[time-nuts] NYTimes: New geoid soon

[Hal Murray]

The US is tilted.  Florida is about right.  Seattle is a meter high.

The U.S. Is Getting Shorter, as Mapmakers Race to Keep Up
https://www.nytimes.com/2020/05/22/science/maps-elevation-geodetic-survey.html

But across the United States, the heights of structures, landmarks, valleys, 
hills and just about everything else are about to change, at least with regard 
to average sea level. Most will get shorter. Parts of the Pacific Northwest 
will shrink by as much as five feet, and parts of Alaska by six-and-a-half, 
according to Juliana P. Blackwell, director of the National Geodetic Survey. 
Seattle will be 4.3 feet lower than it is now.

That's because height is only height compared to a reference point -- and 
geodesists, who calculate the Earth's shape, size, gravitational field and 
orientation in space over time, are redefining the reference point, or 
vertical datum, from which height is derived. It is a fiendishly difficult 
math and physics task that, once completed, will have taken a decade and a 
half to accomplish.

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But making highly detailed measurements of the gravitational field, in order 
to factor them into heights captured by GPS, is no small task. In 2007, the 
National Geodetic Survey launched an ambitious mission -- GRAV-D, for Gravity 
for the Redefinition of the American Vertical Datum -- to accomplish just that.


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