When Philip Minderhoud travelled through Vietnam in 2015, he sensed that something was not quite right. Minderhoud was working on his PhD research on land subsidence in the Mekong Delta, one of the largest deltas in the world. He had brought along a series of maps indicating how high the land was supposed to be elevated: one and a half metres above sea level, and in some places even two metres, according to the maps.

Minderhoud looks around, across the vast delta. The landscape is as flat as a pancake. And almost everywhere he looks, he sees the same thing: the water level is far higher than the maps suggest. In many parts of the delta, the surface water level — directly connected to the sea — is not one and a half metres but only a few decimetres below the land surface. The maps simply do not match reality. And that is not because of the weather or the tides.

The maps Minderhoud consults in Vietnam come from reports by international organisations. How can it be, he wonders, that these reports present such a distorted picture? And how widespread are these inaccuracies? In the years that follow, he delves deeper into the methods and calculations underpinning the relative height of sea level and land elevation.

How sea levels are measured
In the Netherlands, sea level elevation is no mystery. Since the end of the 19th century, the Netherlands has used Normal Amsterdam Level (NAP) as a reference frame for measuring the country's elevation. Nieuwerkerk aan de IJssel is 6.78 metres below NAP, while the Vaalserberg is 322 metres above it. Many people intuitively think that 0 metres NAP equals sea level, but sea level has now risen to about 10 centimetres above NAP. A house 1.50 metres above NAP is therefore actually about 1.40 metres above the current average sea level for the Dutch coast.

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