It is located at the mouth of several major European rivers. As a result, 29% of the country is prone to river flooding. And if it's not enough, 26% of the country is below sea level. It is therefore essential to monitor the most current elevation of the land in as many places as possible. The NAP (Normal Amsterdam Level) arose in the 19th century from the widely used AP (Amsterdam Level) that was designed in the years 1683-1684 by measuring the height of the water at a location in Amsterdam. In the years and centuries that followed, that method was used all over the country.