The Height of Collimation is defined as what?

• A. Height of the instrument above ground.
• B. Line of sight elevation.
• C. Elevation of the optical axis of the telescope at the time of setup.
• D. Distance to observed point.

Answer: (C)

Height of collimation is the vertical position of the telescope’s optical axis above the ground at the moment the instrument is set up. It represents the height of the line of sight itself, serving as the reference point for vertical measurements. When you measure a slope distance and a vertical angle to a point, you use this base height to determine the point’s absolute height above the datum. For example, the height difference to the target comes from the vertical component of the measured distance, and adding that to the height of collimation gives the target’s height relative to the datum. This is why the definition points to the elevation of the optical axis at setup. The line-of-sight elevation in isolation would refer to the angle of sight, not the physical height of the axis; the distance to the point isn’t the reference height, and simply the instrument’s height doesn’t specify the actual optical axis position.