Let’s start again with an example of NAP. But beware!
The threshold depth of the lock at Weurt is NAP +3 m.
The level on the outside of the Weurt lock indicates a water level of NAP + 8.20 m.
How much water is there on the threshold of the Weurt lock?
First sign the NAP line. Then sign the threshold. Arrow 1 indicates the depth of the threshold: Three metres above sea level. Then mark the water level. Arrow 2 indicates that it is 8.20 metres above sea level. So: the passage depth is 8.20 metres minus 3 metres is 5.20 metres.
What is the clearance depth at the threshold at the Panheel lock? The threshold is at KP -3 metre. The water level is KP -0.6 metres. So the depth of passage is 3 metres minus 0.6 metres is 2.4 metres. Have a look at the drawing again!
At a certain place the BP (basin level) is at NAP – 1.20 m. The threshold of a lock at the location is BP – 3.60 m. The water level is NAP – 1.50 m. How much depth is there at the threshold of the lock?
On the drawing below you can now clearly see that the water level is 0.30 m below BP. The threshold is at BP – 3.60 m. So the depth at the threshold is 3.60 m – 0.30 m = 3.30 m.
If the water level could have been read on a BP scale, we would immediately have known that the water level was 0.30 m below BP. Because the water level was indicated on a NAP level scale, a conversion from NAP to BP had to be made.