
two dimensions
three dimensions
four dimensions.