With what kind of instrument does one measure altitude (distance above sea level)? And in the case of mountains, is it necessary to actually be on the summit in order to measure it, or is some sort of triangulation used?
Not surprisingly, an altimeter is the instrument used to measure altitude.
Assuming the question is more along the lines of “how do you measure altitude”?, then there’s a number of ways.
Measure the air pressure, which varies in a predictable way with altitude, although this is only approximate.
If you’re in a plane, you can use a range finder (laser or radar) to determine height above ground or water level.
As for the mountain question, you can use optical means to determine the distance and elevation of a distant point to determine its height relative to yours.
D’oh! I forgot GPS.
Altimeter, which uses barometric pressure, you gotta be there though. Radar, which is accurate but requires an over-flight. Triangulation is how it was done in the old days, this is how Mt. Everest was first surveyed by the British.
It’s worth noting that a standard altimeter is pretty much the same thing as a barometer. It measures the height of the column of air above you. Placed in a fixed spot (say, the top of a mountain) the altitude it reports will vary with changes in weather.
These days, another useful way to measure altitude is with a GPS receiver. For various reasons GPS altitude is not as accurate as horizontal position (lat/lon) but it is pretty good - often within 50 feet. If you leave a receiver in one spot for a while, you can average the readings for improved accuracy. But you would have to take the receiver to the point to be measured.
Trigonometry has been used to determine the height of something, thus altitude, for some time now.
I have used a altimeter, but it’s a PITA, snce air pressure changes can easily put you 500 ft off very easily and quickly.
I now use a gps, the accuracy is reported as being somethign like the 500 ft, but in practice, I find it to be within 50 ft or so, many times within 20 ft. THese altitudes are from known points, like benchmarks, highway signs (altitued 1245 ft, highest point on rt ???, and being close to sea level).
An altimeter as noted above is subject to changes in air pressure.
It is therefore calibrated at a ‘station’ of known altitude and will be reasonably accurate for a matter of hours or days depending on changes in weather conditions, i.e. barometric pressure.
The US Geological Survey Maps, if still available, will give the altitude of any location. TVA at one time had some available for their areas of operations.
GPS is probably the way to go for maximum convenience!
A GPS system or even an altimeter would be expensive. You can get a protractor for about 50 cents, a plumb bob can be made from string and any wieght, and a calculator with trig functions less than ten dollars. Then you just need to know the altitude of some nearby point.
A GPS system or even an altimeter would be expensive. You can get a protractor for about 50 cents, a plumb bob can be made from string and any wieght, and a calculator with trig functions less than ten dollars. Then you just need to know the altitude of some nearby point.
Thomas Twp: But how were they measured?
Reginald Anson: The same way, by comparing them with other hills.
Thomas Twp Too: But who measured the first hill?
Rev. Robert Jones: [whispering] God. God, my boy. God.
(from The Englishman who Went Up a Hill but Came Down a Mountain )
Survey quality GPS units routinely get 1 in 3 million horz. and 1/100 of a ft. vert.
Possibly an urban myth but I was told by a varsity graduate that the question once appeared on a Cambridge University Entrance Examination along the lines of, “Outline three ways in which, if given an barometic altimeter, you would propose to measure the height of the tallest building on the college grounds.”
This got from one candidate the obvious answer involving measuring the air pressure at the base and top and calculating the weight of the column of air necessary to account for the difference. For the second method he proposed dropping the altimeter from the roof of the building and timing with his wrist watch the time taken to hit the ground.
But my favourite was his proposal to find the warden of the college and offer to give him a fine barometer if only he would tell him the height of the tallest building in college!
I understand he got in.