Global Mean Sea Level?


(Timd1971) #1

Curious, is there a point (or points) of the earth that is the basis of Global Mean Sea Level (GMSL)?

I assume that is the median of the highest point of the earth, Mount Everest at 29,029’ and lowest, Challenger Deep at -35,994.1’?

But I guess this get’s even more complicated when factoring in gravity, weather, shifting of the oceans’ waves and terrain, etc. Geoid. So would GMSL be the median of highest point and lowest point on earth or more to it?

Geoid is the basis or zero elevation from what I understand.

So I guess I may or may not be answering my own question? Geoid is the basis of GMSL?

If so, what parts of the earth are at this location? Is there a global elevation Benchmark of the earth then for Orthometric Height for construction? Kind of like Greenwich, London is to time? (GMT).


(Michael Lambert) #2

Mean Sea Level is the mean (average) of low and high tides. Personally I think it is useless except for the fact that our original U.S. vertical datum was based on it way back when. Beyond that the only other place I have been forced to use it is when navigating VRF maps because the FAA thinks it is important for a drone pilot to know… :rofl:


(Timd1971) #3

Seem to agree since based on something that is variable versus stationary? Maybe better to use Mount Everest as the Global Benchmark and just have to climb each time and set your base with that known point? Ha!!! :rofl:

What would you consider to be a better standard for vertical elevation reference other that WGS84 ellipsoid height?


(Michael Lambert) #4

Removed by poster…


#5

It could be very important if your fixed-wing drone was on floats for landing gear!


http://www.barnardmicrosystems.com/UAV/milestones/sea_plane.html


(Michael Lambert) #6

Haha! I think I will stick to land… No :merman: here.


(Timd1971) #7

I think the “exaggerated” model of the earth geoid is so messed up. : /
Initially, I think when "most’ see this 3D model, they immediately think the earth really is “shaped” like this in disbelief, when it is NOT, as it is an exaggeration. HIGHLY EXAGGERATED!. It is very misleading.

In reality, if this model was to scale, the elevations would hardly be noticeable. Therefore the exaggeration.

https://oceanservice.noaa.gov/facts/geoid.html

Great explantion:


(Michael Lambert) #8

That guy has some very good videos. He points out that even a globe like we use to have back in school is highly exaggerated so you can tell where the mountains are. Reality is that the GEOID visual is highly exaggerated, but the GEOID itself is pretty much average in comparison to the real terrain. As seen in the exaggerated models the surface still goes above and below the GEOID.


(Dave Pitman) #9

Not the subject, but… That’s because the FAA has nothing to do with surveying. Elevation at take-off point for a zero reference, like used in drones to convey elevation, is meaningless when traveling very far. The FAA went with what it uses for manned aviation. With manned aviation, it’s more than just how high you are above the surface, which is important for sure. But also, how high you are absolutely due to it’s effect on aircraft performance. We don’t need centimeter level accuracy there, though!


(Michael Lambert) #10

My point exactly and that’s why I separated it in my statement. A simple example of the uselessness of MSL in relation to pretty much anything that we’d normally discuss on these forums.