@bill93?ÿ
As I recall NGVD not being explicitly tied to ??sea level?? is because as sea levels vary sea level would always be zero. So how??d you cite its increase or decrease??ÿ
And of course as we know orthometric heights versus sea level elevations are not synonymous.?ÿ
@dougie NAVD is based on MSL.
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The North American Vertical Datum of 1988 (NAVD 88) is the vertical control datum established in 1991 by the minimum-constraint adjustment of the Canadian-Mexican-United States leveling observations. It held fixed the height of the primary tidal bench mark, referenced to the new International Great Lakes Datum of 1985 local mean sea level height value, at Father Point/Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e., the fact that mean sea level is not the same equipotential surface at all tidal bench marks.
North American Vertical Datum of 1988 (NAVD 88) consists of a leveling network on the North American Continent, ranging from Alaska, through Canada, across the United States, affixed to a single origin point on the continent.
In 1993 NAVD 88 was affirmed as the official vertical datum in the National Spatial Reference System (NSRS) for the Conterminous United States and Alaska (see Federal Register Notice (FRN)). Although many papers on NAVD 88 exist, no single document serves as the official defining document for that datum. Detailed information about NAVD 88 is available here.
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Maybe in Rimouski Quebec, but it's a little different in Tacoma:
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