# Levelling Simply Explained

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

Solution

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

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

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.