# Chart Datums¶

The chart datum is defined using the Lowest Astronomical Tide (LAT) : it is the lowest tide level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions. [LAT]

The different water height are defined using this chart datum (see following figure):

• $$B(x,y)$$ : bathymetry, varying in space,
• $$H(t)$$ : tidal height, given by the tidal model,
• $$\eta(x,y,t)$$ : wave elevation, given by the wave field model,
• $$H(t)+\eta(x,y,t)$$ : free surface position, relatively to the LAT,
• $$H(t)-B(x,y)$$ : ocean depth, relatively to the free surface.

Fig. 3 Chart datum, wave elevation, tidal height and bathymetry definitions

## References¶

 [LAT] RESOLUTIONS of the INTERNATIONAL HYDROGRAPHIC ORGANIZATION Publication M-3 2nd Edition - 2010 Updated to December 2016. SECTION 2.2 – TIDES AND WATER LEVELS para 2 note (i)