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The purpose of the altimeter is to show the pilot the altitude and the pressure above a fixed level. It uses barometric pressure and is 29.92Hg in ISA conditions.

What I dont understand is, how can this be extrapolated to be used in a situation where there is an elevation. For example, say the airplane is in a static position in an airfield at code EGNX, 52°49′52″N 001°19′40″W that has a 10 foot elevation. What would the altimeter indicate in such a situation.

Edit: Added a starting position

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An altimeter is just an adjustable barometer with extra hands.

There is a barometric correction you dial in so that the altitude shown is always feet above sea level. The correction is from the current barometric pressure reference you get with the "altimeter setting" or the weather report.

You dial in the altimeter setting, the current barometric pressure reference for your location, and shows your true height above sea level. In your example, if the altimeter setting is correct, it will show 10 ft above 0 ft.

Or, you can work it the other way. If you know the elevation you are at, you can dial the altimeter so its altitude indication matches your known elevation. Voila!; the current barometric pressure is shown in the setting window on the instrument's face.

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  • $\begingroup$ Could this also work if I'm trying to use DMS to and an elevation to figure out the altimeter reading? $\endgroup$ – SillySquishy Dec 13 '19 at 21:17
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The altimeter, when properly set, shows the height above sea level.

When you're on the ground at Denver International Airport, at 5,434 feet above sea level, your altimeter will show 5,434 on the ground.

When you're on the ground at San Francisco, nearly at sea level, your altimeter will show 13 feet.

Specific notes:

"when properly set" means that it has to be set according to the local barometer setting. If the pilot has set the incorrect barometer setting, it will show the wrong altitude.

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    $\begingroup$ 'when properly set' - the altimeter will only show height above sea-level when set to the area QNH. If properly set to the QFE it will show height above the airfield. If properly set to 1013.2hPa (29.92"" Hg) it will act as a reference for flight level. $\endgroup$ – CatchAsCatchCan Dec 13 '19 at 21:58
  • $\begingroup$ If the initial airfield location is given in DMS form and then the elevation of 10 feet is mentioned. How would the result of the altimeter be calculated. $\endgroup$ – SillySquishy Dec 13 '19 at 22:11
  • $\begingroup$ The altimeter doesn't know anything about location, whatever form you give it in. It's not a GPS receiver. $\endgroup$ – CatchAsCatchCan Dec 13 '19 at 22:35
  • $\begingroup$ Okay in that case, I am really confused as to how to go about solving this question. I will enter the inputs that I am given; Departure: code EGAC, Arrival: code EGNX, the planned flight time is 40 minutes, the total flight distance is approximately 200NM, the cruise flight level is 29,000 feet, the cruise speed is at M0.73. The question states that the flight will take off from the airport code EGAC, and the airport elevation is 150 feet. What does the altimeter currently indicate? @CatchAsCatchCan $\endgroup$ – SillySquishy Dec 13 '19 at 22:41
  • $\begingroup$ @SillySquishy So, this isn't a question about altimeters, but a question about procedure. Strictly speaking, you can't answer the question because you haven't been given the area QNH or the actual altimeter setting. However, as a matter of procedure, ATC will give you the area QNH prior to departure.With that set on your altimeter you'll get height above sea level, which, for your question is 150ft. $\endgroup$ – CatchAsCatchCan Dec 13 '19 at 22:53

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