Levelling is a process of establishing heights relative to a datum or a fixed point. Datum is known as mean sea level and this level varies between countries.
For easy determination of true heights, official or certified benchmarks (certified levels) are placed in various places in your area or country.
Benchmarks are the stated official height of a place with reference to the Datum.
With these benchmarks you can easily determine the height of a particular site by reference to it.
In a situation where the benchmark is far from a site where height or level is to be determined, Temporary benchmarks (TBM)can be established close to the site of interest with reference to the benchmark .
TBMs make it easier to establish levels or control heights on site.
Simple land height determination example
Taking a look at the diagram above, you are to determine the height or level of land at point B
At point A, you have a TBM of 180m ( above mean sea level)
Set up the leveling instrument somewhere around point C ( in such a way that you can potentially sight staff placed at point A and B without obstruction)
With the help of someone, place a levelling staff on top of the TBM in a vertical position at point A
Point your leveling instrument at the staff and take your readings.
Let’s assume that the reading on the staff you took is 2.55m. This is the backsight reading which is always your first reading.
Next, without changing the position of the leveling instrument, point the leveling instrument at point B and take staff reading.
Let’s assume the reading you took was 1.2m
To determine the level of ground at point B, add the TBM reading plus backsight reading.
I.e 180m + 2.55m = 182.55m
Next, deduct the second staff reading from 182.55m
I.e 182.55m – 1.2m = 181.35m
Therefore point B is at a level of 181.35m
As a check. Since point B is higher than point A, deduct staff reading at point B from that at point A
I.e 2.55m – 1.2 = 1.35m
Then add this difference to the TBM at A
I.e 180 + 1.35 = 181.35m.