
Height Modernization: Recalibrating where the U.S. physically sits on the planet - wglb
https://www.nytimes.com/2020/05/22/science/maps-elevation-geodetic-survey.html
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istjohn
Reading this, I wonder if the melting of the ice caps causes a measurable slow
down in the Earth's rotation and an increase in the length of a day by moving
mass farther from the Earth's axis of rotation.

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throwaway2048
Not sure why this is downvoted, this is absolutely the case, but the effect is
small

[https://climate.nasa.gov/faq/30/if-all-of-earths-ice-
melts-a...](https://climate.nasa.gov/faq/30/if-all-of-earths-ice-melts-and-
flows-into-the-ocean-what-would-happen-to-the-planets-rotation/)

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foota
I'm surprised by the magnitude of the effect. I wouldn't have expected things
to move around enough for the length of a day to vary by a millisecond?

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kd5bjo
It doesn’t mention what sort of frequency those variations happen at. If the
variation is caused primarily by plate tectonics, it might take a year or more
to swing between the two extremes.

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supernova87a
If you ever want to be bored out of your mind (at whatever level of detail
that occurs for you), just ask someone in this field about the definitions of
geoid, ellipsoid, etc. etc. and you will have hours (well, maybe minutes) of
fun.

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globular-toast
My PhD supervisor told a funny story about this to illustrate the phenomenon
of learning more, but seemingly knowing less.

First you learn that the Earth is "round". Then you learn about shapes and
that the Earth is actually a sphere. Then you learn more shapes and that the
Earth isn't actually a sphere but is slightly squashed at the poles: it's
spheroid. Then you learn that it is actually more squashed at the top than at
the bottom. It is in fact a geoid. So you look up geoid in the dictionary and
find that geoid means "Earth shaped".

~~~
hug
This reminded me of The Relativity of Wrong.

[https://chem.tufts.edu/answersinscience/relativityofwrong.ht...](https://chem.tufts.edu/answersinscience/relativityofwrong.htm)

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mdturnerphys
It's unfortunate and potentially confusing to readers that they've used the
Washington Monument and the Flatiron Building as illustrations in the article
and suggested that their heights might change. The reference point for
measuring a structure's height is its base, not the geoid. Unless the
definition of a foot changes the height of these structures will not change.

~~~
taejo
Curiously, the definition of a foot _is_ changing. Or rather, one of the two
definitions of a foot (the _U.S. survey foot_ ) is being deprecated in favor
of the other (the _international foot_ ). The two differ by about two parts
per million.

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kd5bjo
My understanding was that an inch is exactly 2.54cm, and a foot is exactly 12
inches ever since the metrification of US customary units. Is that not
correct?

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taejo
No, the original metric definition of US units in 1894 [0] made 1 yard =
3600/3937 metre (i.e. 1 inch = 100/3937 metre). The US survey foot is 12 of
those inches.

The 1 inch = 25.4 mm definition was adopted by the British Standards
Institution in 1930 and by the American Standards Association in 1933 (with
the name "industrial inch"); in 1959 the US, UK, Canada, Australia, New
Zealand and South Africa signed a treaty agreeing on that definition [1] (and
also a definition of the pound). The international foot is twelve of those
inches, but the US allowed (but recommended against) the continued use of the
old foot in surveying.

[0] [https://usma.org/laws-and-bills/mendenhall-order](https://usma.org/laws-
and-bills/mendenhall-order) [1]
[https://en.wikipedia.org/wiki/International_yard_and_pound](https://en.wikipedia.org/wiki/International_yard_and_pound)

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neonate
[https://archive.is/539Hv](https://archive.is/539Hv)

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Rexxar
I would have expected that the recalibration have big impact in middle of
continent and no impact near coasts but strangely it's not the case. How
coastline altitude could have been incorrectly calibrated before ?

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throwaway2048
Oceans do not have the same height everywhere, there can be up to about 2M of
difference.

[https://en.wikipedia.org/wiki/Ocean_surface_topography](https://en.wikipedia.org/wiki/Ocean_surface_topography)

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arethuza
This caused problems when a bridge was being built between Germany (which uses
the North Sea) and Switzerland (which uses the Mediterranean) - someone made a
mistake including the known height difference between the two seas:

[https://www.science20.com/news_articles/what_happens_bridge_...](https://www.science20.com/news_articles/what_happens_bridge_when_one_side_uses_mediterranean_sea_level_and_another_north_sea-121600)

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ARandomerDude
> That’s because height is only height compared to a reference point — and
> geodesists...are redefining the reference point, or vertical datum, from
> which height is derived.

So it isn't getting shorter. We're changing our reference point. Those are
very different things!

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Causality1
We really need a "flag for misleading headline" button.

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dang
If anyone can suggest a better title, we can change it. "Better" means: more
accurate and neutral, and preferably using representative language from the
article itself
([https://hn.algolia.com/?query=representative%20title%20by:da...](https://hn.algolia.com/?query=representative%20title%20by:dang&dateRange=all&page=5&prefix=true&sort=byDate&type=comment)).

Edit: I've taken a crack at it.

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sradman
Perhaps “elevation” instead of height:

 _U.S. Geodesic Elevation Recalibration_

[https://geodesy.noaa.gov/heightmod/](https://geodesy.noaa.gov/heightmod/)

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aasasd
No sweat, the CIA factbook will just define the U.S. elevation as including
its aerospace.

Like it does for area.

