Laser Theodolite

A theodolite is a measuring instrument capable of measuring azimuth and elevation angles to very high accuracy. When computing power and high precision laser distance measuring capability is added, the laser theodolite that results can measure or locate three dimensional "point-in-space" positions within a sphere of 5 millimeter radius.

I had previously surveyed this site with the instrument, including all the buried pipe and conduit runs as I installed them. The lower line of posts for the solar structure are dangerously close to the septic leach field, so it was important to place them accurately to avoid augering into a septic line.

Moreover I believed that precision placement would be critical to the success of erecting the support structure. It's hard enough to thread a lot of heavy precut pipes together in mid-air, but sloppy tolerances would make this 100-foot long monster the mother of all plumber's nightmares.

Using the laser theodolite is a 2-person procedure - one at the instrument and one at the cooperative retro-reflector target. When the laser theodolite locates a stakeout point it displays the distance to that spot and projects a pair of parallel visible laser beams right to the spot. One beam blinks and the other is solid. The target person can pace the indicated distance, then look back and move from side to side to reach the point where the beam transitions from blink to solid. The target is thus placed and another measurement reveals the error for another try. This iterative procedure quickly converges on the precise location.

Elaine (my wife) is not a technocrat and therefore has limited patience as the target person. So I constructed a replacement for her. It is a simple XY table, made of waxed MDF, with leveling feet and a mount for the retro-reflector target.

The XY table is placed at an approximate location facing the laser theodolite, which measures its location and indicates the error in XY movement. If the error is large, the table must be relocated by the approximate amounts. If the error is small, the table is moved in its ways by the indicated amounts. Another measurement is taken and the procedure repeats. When the measured error is acceptable (usually after three measurements) the retro-reflector is lifted from the table and a marker spike is dropped through the hole into the ground.