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height datum

This is the tide level used to specify the height of charted lights and the vertical clearances under bridges. For US charts, it is mean high water (MHW). This value can changes somewhat even on the same chart. It is not listed in tide books, but it is always printed on paper charts and the RNC based on them. There is a table on the chart listing the value for several locations on the chart.

That MHW data on RNC, however, is no longer being updated in preparation for the new ENCs. Unfortunately, the ENC do not present this is a very direct manner. To get it, do a cursor pick of the green foreshore and you will get a depth area of 0 to some negative value, which is the MHW for that chart.

The green foreshore is bounded by two datums. Where the green meets the blue is zero tide, so a sounding is 0. This is where water meets land at zero tide. On the inland side of the green, where it meets the tan, is where the water meets the land at the height datum, MHW. But since these are "soundings" relative to zero tide, they go negative in the air.

in short, to get the MHW use the negative value of the depth area of the green foreshore on the chart. Although NOAA uses only one value for the full chart, which is not as accurate as it was for RNC, this will become less of an issue when they rescheme the charts to smaller cells.

Not to be confused with elevation datum

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