Sea Level as a Point of Measurement

Discussion in 'Earth Science' started by Acitnoids, Sep 18, 2018.

  1. exchemist Valued Senior Member

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    I do not have a definite answer, but this article: https://en.wikipedia.org/wiki/Sea_level_rise explains that sea level is measured by satellites in Earth orbit. It is not explicitly stated, but I presume the deviations in this orbit from circular are known and thus such a measurement fixes the height of the sea relative to the centre of the Earth. This, after all, is the one fixed point in the system we can rely on.

    According to my information the rate of rise of the Himalayas (relative to the centre of the Earth, I presume) is >1cm/yr and the mean rate of rise of the sea is ~3mm/yr.

    But you are right. The articles I have seen on the web do not explicitly state what the rates of rise are measured relative to, so it is bit frustrating. Perhaps if a geologist reads this thread, he or she can confirm or explain further. Post 18 from Iceaura suggests it is not that simple.
     
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  3. Acitnoids Registered Senior Member

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    Thank you exchemist,

    I agree, it is not a simple question which is why I asked here.

    The center of the Earth seems like an ideal start point but that insures sea level is still the dominant factor.

    I appreciate your take, .... thx

    Maybe that start point could add to predictions but .... *shrug* ... as of now not accurate enough.

    Seriously, thx=> <=
     
    Last edited: Sep 18, 2018
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  5. iceaura Valued Senior Member

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    No. I'm saying there is more than one possible standard or reference, and to find out which one was used one should consult the published work.
    ? Not sure how I suggested that.
     
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  7. iceaura Valued Senior Member

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    At a given time and location.
    That is a good start, but mind the earth is not spherical and it is spinning. So there is no single distance from the level surface of the sea to a center point of the planet - no "the" planetary sea level in that sense. Then enter currents, the moon, various interactions of geography and spin, and so forth. (no small factors)

    Brief nod in pop sci: https://curiosity.com/topics/mount-...-the-tallest-mountain-in-the-world-curiosity/
    Note the size of the difference: Everest is more than 2500 meters taller measured from it's local virtual sea level, but it's peak is closer to the center of the planet.
     
    Last edited: Sep 18, 2018
  8. sweetpea Valued Senior Member

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    My mistake.
    flip
     
    Last edited: Sep 18, 2018
  9. Acitnoids Registered Senior Member

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    660
    Let me give an example of the O.P. and maybe you can tell me if my conclusion is accurate.

    https://www.nationalgeographic.com/environment/global-warming/sea-level-rise/

    This link from National Geographic states that the mean sea level has risen over the past 20 years at a rate of 3.2mm per year on average. We have already stated in this thread that the elevation of Mt Everest is rising by 3-4mm per year on average.

    Given these facts would I be wrong to conclude that the elevation of Mt Everest has essentially stayed to same over the past 20 years because of the rising mean sea level?
     
  10. Acitnoids Registered Senior Member

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    660
    Please, I beg you not consider this as a snarky comment. The title of this thread is called "Sea Level as a Point of Measurement". I was not asking about satellite data or the center of the Earth as a point of reference. I work with different points of reference everyday do to my occupation.

    Elevation is based on "sea level" (I thought "mean" was implied). With all do respect I didn't care about Altitude and what not regarding this thread. I am only concerned with "Sea Level as a Point of Measurement" and what that means as a definition.

    Thank you for trying to understand my POV.
     
    Last edited: Sep 19, 2018
  11. iceaura Valued Senior Member

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    "Mean" is not exactly what would be involved. Sea level is a complicated calculation, and it varies by location.
    Probably, yes. Depending on how those facts were calculated. Everest's "elevation" (not the same as height, necessarily) is measured - usually, but not inevitably, as my link pointed out - from a virtual sea level calculated for its latitude. The amount that particular sea level has risen is not likely to be the "mean sea level" rise of the planet as a whole.
     
  12. Acitnoids Registered Senior Member

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    660
    "Mean sea level" is just another way of saying "average sea level" yes?

    https://en.m.wikipedia.org/wiki/Sea_level

    This is an "Ideal sphere" that gives a constant water level which makes calculation easier. Or do we have a different definition for "mean sea level"?
     
    Last edited: Sep 19, 2018
  13. Acitnoids Registered Senior Member

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    660
    Last edited: Sep 19, 2018
  14. Write4U Valued Senior Member

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    It would seem a fairly stable standard and even on dry land, "sea-level" measurements do provide a relatively precise global "mean", IMO

    It is used in Geographic mapping a lot
    https://researchguides.library.wisc.edu/GIS

    And specifically in topology and topography, if I recall .

    https://viewer.nationalmap.gov/advanced-viewer/
     
    Last edited: Sep 19, 2018
  15. iceaura Valued Senior Member

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    No. Elevation above sea level (mean sea level) is above

    a calculated virtual sea level for that location,
    or the local sea and its level,
    or relative to some other surveyed point whose elevation was determined by one of those two.

    Quoting your link, first paragraph, key aspect highlighted by me:
     
  16. Write4U Valued Senior Member

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    I agree, it has nothing to do with ocean depth but with the physical tendency of water to seek gravitational leveling at the surface (sea-level).
     
  17. iceaura Valued Senior Member

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    The shape of that surface is called a "geode". If one wishes to measure Everest's absolute height from its carefully defined "sea level" to within 4mm, that is what one would use.
    https://en.wikipedia.org/wiki/Figure_of_the_Earth
    Note that comment about mass distribution: since massive amounts of rock have significant gravitational pull, the sea will be attracted to ("sideways"), pile up against, and attain a higher level near, features such as Greenland with its ice cap.

    Note that if one assumes the earth to be a sphere, and mean sea level to be some distance from the center of that sphere, Greenland would be entirely below sea level.
     
    Last edited: Sep 19, 2018
  18. Gawdzilla Sama Valued Senior Member

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    The last time I was in Panama City the tides raised and lowered the cruiser by nearly six feet. The dock was over our heads and then level with our main deck. The locals told me that "Sea level" there was at the point of highest tides. It's just a convention people agree on.
     
  19. iceaura Valued Senior Member

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    No. "Geoid".
    A geode is something else rattling around in the mental junk drawer.

    There is no single level of highest tide - that convention will not allow one to record the height (elevation) of a nearby mountain to within a few millimeters.
     
  20. Gawdzilla Sama Valued Senior Member

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    You've never seen "The Gate".
     
  21. RainbowSingularity Valued Senior Member

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    it is a subjective hyperthetical equilibrium of relative matter fields in varying correlative states.

    it is not a point in space because it only exists because of "relative matter fields in varying correlative states".


    i was about to give you a serious scientific answer(i was formulating a paragraph or 2 in my head) until i read your last 2 sentences.
    at which point i realised your a climate science denier & likely a trumpsterdiver

     
  22. Gawdzilla Sama Valued Senior Member

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    Acitnoids, how would you measure a six foot change in the height of Mt. Everest?
     
  23. Acitnoids Registered Senior Member

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    I didn't think I was publishing a thesis here heh. I guess I could have said:

    "An ideal circular-ish average that depicts a hypothetical surface tension between the atmosphere and global bodies of water while taking into account geographic and astrophysic influences none of which reflects reality because this is just a mean average after all yet this allows for easier calculations".

    I'm sorry but I thought saying "ideal sphere" placed in parentheses was much easier ☺
     
    Last edited: Sep 20, 2018

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