Marine Glossary

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mean high water

A tidal datum abbreviated MHW. The MHW at a specific location is the average of all the high water heights observed over the 19-year National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent of a 19-year datum.

This is an especially important reference in US charting because it is also the vertical height datum used on charts. The heights of lights, the vertical clearances of bridges, and the drying heights of rocks are specified as above MHW. Thus when the tide is lower than MHW you have more clearance under a bridge, lights are higher above the water and can thus be seen a bit farther off, rocks are higher above the water than stated on the chart. All of these are reversed when the tide is above MHW.

On US RNC, we find the value of MHW from tables printed on the chart. It is generally listed for several locations. On a smaller scale chart (1:80,000 or less) this can vary up to 1 ft or so.

On US ENC, we find the value of MHW from a cursor pick of the green foreshore that will show a depth area extending from 0 to -MHW, which is from the soundings datum to the height datum. The value is given as negative because soundings are listed as positive numbers and this is going the opposite way from zero tide, which is the reference point. In contrast to RNC, US ENC only list one value of MHW for the full span of the ENC cell.

Refer to our textbook for more discussion of this detail, and how other nations choose to treat this.

See height datum