# Sea Level and its variability with regard to 'Altitude'

I'm wondering how we are managing 'things' that relate to 'sea level'.

Mean Sea Level is taken to be the halfway point between high- and low tide... but as the sea level rises, presumably this will change over time...

Within the world of cartography, where elevations can often be given in 5m increments, has someone decided what '0m' is, and fixed it?

Presumably cartography and systems like GPS have the same problem...

I'm also curious with regard to barometric pressure... With the rising sea level one could reason that the absolute value (e.g: in bar) of the pressure at sea level will be falling (the atmosphere is now surrounding a 'larger' planet, and thus is spread more thinly).

I don't expect this to be of significant concern to aviation (presumably they are working with large-ish tolerances), but pressure sensors in consumer applications (e.g: mobile phones) have a high enough resolution that they can detect a change in pressure over 1-2 metres or less. I wonder where else they are employed...

I thought I'd follow up (24th Oct 2017) with a link to Tom Scott's video, where he mentions variations in gravity causing problems for "sea level" too: What is sea level, anyway?

• Can you provide a reference for portable barometric altimeters with the 1m resolution you quote? Most mobile phones use GPS data for their altitude readings, and the weather-driven variation is bigger than that. – Emilio Pisanty Apr 9 '17 at 13:43
• Apologies - I didn't mean that these pressure sensors can provide an absolute altitude alone (as you state, the weather alone would make that impossible), but rather that they are able to detect the change in pressure over a 1-2m vertical range. – Attie Apr 9 '17 at 13:46
• In that case, you should clarify that statement in the question. – Emilio Pisanty Apr 9 '17 at 14:23
• @Emilio updated – Attie Apr 9 '17 at 14:43

Sea level is defined by the "reference geoid ellipsoid"--the current version is WGS 84. This has sub-1 meter resolution, and its value is $6378137.0$ meters at the equator and $6356752.314$ meters at the poles.

Note that the polar radius is calculated by the flattening of $1/298.257223563$ (as defined by the WGS 84 standard; see the linked page for information on how that is determined). NOTE that the equatorial radius (semi-major axis) and the flattening are defined; the polar radius (semi-minor axis) is calculated.

Modern technologies such as GPS and similar can be used to measure sea level (and variations thereof) with accuracy down to centimeters (maybe more). Also, GPS uses the reference ellipsoid when telling your smartphone or handheld GPS thingy what altitude you're at.

As for the barometric pressure, I intuitively say yes, it would decrease, but the amount would be negligible--way below any current means of measurement. I'm certain other environmental factors would have a bigger impact on average barometric pressure. Note that the chemical composition of air has changed over geological timescales, and this affects the density (and thus the mass and pressure).

ADDED: This is an interesting page to read regarding the ellipsoid and what we know as "sea level".

Side note: The Panamá Canal empties into the Pacific Ocean 20 cm higher than at the Atlantic Ocean (and this is not due to diurnal tides, etc.).

• So by the sounds of it, sea level is becoming a reference point that was historically related to the sea, but may no longer be related? i.e: It's fixed, and maps, etc... don't need to be updated. – Attie Apr 9 '17 at 14:06
• Thanks for pointing out the composition of the atmosphere - not something I'd considered here!.. – Attie Apr 9 '17 at 14:07
• WGS84 is a reference ellipsoid--the approximate shape of a spinning planet. A geoid is different from an ellipsoid. A geoid is an equipotential surface, and on the Earth is not an ellipsoid--as there are density variations in the planet. EGM96 and EGM08 are commonly used unclassified geoids; they are available as look up tables with 1 degree and 1 minute postings, respectively, or a as spherical harmonic expansions. The mean sea level should approximate a geoid. – JEB Apr 9 '17 at 15:37
• @JEB - thanks, fixed. Apparently I'm confusing the two terms. – pr1268 Apr 9 '17 at 15:50
• @Aditya: I believe it's because the ellipsoid is a mathematical model that can easily be calculated with software and hardware (thinking smart phones and GPS devices here). On the other hand, a geoid is a model of the Earth with all its irregularities. Check the "interesting page" link above and the first image on that page shows an example (albeit highly exaggerated) of all the "bumps" and protuberances of the Earth. – pr1268 Jul 8 '17 at 23:13