# 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.

### Standard Equipment

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

1. Choose your Benchmark, with a known or assumed elevation. This is your Backsight.
2. Choose the point that you require the elevation at. It should be no more than 100 meters away, preferably less. This is your foresight.
3. 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 readible.
4. 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 correctly.
5. Add the backsight reading to the backsight elevation. This gives you the Height of Plane of Collimation (HPC).
6. 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.
7. Subtract the foresight reading from the HPC. This gives you the elevation of the foresight.
8. 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 required.

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.