The purpose of a vertical survey is to establish an elevation relative to a reference surface.

The most common reference surface - called "mean sea level" - is the one used for our national topographic maps.

Small scale, local surveys, such as construction surveys, may not need a height above mean sea level, only the differences in heights between points.

The most common methods to determine the difference in heights or to establish elevations are differential and trigonometric levelling.

Differential Levelling

Most often referred to as 'levelling', this method of establishing elevations uses a tripod mounted telescope, called a level, which is aligned at right angles to the direction of gravity.

The elevation is determined by measuring the difference in readings taken on two graduated rods similar to very large rulers.

Points with known elevations are referred to as "benchmarks". If we know the elevation of one point, we can "transfer" the elevation for a new point by applying the difference in elevation between the known (or assumed) elevation and the new point.

Example: We have a known elevation at one point (benchmark) and need the elevation of another point, say, 50 metres away.

• First, we would have an assistant hold one of these rods perfectly vertical - straight up and down - on top of the point of known elevation.
• Next, we would set up our level (tripod mounted telescope) about half way between the point of the known elevation (benchmark) and the point for which we need an elevation.
• We would then aim the telescope at the rod, and using the crosshairs in the telescope, we would record where on the rod the crosshairs appear.
• If we add this measurement to the elevation of the known point, we could compute the elevation of our instrument. This is called our HI or height of instrument.
• By moving the rod to the new location and 'reading' the rod again, we can subtract this rod measurement from our HI and then compute the elevation for our new point.

Trigonometric (or Trig) Levelling

This form of transferring elevations uses the mathematics of trigonometry. Rather than using a level, we can use the same theodolite we used for our horizontal surveys.

The theodolite is set up over a point of known elevation (benchmark) and the height of the centre of the telescope above the reference point is carefully measured and added to the known height to produce an accurate HI, or height of instrument.

The vertical angle and distance to the target station is measured and using trigonometry the difference in elevation is computed. This difference is added to the height of the telescope to give us the height of the target station.

Other Vertical Survey Methods

Generally speaking, the same methods used for horizontal surveys can also be used for vertical surveys - theodolite, total station, Doppler, inertial surveys, GPS, LIDAR, and photogrammetric surveys.

Observations or measurements from these types of surveys vary in accuracy.