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36Geoid modelsIn ancient times, people believed the world was flat. Around 500 BC, the Greek philosopher Pythagoras proposed the idea of a spherical earth, but without any scientific evidence to support his idea. It turns out that Pythagoras%u2019 gut feeling was sort of right. In modern times, we%u2019ve learned that the earth is not perfectly spherical, but rather an irregular ellipsoid. Of course, the earth%u2019s surface is not perfectly smooth due to the presence of mountains and valleys. However, for calculating depths and distances, scientists use an imaginary, irregular ellipsoid called a geoid to represent the earth%u2019s mean sea level. Prof. Chen and his team achieved centimetre-level accuracy in their Hong Kong geoid model for GNSS heighting, a technique that uses satellites to measure the precise location of a point on earth, including its height above the geoid. This contributes to high-precision surveying of the earth%u2019s surface and topographic mapping.ON THE BUTTONDIGGING DEEPER