What would a theodolite be used by?

Answer 1

The vertical angles are all measured from an assumed horizontal plane....e.g. level ground. So the instrument must be carefully "leveled" when you set it up (usually on a tripod). The course adjustment for the instrument starts with the tripod legs, then the leveling screws of the base, and the leveling of the sight tube. Rotating the instrument to various horizontal azimuths helps to confirm the instrument is level no matter which direction it faces.

Horizontal angles are often referenced to North (which can be either true north or magnetic north) but on building construction sites, its more important to align to a structural element of the building, usually a side of the foundation. So many times a corner point serves as the start and all horizontal angles are measured relative to a side of the structure starting from the corner point to another corner point.

The precision of the theodolite lies in the direct reading micrometer scale in the telescope (sighting tube). The transit (the poor brother of the theodolite) often uses vernier scales for measurement. When you sight on the target with the telescope, the horizontal angle is read off the circle (scale at the rotating base of the telescope), and the vertical angle is read directly from the micrometer scale seen in the telescope.

Its a good practice to take a reading and record both angles. Then rotate the telescope completely in one direction (say clockwise) and take a second reading and record both angles. Then rotate the telescope completely in the opposite direction and take a third set of readings and record them. Then calculate the average vertical angle and the average horizontal angle...and it is more likely that if anyone were to repeat these measurements, they would probably come closer to the averages of your measurements than to any one of the three individual measurements you took.

Keep in mind that as the day goes on, temperatures tend to rise..and heat causes most materials to expand and could throw things out of whack. Also, over long distances, heat shimmer can make it difficult to accurately sight on the target even with magnification of the telescope. So shading the instrument and planning your work earlier in the day when temperatures are cooler can also factor into the precision of your measurements