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Thread: How can one determine an altitude of a location?

  1. #1
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    How can one determine an altitude of a location?

    Say I want to determine the altitude of a specific place by going there(ie not using maps) accurately, how could I do that?

    Can GPS on some smart phones do it?
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    GPS is what people commonly use these days, but its vertical error is higher than its horizontal error, so you can be off by tens of metres if you get a bad constellation of GPS satellites. You also need to be sure that your GPS is using your local vertical datum, otherwise you get a geoid error, too (ie, the GPS gives you your height above a reference level you're not interested in). You can download GPS elevation apps for smartphones, but I've no experience of them. I use a dedicated Garmin unit that's waterproof and shockproof and keeps its maps onboard rather than needing a phone signal.

    Differential GPS adds a ground station signal, and in the right hands gets your vertical error down to tens of centimetres.

    I regularly use an altimeter for navigation in the hills, but I carry a barometric altimeter, which I find is usually (but not always) more accurate than GPS over relatively short distances and times (on the scale of a few miles and a couple of hours). I reset it each time I pass over a point of known height. A fast-moving weather system will throw it off, though. And it's useless if you don't calibrate it, which many people seem to forget to do.

    Another common option, which may happen in the background with some GPS units, is that they try to improve vertical accuracy using a terrain map (onboard or downloaded), commonly prepared from satellite altimetry. They match your horizontal location to a grid of elevation data, and interpolate to give you a height. They can go badly wrong if your horizontal position is a little off (for instance if you're surrounded by trees or buildings on sloping ground).

    Grant Hutchison

  3. #3
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    Yes, GPS does it, and thus smartphones can do it. You just need an appropriate app.

    If you know the current air pressure at the surface, you can use air pressure to determine your current altitude. That's how it's been done traditionally in aircraft.

    ETA: But, of course, Grant beat me to it.
    Selden

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    Quote Originally Posted by selden View Post
    If you know the current air pressure at the surface, you can use air pressure to determine your current altitude. That's how it's been done traditionally in aircraft.
    Yes, it's very effective in aircraft, because they use what's effectively "differential barometry" at takeoff and landing - the airport tells you the exact barometric pressure at that location.
    Less good for terrain avoidance in bad weather, though, since a falling barometric pressure will make your altimeter think you're higher than you actually are. Hence the old pilot's adage: "High to Low, beware below."

    Grant Hutchison

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    Siri, what is my altitude?

    I can't comment on the accuracy of the answer you get.


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    Quote Originally Posted by Extravoice View Post
    Siri, what is my altitude?

    I can't comment on the accuracy of the answer you get.
    I'm reminded of the story of a question on a physics exam, asking how to determine the height of a building using a barometer. Of course there's the answer the professor is looking for, looking at the difference in pressure between the top and bottom, and using that to computer the height, but there are a host of other ways, including throwing the barometer off the building and timing how long it takes to hit the bottom, and my personal favorite, going to the building superintendent and saying, "I'll give you this nice barometer if you'll tell me how tall your building is".
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    Quote Originally Posted by grant hutchison View Post
    Yes, it's very effective in aircraft, because they use what's effectively "differential barometry" at takeoff and landing - the airport tells you the exact barometric pressure at that location.
    Less good for terrain avoidance in bad weather, though, since a falling barometric pressure will make your altimeter think you're higher than you actually are. Hence the old pilot's adage: "High to Low, beware below."

    Grant Hutchison
    Related to "Beware of flying into clouds. They are stuffed with rocks."

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    Quote Originally Posted by swampyankee View Post
    Related to "Beware of flying into clouds. They are stuffed with rocks."
    During the Burma Campaign, the RAF had to fly ground support during the monsoon. My father's flight found they were blocked from returning to their base by a wall of thunderclouds that spanned from one horizon to the other. Their leader took them into the clouds. My father then spent quite a long time being violently rattled around with his artificial horizon gyro toppled, before he was spat out of the base of the cloud, to find himself flying down a valley, partially inverted, with mountains rising into the sky on either side.
    (He had three near-death experiences during the Burma campaign, and none of them involved the Japanese.)

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    Quote Originally Posted by grant hutchison View Post
    Yes, it's very effective in aircraft, because they use what's effectively "differential barometry" at takeoff and landing - the airport tells you the exact barometric pressure at that location.
    Less good for terrain avoidance in bad weather, though, since a falling barometric pressure will make your altimeter think you're higher than you actually are. Hence the old pilot's adage: "High to Low, beware below."

    Grant Hutchison
    Yes, whenever I contacted a flight information centre (or Flight Service), the first bit of information they always provided was the local altimeter setting. Years ago, they prefaced it with "Kollsman", eg they'd announce "Kollsman altimeter two-niner-niner-two". I'm assuming that this was meant to be an explicit reference to the Kollsman window on the altimeter.

    When I moved to another part of the province, where there was no local information service, I kept the plane on the river. I'd always set the altimeter to read the river elevation before commencing a flight.

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    Quote Originally Posted by grant hutchison View Post
    I regularly use an altimeter for navigation in the hills, but I carry a barometric altimeter, which I find is usually (but not always) more accurate than GPS over relatively short distances and times (on the scale of a few miles and a couple of hours). I reset it each time I pass over a point of known height. A fast-moving weather system will throw it off, though. And it's useless if you don't calibrate it, which many people seem to forget to do.
    I used to use an altimeter when I was doing reconnaissance work in coastal BC. I was usually working right off the coastline, so every time my exploration brought me back to sea level, it was reset to zero. I sometimes kept note of the time of resets as well as time of observation to see how much it could be off by, but this was more out of curiosity than a genuine need for accuracy.

    Unfortunately for one of those altimeters, it was in the pocket of my vest that I set on the bench in the boat at the end of the day. The vest slid off the bench into the salt water in the bottom of the boat, and that was the end of that instrument.

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    Quote Originally Posted by Torsten View Post
    Yes, whenever I contacted a flight information centre (or Flight Service), the first bit of information they always provided was the local altimeter setting. Years ago, they prefaced it with "Kollsman", eg they'd announce "Kollsman altimeter two-niner-niner-two". I'm assuming that this was meant to be an explicit reference to the Kollsman window on the altimeter.
    My recollection is that we used QFE for take-off and landing, which was the barometric pressure at runway altitude, making the runway match altimeter zero. Then we used QNH for cruising (barometric pressure at sea level), which gave you an altitude setting that matched the charts. There were regional QNH settings, updated regularly as the weather changed, and everyone cruising in that area used the current regional QNH, to maintain flight level separation.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    During the Burma Campaign, the RAF had to fly ground support during the monsoon. My father's flight found they were blocked from returning to their base by a wall of thunderclouds that spanned from one horizon to the other.
    Nothing to do with altitude, but my dad was in Burma with the US Army. He was in the Signal Corp and ran a crypto-machine. I wonder if your dad gave him a lift.
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    Quote Originally Posted by grant hutchison View Post
    My recollection is that we used QFE for take-off and landing, which was the barometric pressure at runway altitude, making the runway match altimeter zero. Then we used QNH for cruising (barometric pressure at sea level), which gave you an altitude setting that matched the charts. There were regional QNH settings, updated regularly as the weather changed, and everyone cruising in that area used the current regional QNH, to maintain flight level separation.

    Grant Hutchison
    I had to look up those "Q" codes.

    I don't recall regional settings, and I certainly don't know what the current practice is here. But if I recall correctly, if flying above a certain altitude, possibly within Class B airspace (between 12,500 and 18,000 ft), or at the flight levels (18,000 ft and above), then a setting of 29.92 (inches Hg) was universally used.

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    Oh, and to answer the OP, there are ways of doing it by surveying it directly. One of the tasks I routinely did earlier in my career was known as a "deflection line". A straight line up a slope was traversed, with measurements of slope distance and slope inclination measured at intervals. We used simple tools; just a nylon "chain" with marks every metre, and an inclinometer, such as a Suunto. The observations were reduced into their horizontal and vertical components, and then plotted (usually right there in the field) to get a sense of the shape of the slope. The method yields a very good estimate of the relative difference in elevation.

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    Quote Originally Posted by Swift View Post
    Nothing to do with altitude, but my dad was in Burma with the US Army. He was in the Signal Corp and ran a crypto-machine. I wonder if your dad gave him a lift.
    My father was in fighters with 135 Sq. RAF, 1944-45. A little unusually for the RAF, he was flying P-47 Thunderbolts, in their "Razorback" incarnation (which the RAF called the "Thunderbolt I").

    Grant Hutchison

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    Quote Originally Posted by Torsten View Post
    I had to look up those "Q" codes.

    I don't recall regional settings, and I certainly don't know what the current practice is here. But if I recall correctly, if flying above a certain altitude, possibly within Class B airspace (between 12,500 and 18,000 ft), or at the flight levels (18,000 ft and above), then a setting of 29.92 (inches Hg) was universally used.
    Ah. That's QNE. If you're not in danger of fouling the terrain, then it makes sense for everyone, everywhere, to fly as if they're in a standard atmosphere, and not worry about their actual relationship to sea level. This has obvious disadvantages if you're down where the rocks are, so the regional QNH allows everyone to consistently compare their altimeter to the charts.
    To link back to the OP, what I'm doing with my altimeter in the mountains is implementing my own personal QNH, using map heights for reference. (And when I camp at night, I can use my altimeter as a barometer. It's always a source of depression to wake up in the morning and find that the tent's altitude has increased markedly overnight. If you'll pardon the pun.)

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    My father was in fighters with 135 Sq. RAF, 1944-45. A little unusually for the RAF, he was flying P-47 Thunderbolts, in their "Razorback" incarnation (which the RAF called the "Thunderbolt I").

    Grant Hutchison
    My father was in Lancasters as Navigator and one sortie number 12 I think, his pilot made a barometer error and ended up away from the squadron and the mission was not allowed, so the crew had to redo 12 as 13. With the high loss rate the crew were not amused but they went on to do the (I think it was) 40 missions, 41 actually.
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    Quote Originally Posted by profloater View Post
    My father was in Lancasters as Navigator and one sortie number 12 I think, his pilot made a barometer error and ended up away from the squadron and the mission was not allowed, so the crew had to redo 12 as 13. With the high loss rate the crew were not amused but they went on to do the (I think it was) 40 missions, 41 actually.
    The rate was so high I remember when they finally declassified it back in the '70's. It came out in Air & Space, or Proceedings, I'm not sure which one I read it in. The bombing campaign against Germany cost allied aircrews 47,000 bomber crew men killed in action over 6 years.

    One of the things I remember in a post war analysis that struck a cord in me as someone who was raised on "Hogan's Heroes" was that bombing ball bearing factories is a completely futile undertaking. Oh sure it sound s important, but ball bearings are too easy to make as are the facilities to make them.
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    There was an incident in the Atlantic during World War II in which the altimeter on a carrier-based antisubmarine plane was maladjusted 1,000 feet high on a night mission. They came down to what was supposed to be 1,000 feet above the water to start hunting u-boats and smacked the water about 10 miles from the carrier. Fortunately the crew got out alive and the radar operator was able to coach a destroyer into position to rescue them.

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    Quote Originally Posted by Hornblower View Post
    There was an incident in the Atlantic during World War II in which the altimeter on a carrier-based antisubmarine plane was maladjusted 1,000 feet high on a night mission. They came down to what was supposed to be 1,000 feet above the water to start hunting u-boats and smacked the water about 10 miles from the carrier. Fortunately the crew got out alive and the radar operator was able to coach a destroyer into position to rescue them.
    On the other hand...

    Whenever we went to Nellis Airforce Base in Nevada, from our usual base in then NAS Miramar, we awaited the usual gripe from the newby pilots.

    "Altimeter showing 5,000 feet on ground." You never wanted to be "that guy" as not only other aircrews, but the enlisted as well were allowed to "make comment" on it.
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