What sort of data does nautical chart datum largely depend on?

Nautical chart datum primarily depends on physical surveys, which involve the direct measurement of the water body and its features. These surveys typically include depth soundings and the assessment of underwater topography, conducted using techniques like echo sounding and multi-beam sonar. The data collected during physical surveys is crucial in establishing the vertical reference points for the chart datum, which is a standard reference for nautical charts.

Physical surveys provide the accurate, localized data needed to ensure that the hydrographic information is reliable and can effectively guide navigation. The results from these surveys are used to create and update nautical charts, ensuring that they reflect current conditions and allow mariners to navigate safely. While other methods like numerical modeling, astronomical predictions, and remote sensing can contribute to data collection and analysis in hydrographic surveying, they are not the primary sources for establishing nautical chart datum.