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Elevation is often measured in terms of "[unit of length] above sea level." Considering that the oceans aren't a uniform height, how is this measured?

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It's actually an inordinately difficult problem. Neither the land nor the sea level is stable. A tide gauge along a coast needs 30 to 40 years of data to filter out the very noisy tidal and other weather signals. By the time one has homed in upon a precise average, one is already within the time-frame of land uplift or subsidence, and that is before we even think about the complexities of climate-change generated sea level rise, or quasi-decadal oceanic oscillations. Then there is the question of Earth's non-homogeneous gravity which affects local sea levels. For example if we correctly measure the elevation of a Himalayan peak relative to average sea level at the Gurarat coast, west India, we get an elevation of 'h1'. We can then measure the same peak relative to sea level in the Ganges delta, east India, which we will call 'h2'. We can make all the usual corrections for Earth's curvature etc., yet h1 will be different to h2 - though both are 'correct'.

There is no stable bench mark on 'God's Earth', so maybe the answer is to get off the planet completely. Satellite orbits are not perfect, but they are predictable, so satellite-based measurements of elevation are the best available.

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  • $\begingroup$ I still don't understand. How then do they measure elevation if there could be multiple answers that are all correct? (That last point about the Himalayas was actually what I was thinking of when I posed the question. Well, it was actually the Rockies, but the point remains the same.) $\endgroup$
    – user6422
    Commented Aug 8, 2016 at 16:59
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This question is more about surveying than earth science.

Every country with an ocean coastline determines it's own standard for where sea level is. A mean point between the lowest tide mark and highest tide mark is generally taken to be the average sea level.

A surveying datum station is place at that point and all elevations above sea level, for those countries are referenced back to the national standard datum.

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  • $\begingroup$ This still doesn't really help anything. Take a look at Gordon's answer and my reply. How do they determine where to place "sea level" when there could be numerous sea levels, especially in countries with multiple oceans bordering them? $\endgroup$
    – user6422
    Commented Aug 8, 2016 at 17:02
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    $\begingroup$ Where to site a national datum is arbitrary, decided on pragmatic reasons of ease of access, ease of construction, and shelter from waves. $\endgroup$
    – Gordon Stanger
    Commented Aug 8, 2016 at 23:57
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    $\begingroup$ @DonielFilreis the various national geologic survey groups (e.g. USGS) agree on a common datum to call "mean sea level" and put down standards to measure this from. See ngs.noaa.gov/datums/vertical for examples. In a nutshell, its arbitrary and it doesn't really matter how it relates to where the water actually is as long as everyone is measuring from a common accepted reference datum. $\endgroup$
    – casey
    Commented Aug 9, 2016 at 2:41