What really is Sea Level?

[zdas04]

At lunch today my wife asked me "if the sea level rises, will Denver be at a lower elevation?" I stammered a bit and said that sea level is a fairly arbitrary plane equidistant from the center of the earth, blah, blah, blah. She didn't buy it, she wanted to know if the earth warmed up (for whatever reason, not necessarily AGW) and Florida was under water, Little Rock was a beach front community, etc, would "they" change the official elevation of Denver?

My reasoning was that you don't really measure the elevation above sea level, you measure the height of gas above a point to determine atmospheric pressure (and subtract that from official sea level pressure to get elevation) and that what we really measure is height below space. Not sure that she bought that either. Anyone have another explanation?

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[MintJulep]

From the ever-trustworthy Wikipedia; World Geodetic System article.

http://en.wikipedia.org/wiki/World_Geodetic_System[/URL]]The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the error is believed to be less than 2 cm.[2]
The WGS 84 meridian of zero longitude is the IERS Reference Meridian,[3] 5.31 arc seconds or 102.5 metres (336.3 ft) east of the Greenwich meridian at the latitude of the Royal Observatory.[4][5]
The WGS 84 datum surface is an oblate spheroid (ellipsoid) with major (transverse) radius a = 6378137 m at the equator and flattening f = 1/298.257223563.[6] The polar semi-minor (conjugate) radius b then equals a times (1 - f), or 6356752.3142 m.[6]
Presently WGS 84 uses the EGM96 (Earth Gravitational Model 1996) geoid, revised in 2004. This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 360 (which provides about 100 km horizontal resolution).[7] The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid range from about −105 m to about +85 m.[8] EGM96 differs from the original WGS 84 geoid, referred to as EGM84.

 

[zdas04]

So it is an arbitrary plane equidistant from the center of the earth that pretty much has nothing to do with where the level of the sea is. Would they change it if the actual average sea level changed 500 ft?

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[MikeHalloran]

Locally, sure, it's a plane.
Globally, it's an oblate spheroid. ... and arbitrary.


Mike Halloran
Pembroke Pines, FL, USA

 

[dpc]

We'll be changing a lot of data if the sea level rises 500 feet.

 

[VEBill]

Google (as an example): North American Vertical Datum of 1988

It's complicated. The change in sea level (due to global warming) is the least of the complexities. Your local mean sea level (the actual water) might be meters above or below your country's "sea level" datum (ref: ocean surface topography).

This topic ("Sea Level") would be an interesting subject for a one-hour science documentary.

 

[msquared48]

Sea level is tanamount to Middle C.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[fattdad]

I'd like it simple like the OP's wife. Surveyors really complicated this one, eh?

f-d

¡papá gordo ain’t no madre flaca!

 

[patprimmer]

If sea level rises, does atmosphere level also rise.

Seriously, I think if sea level rises, the datum point called sea level should not change otherwise we would need a date to which all altitudes where tied. A change of date throws instrument calibrations out. That might be problematic for aircraft navigation at times.

We kinda handle that with shift between true and magnetic north with dates and annual shift data on maps, but it's far from ideal.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[MintJulep]

Per my cited reference sea level already has a date associated with the datum surface.

The relationship between the hypothetical standard atmosphere and the arbitrary sea level datum suggests that IFR near MSL could result in wet feet.

 

[patprimmer]

If they changed the term from Sea Level to Standard Theoretical Traditional Sea Level? Verbose and pompous but fixed as a reference. Yes one could get wet feet even at above sea level. But even now, one can have dry feet standing by the Dead Sea at well below Sea Level.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[CorBlimeyLimey]

I'd like it simple like the OP's wife.

Hmmm! You better hope that Mrs. Simpson doesn't read that.

 

[zdas04]

Yeah, she's anything but that.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[patprimmer]

Her question sounded pretty switched on to me.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[fattdad]

I guess we have to republish our maps any how with the Atlantic Riff, San Andreas and all that plate confusion in the India Ocean. Anyway, isostatic adjustment heaves the Appalachian Mountains a few inches per year.

I guess there's simplicity in the WGS 84 too.

f-d

¡papá gordo ain’t no madre flaca!

 

[SomptingGuy]

GMT anyone?

- Steve

 

[BigInch]

I'm glad to hear that somebody in your family is finally taking global warming seriously. Apparenty very seriously.

In loose terms MSL reference at a given location is the mean of sea levels at the closest place to your location where a sea level exists.
MSL varies considerably, with the oblate spheroid surface of earth and with gravitational attraction of the adjacent land mass.

Chimborazo in Ecuador has an altitude of 6,310 meters (20,703 feet). Mount Everest has a higher altitude. Mount Everest is 8,850 meters (29,035 feet) above sea level.

Mauna Kea is "taller". However Mauna Kea rises over 10,000 meters above the ocean floor making it taller than Everest.

Chimborazo has the distinction of being the highest mountain above Earth's center. This is because Earth is not a sphere - it is an oblate spheroid. As an oblate spheroid, Earth is widest at its equator. Chimborazo is just one degree south of Earth's equator and at that location it is 6,384 kilometers from Earth's center or about 2 kilometers farther from Earth's center than Mount Everest.




"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[ivymike]

sooo... I gather the air should be much thinner at 20kft on Chimborazo than at 20kft on Everest?

 

[Latexman]

I was in Ecuador on business many years ago. Over the weekend there, I went to a wedding in Quito. A friend and I drove his Jeep to the snow line on Chimborazo, then hiked up past the snow line. Ivymike, the air was so thin we had to stop every few steps to rest. Once we got as high as we could go, we built a small snowman. The views were . . . breath taking! And, the annisette, "to clear our ears", was delicious. It was a bucket list experience.

Good luck,
Latexman

 

[ivymike]

any idea what altitude?