# Thread: How does an altimeter work?

1. ## How does an altimeter work?

I see all types of electronic gadgets (watches, gps, etc) now advertising electronic/barometric altimeter capabilities. I always thought the only way to find elevation (besides using a ruler) was by measuring the change in barometric pressure, which is another thing I don't totally understand because pressure is coninuously changing in the atmosphere. I think the barometric altimeter developed in the early 1900's (1920-1940??) by a pilot or someone in the aircraft industry. I also remember reading somewhere that you could read elevations by bouncing radio waves, maybe work in regard to aircraft elevation measurement, but not sure how well that would work if you were standing on top of a mountain. What else do they have and how does something as small as a watch have the components to read pressure? What does it take to read pressure, electronically speaking?

Just wondering if anyone can clear the air.
bnr

2. I think GPS units calculate altitude by triangulating from at least three satellites. My aviation Garmin is perfectly accurate and never requires adjusting to the current barometric pressure.

My old e-trex Summit has a barometric altimeter. It requires calibration every time you turn it on. It's worthless as far as altitude reporting goes.

3. ## Altitude for real!

ALtimeters work by measuring altitude based on sea level, configuration.
An altimeter does not actually measure altitude directly, but rather just atmospheric pressure. So an altimeter is actually a barometer created for a specific purpose. The altimeter uses the changes in atmospheric pressure to determine the changes in altitude. Why does pressure change with altitude? Simply put, the pressure at any given point in the atmosphere around the earth is a result of the weight of the atmosphere above it (pulled down by gravity). For this reason, the higher in the atmosphere you are, the less atmosphere you have above, and the less pressure exerted on you.

4. Barometric altimeters require setting to a known pressure. If you know your altitude the barometer will report the pressure. If you know the pressure the barometer will report the altitude. If you don't know one or the other your altimeter can't be set accurately.

5. Good question Buck, I've noticed my garmin fluctuating hundreds of feet from day to day. That explains it.

6. How does an altimeter work?

Some GPS units, Magellan, I believe, include barometric altimeters. Such altimeters are very accurate, since the atmospheric pressure changes with altitude, but they still require calibration. I imagine that Magellan and others must include an automatic electronic calibrator in their units. A ruler can only tell you pre-recorded altitude numbers on a map, but if you are using an old map, the altitude on it may be different than the one recorded on a new map. The reason for this is that terrain altitudes change with time. For example, some mountains keep on growing through the years, while others stop growing and get smaller.

You can read some altimeter details here:
http://www.thealtimeterstore.com/howtheywork.html
An altimeter does not actually measure altitude directly, but rather just atmospheric pressure. So an altimeter is actually a barometer created for a specific purpose. The altimeter uses the changes in atmospheric pressure to determine the changes in altitude. Why does pressure change with altitude? Simply put, the pressure at any given point in the atmosphere around the earth is a result of the weight of the atmosphere above it (pulled down by gravity). For this reason, the higher in the atmosphere you are, the less atmosphere you have above, and the less pressure exerted on you.
I also found some details on the altimeter used by Magellan:
http://gpsinformation.net/main/coloralt.txt

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