MARITIME – TIDE & TIDAL TREAM CALCULATOR
Chart Datum Height
A precise tool.
What is the Chart Datum Height & How does it work?
Chart datum is the reference level printed on nautical charts, typically the Lowest Astronomical Tide (LAT). It provides a consistent baseline for mariners to assess water depth and safe navigation. Observed water level is measured relative to a more common datum such as Mean Sea Level (MSL). To translate this measurement to the chart datum, the offset between the two datums must be applied. The resulting height above chart datum indicates how much water is present relative to the chart’s reference, which is essential for evaluating tidal windows, clearance under bridges, and grounding risk.
H_{CD}=H_{obs}-D_{CD}
H_{CD} = Height above chart datum
H_{obs} = Observed water level (relative to reference datum)
D_{CD} = Chart datum offset (reference datum minus chart datum)
H_{obs} = Observed water level (relative to reference datum)
D_{CD} = Chart datum offset (reference datum minus chart datum)
Parameters
Result
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Frequently Asked Questions
What is chart datum in maritime navigation?
Chart datum is the reference level on nautical charts, usually the Lowest Astronomical Tide (LAT), used for measuring water depth.
How do I convert observed water level to chart datum?
Subtract the offset between Mean Sea Level (MSL) and Chart Datum from the observed water level to get the height above chart datum.
Why is it important to know the chart datum?
Knowing the chart datum helps mariners assess water depth accurately for safe navigation.
What is the difference between Lowest Astronomical Tide and Mean Sea Level?
Lowest Astronomical Tide (LAT) is the lowest tide predicted to occur, while Mean Sea Level (MSL) is an average of the tides over a long period.
Can I use this calculator for any type of water body?
This calculator is primarily used for marine environments where nautical charts are applicable.
How often should I update my chart datum information?
Chart datum information can change due to land subsidence or other factors, so it’s important to consult the latest nautical charts and updates from maritime authorities.
What if I don’t have the exact offset between MSL and Chart Datum?
You should obtain this information from local maritime authorities or the chart’s metadata, as it is specific to the area and can vary.
Results are for informational purposes only and do not constitute professional advice.