Mean Tide Level sits exactly halfway between mean high water and mean low water. In open water with a symmetric tide it sits close to mean sea level, but in shallow estuaries where the tide rises fast and falls slowly, or the reverse, the two can differ by several centimetres. A related value, the diurnal tide level, is the midpoint of mean higher high water and mean lower low water.
MTL and MSL are 4 centimetres apart up the river, that's the overtides skewing the curve.
Also heard as
- mean tide level
- half-tide level
Related terms
MSL
Mean Sea Level: the plain average of a station's hourly water levels across the tidal datum epoch.
MHW
Mean High Water: the mean height of every high tide across the datum epoch, often the line that defines the legal shoreline.
Overtide
A higher-frequency constituent, such as M4, generated when a tide distorts in shallow water.
Tidal datum
A local zero for heights, computed by averaging one stage of the tide, such as high or low water, across many years.