Ask A Scientist General Science Archive

Question:
In Earth Science (9th Grade), we are learning about
contour maps. We were given a scale model of a
volcanic mountain in New Mexico.
To make a contour map, we immersed the mountain in
water and recorded where it reached on the model
(for each height of water).
Obviously, this is not how actual contour maps are
made.
How is the height above sea level measured in reality?
I am interested in both historical ways this was measured and
new technological ways.
      Waiting anxiously for an answer,
                                   Kalman Strauss

Answer:
Airplanes use devices called Pitot-Static tubes (pronounced PEE-TOE-
Static), which measure the static and dynamic air density.
The static air density is measured at a spot on the airplane where
there is relatively dead air.  The dynamic density is measured where
the air is rapidly passing by.  Actually, I think P-T S tubes
measure pressure and then calculate the density.  At any rate,
air density decreases the higher you go up.  So, once the density is
measured in the air, or at the plane, it can be compared to a reference 
point on the goround.  The height can then be calculated above the ground
or above sea level.
 
It can also be done using satellites I believe.  I'm not familiar on
the exact details, but I think radar is the basic idea.  A beam of
radiation is sent to the Earth, and bounces back. The time it takes
to bounce back and the orbit height of the satellite tells you the 
elevation at that point on the Earth.
 
I'm sure it can get more complicated than that.  Hope this helped some.
 wildman jackson