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  1. #1

    Default Mountain elevations

    This may sound dumb but what is standard for measuring the height of mountains above sea level? Is it the same for both coasts and if not, where is the dividing line?

  2. #2
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  3. #3

    Default Not sure of the question, but......

    .......I'll take a stab at it.

    Howdy neighbor (Foy from Raleigh here),

    Bob,

    While I'm familiar with the notion that there are overall elevation differences between sea levels in the Pacific basin and the Atlantic basin, I believe they're nominal. The locks along the Panama Canal are primarily for the purpose of following river canyons up one side of the dividing ridge and down the other, rather than to compensate for the sea level differences between Pacific and Atlantic.

    So let's assume there is but a nominal (well less than 1 meter?) difference between the level of the Atlantic and the Pacific from Baltimore to San Francisco. To the best of my knowledge and recollection from geology school, elevation contours are plotted onto USGS topographic maps from the datum of Mean Sea Level (MSL), where MSL is determined by international convention.

    I am not aware of any variances in the standards from one coast of the US to the other. Every feature carries elevations expressed as feet or meters above MSL. Throughout the US there are thousands of Benchmarks (BMs on USGS 7.5 minute topographic maps) where the precise point and its elevation above MSL have been surveyed by standard transit, rod, and chain techniques, and thus have a high level of engineering control. One can find the BMs on the ground, generally seen as a metallic disc attached to bedrock outcrops or placed on a concrete monument firmly embedded in stable soils. Plotting of elevations are then keyed to the BMs. Any specific point's elevation NOT backed up by a BM has been plotted via high-resolution airphoto interpretation where focus can be adjusted mechanically to depict lines of equal elevation across the field of a set of steroscopic air photographs, lines which are of course contour lines. The BMs, then, provide control for the airphoto plots.

    Is this what you're driving at?

    Foy

  4. #4
    Join Date
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    Tucson, AZ
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    Default "Sea Level"

    Without getting too technical, the standard for elevation above sea level is the same everywhere, but 'sea level' is not the same everywhere! That is, 'sea level' is not everywhere at the same distance from the center of the Earth. The definition of sea level is an equipotential surface in the Earth's gravitational field. Some first order differences in this surface from an equal distance from the Earth's center would be the poles vs. the equator. At the equators, the surface is spinning at about 1000 mph inducing a centrifugal force that counteracts gravity and raises sea level. At the poles, there is no spin and 'sea level' is lower (closer to the center of the Earth). Similarly, small local variations in gravity due to mass concentrations (mascons) will affect (very slightly) that equipotential surface. But the most obvious change in sea level due to changes in gravity occurs twice a day as the local gravity equipotential surface moves up and down in response to the gravity of the moon and sun - the tides - which is why the reference is usually made to mean (or average) sea level. And of course, sea level moves up and down everywhere as there is more or less water in the oceans. So, the sea level gravitational equipotential surface is by no means a perfect sphere or even a smooth surface.

    AZBuck

  5. #5

    Default Yep

    And thus the resident Earth and Planetary scientists of the RTA Forums have spoken. At least, those who ADMIT to being such scientists.........

    Talk about a moving target!

    Foy

  6. #6

    Default

    just as a comment there is about 20 cm difference in water level between the atlantic and pacific. if the locks on the panama canal were blown, immediately a new ocean current would move through from the pacific going into the atlantic. you can see the same thing happening at the southern tip of south america

  7. #7

    Default

    Quote Originally Posted by scotishbob View Post
    just as a comment there is about 20 cm difference in water level between the atlantic and pacific. if the locks on the panama canal were blown, immediately a new ocean current would move through from the pacific going into the atlantic. you can see the same thing happening at the southern tip of south america
    Good stuff, Bob. I couldn't remember the figure on the average difference between MSL but thought it was pretty modest.

    I just looked at Gatun Lake's Wikipedia entry. It's 85' above sea level and carries Panama Canal shipping for 20 of the canal's 48 miles. The Gatun locks raise/lower ships the entire 85' on the Atlantic side, while the Miraflores and Pedro Miguel locks on the Pacific end account for 54' and 31', respectively.

    With the operative channel depth being 50', I wonder if removal of the locks would in fact allow free flow from the Pacific to the Atlantic. Seems as though the bottom elevation through the Gaillard Cut would still be around 35' above sea level.

    There must be some awesome currents at the tip of South America!

    Foy

  8. #8
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    Default

    With the operative channel depth being 50', I wonder if removal of the locks would in fact allow free flow from the Pacific to the Atlantic. Seems as though the bottom elevation through the Gaillard Cut would still be around 35' above sea level.
    Nope, but it sure would drain most of Gatun Lake!

    There must be some awesome currents at the tip of South America!
    From what I can find, they are 5 to 7 knots.

  9. #9
    Join Date
    Jan 1998
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    Default I love this site

    What a cool thread -- at some level I knew much of this -- but what a joy to have my somewhat stale memory banks shaken a bit. Pretty fascinating stuff.

    Mark

  10. #10

    Default Oh yeah, it'd drain ALL of Gatun Lake

    Quote Originally Posted by glc View Post
    Nope, but it sure would drain most of Gatun Lake!



    From what I can find, they are 5 to 7 knots.
    Gatun Lake was formed by damming the Chagres River at a point some 8 miles from its mouth on the Atlantic side. It flooded the river and its tributaries back to the foot of the dividing ridge, where the Gaillard Cut (Culebra Cut) drove through the ridge to the upper end of manmade Lake Miraflores on the Pacific side of the divide. It's unlikely any construction-related excavation altered the underlying topography of the Chagres River valley in such a way to prevent a complete emptying of the lake if the locks and dam were removed. Might empty Miraflores, too, for that matter, at least down to the 35' above sea level contour.

    A 5 to 7 knot current (5.75 to 8.05 mph) with the wind opposite the current creates what we call a "washing machine". Don't want any part of that in a small vessel!

    Foy

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