Global Sea Level

[Terry]

Help me out, fellow pilots.

True Altitude = Height above sea level

Absolute altitude = Height above ground

Pressure altitude = Height above sea level according to barometric pressure or datum plane.

Does True altitude take into account the mean sea level?

If it does, how do they measure this or arrive at this mean sea level?

The picture, Figure 1-5, of Bob Gardner's The Complete Advanced Pilot, shows the datum plane slightly below the sea level.

Thanks,
Terry
ASEL-IR

http://oceanservice.noaa.gov/facts/globalsl.html is the referenced article.

 

[comanchepilot]

What it means is that you never really know how high you are - and you never really know what MSL is - and if you use GPS, you are even more confused.

 

[write-stuff]

The datum plane is an average. Since the seas bulge from tidal forces and wind, it is not perfectly spherical. Sea level is a local measurment. That's why we use mean sea level (MSL) when describing altitudes. I suspect this is why Bob's book shows it slightly below "sea level".

 

[Terry]

What it means is that you never really know how high you are - and you never really know what MSL is - and if you use GPS, you are even more confused.

I love it! Great answer.

The reason I went down this road is because I am taking my flight review Thursday and was going over altitudes in my FAR/AIM.

Then I got to thinking that the seas were different levels and was curious if this affected orbital and space calculations.

Terry
ASEL-IR

 

[Capt. Geoffrey Thorpe]

Somewhere on your sectional the datum for latitude / longitude is listed. The specific datum for elevation is not, but you can find it via google.

 

[MAKG1]

Then I got to thinking that the seas were different levels and was curious if this affected orbital and space calculations.

It sure does. There are a bunch of datums; GPS (WGS84) is excessively simplified for precision measurements (though it's fine for the scales it was intended for). That's an oblate spheroid. The real geoid is a whole lot bumpier than that. There are no really good global datums. Either they are global or they are good. Or maybe neither.

Higher order orbital terms are affected by bumpiness in the geoid. The leading term is not. It's less of an issue for very high (such as geosynchronous) orbits than it is for low earth orbit. But it even affects the moon -- that's why we always see the same face.