Vertical Distance Measurement
Using a level to measure height differences
Levelling is the name given to the process of measuring differences
in elevation (height) between
two or more points. It is
based around knowing the elevation of one point, and then relating the
other points to it.
An Automatic Level (with tripod),
and a Levelling Staff. Levelling
requires one skilled operator for the Level itself, but the individual
holding the staff may be unskilled.
Levelling between two points
- Choose your Benchmark,
with a known or assumed elevation. This is your Backsight.
- Choose the point that you require the elevation at. It should be
no more than 100 meters away, preferably less. This is your foresight.
- Set up your level in a position
roughly equidistant, (and not more than 50 metres) from your backsight
and foresight. Ensure that from here, a staff held on either point is
- Sight onto a staff held on your
backsight, and carefully take a
reading. Look away, and record your
sighting. Look back, and check that you have read and recorded
- Add the backsight reading to the backsight elevation. This gives
you the Height of Plane of Collimation
- Sight onto a staff held on your foresight, and carefully take a
reading. Look away, and record your sighting. Look back, and check
that you have read and recorded correctly.
- Subtract the foresight reading from the HPC. This gives you the
elevation of the foresight.
- If you wish to move on to a 3rd (or further) point, you can repeat
steps 2 - 7, except that instead of using a benchmark, you use the
foresight information you have just gained.
Change Points and Intermediate Sights
If the distance between your points is greater than 100 metres, you
will need to create a Change Point..
This is simply an arbitary point between your points of interest, that
you take a level at. Treat it exactly the same as any other point,
using it as a backsight when you move on to the point you actually wish
to measure. Over large distances, large chains of change points may be
You may require the elevation of a number of points near to the
backsight. These are known as Intermediate
Sights. Having found the HPC using the backsight, simply treat
these as you would a foresight. Leave the level in position, and take
Intermediate sights to all the points required. Subtracting these
readings from the HPC will give you the elevations of these points.
Or select Back to return to the main selection screen.