What Chart Datum Does Savvy Navvy Use for Depths?

Savvy Navvy shows depths in chart datum, the same reference used by the official hydrographic charts for your region. In the US, that means NOAA charts referenced to Mean Lower Low Water (MLLW).

The Detail

• Chart datum is the baseline depths are measured from. It represents a low water reference point, so the actual depth on the water at any moment is the charted depth plus the current tidal height.
• Savvy Navvy follows the datum used by the official hydrographic office for each region, rather than converting every chart to one global standard.
• In the US, depths are referenced to NOAA's MLLW (Mean Lower Low Water), consistent with standard US charting practice.
• Outside the US, depths follow whichever datum the relevant national hydrographic authority publishes for that region's charts.
• Bridge and overhead clearance heights come from the same official hydrographic source as the chart's depth data.

Edge Cases and Caveats

• Because Savvy Navvy licenses chart data from multiple hydrographic sources worldwide, the datum can differ from country to country. This is expected and matches standard nautical charting practice, it isn't an inconsistency in the app.
• Chart datum is a fixed baseline. The water depth you'll actually find also depends on the tide at that moment, so always factor in the current tidal height on top of the charted figure. To learn more about Tidal Heights view this help article.
• If you're comparing a depth reading to a different app, chart, or plotter, small differences can occur if that source references a different datum for the same area.

Updated on: 11/09/2026