Is Chart Datum considered fixed across different charts?

Chart Datum is defined as the reference level for measuring depth and heights on nautical charts, and it is not universally applicable or fixed across different charts. The reason for this variability is that Chart Datum can be influenced by local tidal conditions, land uplift or subsidence, and specific operational practices in different regions.

In practice, this means that measurements taken at one location may not be directly comparable to measurements taken at another, as each chart may use a different Chart Datum based on local conditions. For example, a chart used in a coastal area with significant tidal movements may adopt a different datum compared to one used in an inland river system where such tides have minimal effect.

While some charts, particularly those associated with large bodies of water or major ports, may tend to use similar datums, this is not the case for all charts, and thus the concept of a universally applicable datum does not hold. Additionally, as conditions can change over time, data adjustments or updates can affect the relevancy of a given datum, reinforcing the variability across different charts. This context illustrates why the correct understanding of Chart Datum is crucial in hydrographic surveying and navigation.