In order to be able to indicate the depth of waterways, we must assume a certain area as ‘zero point’. This also applies if we want to indicate the height of towers, mountains, bridges or buildings. The depth or height is compared to this ‘zero point’. It’s called the ‘plane of comparison’.

In this chapter you will learn how to calculate how much the height under bridges or the depth over thresholds can be calculated. We only look at waterways that are not in tidal areas.