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Thread: Altitude Readings - GPS or Altimeter (barometer) ?

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    Last edited by Thala Dan; 15-08-19 at 01:23 PM.



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    Quote Originally Posted by ol' boy View Post
    Question, what are the 2 barometric pressure readings mean either side of the height?
    Or why is there 2?
    I think the answer is in the specs you provided in your next post:

    Here are the main features :

    - 3 hands analog altimeter (Hundreds, thousands & tens of thousands).
    - Sensor "Autostart" feature (see preferences).
    - Automatic sensor stop on exit.
    - Preferences save on exit (including calibration data).
    - 2 altitudes providers : Barometric sensor(*) or GPS chip (no network).
    - Robust Barometric Algorithm.
    - Units are feet or meters for altitude and hPa or inHg for pressure (*).
    - Displays QFE / Altitude / QNH (*).
    - Altitude speech.
    - Calibration function(*) with altitude or pressure (QNH) including a reset function.
    - User interface in English, French, German (Thanks Peter), Italian (Thanks Cosimo !).
    - Free application... no ad !
    And here's the answer to your next question

    What is QNH and QFE?

    QNH is sea-level pressure. It’s used to cause the altimeter to register height above sea level. When sitting on the ground at an airport, dialing QNH into the altimeter will cause it to display the airport’s altitude above sea level.

    QFE is air pressure at the current ground level. It’s used to cause the altimeter to register height above the ground (for a certain area). When sitting on the ground at an airport, dialing QFE into the altimeter will cause it to display zero feet.

    Western countries largely standard on using QNH during takeoff and landing. It allows you to accurately know your height above sea level, but you must know the height of the local terrain to know if you are at risk of impacting terrain. Eastern countries have sometimes standardized on using QFE, which allows you to know your height above the airport by just reading the altimeter.

    This should hopefully make the risk obvious. Imagine you’re landing at an airport that is 300 feet above sea level. You've dialed in QNH thinking that you dialed in QFE. Your altimeter reads 400 feet. Because you think you dialed in QFE, you believe you’re 400 feet above the ground, but since you actually dialed in QNH, you’re only 100 feet above the ground, and at risk of impacting obstacles.



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  • #123
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    Quote Originally Posted by ol' boy View Post
    Not sure what the "1285" number represents, its stamped over the tower in the above screenshot
    The address


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    I think this was already posted??? Can't remember

    If u want to go on an expedition get a Land Rover, if u want to come home from an expedition get a Landcruiser!

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    Some words to go with the picture, OB.



    Haven't read it yet, but it looks to be very informative.

    Some people take their hobby pretty damn seriously, don't they.

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    Yup, i was fried days ago....
    Pool Cue is back in the rack

    I've concluded there is 2 ways to have a multiple guess at your elevation
    If u want to go on an expedition get a Land Rover, if u want to come home from an expedition get a Landcruiser!

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    Quote Originally Posted by eeprommemory View Post
    WTF those orange markers are supposed to be accurate.
    is there a reflector strapped to the edge of a cliff ???.
    the 1285 is the target number for the bombing order during the Chinese invasion.
    Ok, i have just returned from site.... And your comment about the Chinese couldn't be more correct.

    First off, i scoured the area and found no Datum point or height marker or radar reflector, nothing..... Not even a courteous Parks Vic sign.
    Just a comms tower, chain mesh fence and the Global Warming Datum point.

    Next, Mt Cowley elevation.... i took some readings (GPS based)








    Using GPS we have 3 different heights
    667m, 658m, 652m for an average of 659m

    Using the Internet, we have 3 different heights
    654m, 689m, 660m for an average of 667.6m

    654M
    689M
    660M

    Would have been nice to have a Barometer based device up there, but i guess it would be around the same reading
    And as there is no actual way of telling which reading is correct, they all win


    And lastly, your ode to the Chinese were on the money good sir
    Missile Guidance system being installed this week
    Spyware being installed next week



    Oh yeah, that's the Global Warming Ocean Height datum thingy.

    Last edited by ol' boy; 19-08-19 at 10:24 AM.
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  • #128
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    Quote Originally Posted by ol' boy View Post
    Ok, i have just returned from site.... And your comment about the Chinese couldn't be more correct.

    Using GPS we have 3 different heights
    667m, 658m, 652m for an average of 659m


    Would have been nice to have a Barometer based device up there, but i guess it would be around the same reading
    And as there is no actual way of telling which reading is correct, they all win

    The pressure sensor in my Chinese spy phone(with the app I suggested) would have given you the precise height down to 30cm if you had a mate down the bottom to radio/phone you any change of the barometric pressure since your last calibration.
    As I have WiFi/Data off, the Chinese would have never known
    Last edited by Uncle Fester; 19-08-19 at 11:23 AM.
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    Quote Originally Posted by Uncle Fester View Post
    The pressure sensor in my Chinese spy phone(with the app I suggested) would have given you the precise height down to 30cm if you had a mate down the bottom to radio/phone you any change of the barometric pressure since your last calibration.
    Now that i'd like to see, before i believe
    And once again, there would be no way of proving the result, its just another result
    At least it might be a stable result for the time you were there (a 20 minute window at least)
    Last edited by ol' boy; 19-08-19 at 11:35 AM.
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    Quote Originally Posted by ol' boy View Post
    Now that i'd like to see, before i believe
    And once again, there would be no way of proving the result, its just another result
    At least it might be a stable result for the time you were there (a 20 minute window at least)
    That is why you need to include any weather related barometric changes that the mate below can record.

    It is accurate. I regularly do the walk down to the beach (500m line of sight) and get precisely the same results if I go down to a certain rock I use as reference that is barely submersed at mid tide.

    Those Internet results are rubbish, unless you are fine with a +/-20m deviation.
    GPS, at least what I have on the phone, is rubbish too.

    My little drone does a good job but it is only allowed up to 120m. I think it uses IR sensors or Laser for ground proximity though.
    Last edited by Uncle Fester; 19-08-19 at 12:05 PM.
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    Quote Originally Posted by Uncle Fester View Post
    It is accurate. I regularly do the walk down to the beach (500m line of sight) and get precisely the same results if I go down to a certain rock I use as reference that is barely submersed at mid tide.
    So, without any calibration, you get the same result at this rock, day in, day out.... around 1 or 2m i guess if you're holding it?
    If u want to go on an expedition get a Land Rover, if u want to come home from an expedition get a Landcruiser!

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    Three observations.....and I'm not sure what they all mean....other than to possibly to confuse the issue

    1. On the trip to Bright on Friday, I had the Oregon and the eTrex running in the cabin of the campervan.
    One thing was different from the August 13 test.....I had both units set to GPS+GLONASS.
    A check at Bright parked down by the river revealed 317m on both units.....according to the web, Bright is 319m (but no indication of the location of that reading) .
    Moved the vehicle up into the shopping centre....elevation shown on both units 323m.....which would pretty much represent the change in elevation from down by the river.
    Arrived back home.....both units showing 271m
    The real standout of this non-scientific test was that both units were tracking elevation like siamese twins....a different result to my test on August 13.

    2. Yesterday we had a weather front go through......between 1:18 pm and 6:18 pm barometric pressure dropped below 980hPa and bottomed at 973 hPa at 4:48pm.
    That minimum pressure is a full 20+ hPA below the barometric pressure during my test on August 13 (which ranged 994-996).
    By rights, the barometric altimeter on the Oregon 600 should have shown, based on that lower barometric pressure, my Elevation as 323m.....but it didn't......it still showed between 267 and 271.
    Bear in mind, I had no way of inputting a reduced sea-level baseline for it to work on.
    So, something strange is happening in that barometric altimeter....I'm suspecting that it gets a little helping hand from its GPS mate.

    3. I've currently had the eTrex out on the verandah for almost three hours this morning.....set to GPS+GLONASS.
    A check of elevation every 15-20 minutes has revealed a consistent reading of 266-269m.....with one outlier at 270m.
    So far this has been a tighter range than achieved on August 13 with the unit set to GPS only.
    GPS accuracy is showing as 2m.....the best of I've ever seen on this unit.....so there definitely something to this GPS+GLONASS.

    Might be interesting to redo your Cowley test one day with GPS+GLONASS set on that app....if it has that facility.

    And OB........I hope you smiled while you were up on Cowley.......your photo is probably all over Wechat by now

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  • #133
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    Quote Originally Posted by ol' boy View Post
    So, without any calibration, you get the same result at this rock, day in, day out.... around 1 or 2m i guess if you're holding it?
    I calibrate to zero when I am standing on the rock while holding the phone exactly at my shirt pocket and when I come home I read the value when I reach my portico.
    Another day I calibrate to zero when I stand on the portico and when I reach the rock I get the near same value but negative, +/-2 foot not metres.
    I don't walk to the beach, when it is cold and windy.

    A difference of 1 ft shows when I hold it at my waist instead of my shirt pocket. Takes around 3 seconds to respond and stabilise.
    Last edited by Uncle Fester; 19-08-19 at 01:16 PM.
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    Quote Originally Posted by Uncle Fester View Post
    I calibrate to zero when I am standing on the rock while holding the phone exactly at my shirt pocket and when I come home I read the value when I reach my portico.
    Another day I calibrate to zero when I stand on the portico and when I reach the rock I get the near same value but negative, +/-2 foot not metres.
    I don't walk to the beach, when it is cold and windy.

    A difference of 1 ft shows when I hold it at my waist instead of my shirt pocket. Takes around 3 seconds to respond and stabilise.
    Yeah ok, that is cool.
    I like that
    If u want to go on an expedition get a Land Rover, if u want to come home from an expedition get a Landcruiser!

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    Quote Originally Posted by Thala Dan View Post

    And OB........I hope you smiled while you were up on Cowley.......your photo is probably all over Wechat by now
    Don't say that, there is a new camera up there too.
    We Chat... wouldn't you have to own a smartphone to know what that is ol mate
    Last edited by ol' boy; 19-08-19 at 01:32 PM.
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    Quote Originally Posted by Thala Dan View Post

    Might be interesting to redo your Cowley test one day with GPS+GLONASS set on that app....if it has that facility.
    The App i'm currently using doesn't give that option
    Just an option to use Geoid or not

    Amazing testing with your units though.... so once again, one was on GPS and the other on Barometric?
    And they concurred so often... That is amazing
    If u want to go on an expedition get a Land Rover, if u want to come home from an expedition get a Landcruiser!

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    Quote Originally Posted by ol' boy View Post
    Don't say that, there is a new camera up there too.
    You know what they say, OB - "Smile....you're on Canton Camera"

    Quote Originally Posted by ol' boy View Post
    We Chat... wouldn't you have to own a smartphone to know what that is ol mate
    Nah, mate ....I just try to keep up to date so that my Social Credit Score is acceptable when our new Overlords take over.

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    Quote Originally Posted by Thala Dan View Post
    You know what they say, OB - "Smile....you're on Canton Camera"
    Now you are scaring me!
    There is a newly erected sign that says that as you climb the tower, just near the camera!!!!
    If u want to go on an expedition get a Land Rover, if u want to come home from an expedition get a Landcruiser!

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    Quote Originally Posted by ol' boy View Post
    Amazing testing with your units though.... so once again, one was on GPS and the other on Barometric?
    Yes……and no

    On page 5, the Garmin Oregon manual states:

    Calibrating the Barometric Altimeter
    Your device was already calibrated at the factory, and the
    device uses automatic calibration by default.
    You can manually
    calibrate the barometric altimeter if you know the correct
    elevation or the correct barometric pressure.
    1 Go to a location where the elevation or barometric pressure
    is known.
    2 Select Elevation Plot > Calibrate Altimeter
    3 Follow the on-screen instructions.
    Notice it refers to “automatic calibration” but does not elaborate….there is no mention of “GPS” anywhere.

    But then on Page 11, we get:

    Altimeter Settings
    Select Setup > Altimeter:
    Auto Calibration:
    Automatically calibrates the altimeter each
    time the device is turned on. The device continues to
    automatically calibrate the altimeter as long as the device
    has GPS signals acquired.
    Barometer Mode:
    Variable Elevation allows the barometer to
    measure changes in elevation while you are moving. Fixed
    Elevation assumes the device is stationary at a fixed
    elevation, so the barometric pressure should only change
    due to weather.
    But now I’ve found an aftermarket document with a lot more information, and it describes in more detail what “automatic calibration” entails:

    Auto Calibration:
    1. Off: auto calibration turned off
    2. At Power On: automatic calibration after starting the gps device (requires gps position and gps height)
    3. Continuous: using GPS continuously for calibration
    So, the Oregon Barometric Altimeter can act:
    1. as a purely barometric device (Auto calibration off), or,
    2. as a barometric device after it has been initially calibrated by the GPS (At power on), or
    3. as a barometric device with continuous GPS calibration (Continuous)

    My device was set to "At power on" - (a setting not even mentioned in the Garmin manual).......which means that the barometer was calibrated to the GPS Elevation (once GPS signals were acquired), and thence acted as a barometric altimeter thereafter.

    This would explain why the Oregon tracked the eTrex throughout the drive on Friday, because it started with the same elevation starting point (courtesy of the GPS), and then tracked the barometric variation correctly as I proceeded to Bright and back.

    It also explains why, when I switched the Oregon on during the passage of a low pressure system on Aug 18, it still showed the correct elevation - because again, at switch-on, the GPS gave it the correct elevation starting point.

    So, if I were to select the "Continuous" setting, the Altimeter would then essentially act as a GPS Altimeter, deriving continuous calibration from the GPS.

    But, as it's been set to "At power on" all this time, it has tracked barometric pressure to determine elevation, but only after it had been told the initial elevation by the GPS at switch-on.

    See.....simple......

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