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How to Calculate True Altitude

True altitude, defined as a point's elevation above mean sea level (MSL), may seem like a piece of information that every pilot must know while in the air. Surprisingly, it's not -- no tool on an airplane measures it with spot-on accuracy, though a flight computer can closely approximate it. (It helps to know that architects define true altitude as the angle between the rays of the sun and a horizontal plane.)

Things You'll Need

Sun Calculator
Pencil
Grid Paper
Compass
Graphic Calculator (if desired)

Instructions

Calculate The Sun's Highest Position
- 1
  Using a sun calculator, plot your true sun angle versus Universal Time (GMT).
- 2
  With your compass, draw a curve through the plotted points. The high point of the curve gives the sun's highest position on this day and indicates when meridian passage occured.
- 3
  The highest angle of the sun on your plot indicates local noon. Make a note of it.
Correct For Refraction
- 4
  As you have calculated it, this position has been artificially inflated by atmospheric refraction. The atmosphere has bent the sun's rays in a way that makes them appear to have come from a higher altitude than is mathematically true.
- 5
  To counteract this inconsistency, estimate the amount of refraction currently affecting your measurement of high noon.
- 6
  Subtract the amount of the refraction correction from your highest measured solar altitude. This corrected figure is your true altitude.

Aviation