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Technical explanation for why your GPSsucks at elevation (Read 850 times)


Patzer

    We've all learned the hard way that our expensive (and cool) GPS gear, while accurate at latitude and longitude, is useless when it comes to altitude.  Now I know why:

    http://www.somebits.com/weblog/aviation/gps-altitude.html

     

    Maybe a bit more geeky than most care for, but I liked it.

    Current goal: an Olympic distance triathlon. Did two of them! New goal: a half-ironman?? Did that too. New goal: I'll have to get back to you.

    Trent


    Good Bad & The Monkey

      Very interesting.  But I don't buy that this explains most of the elevation variation I see in GPS.  Gravimetric bumpiness at the scale of an entire planet would not explain the GPS-derived elevation error that I see on a scale of a few hundred feet.  I would think it just has to do with problems of triangulation in a perpendicular plane reducing precision.


      Outside Lane

        There are Garmin GPS devices that do a great job measuring elevation - but they aren't built for runners (trail hiking, yes - running, not really).  Maybe someday your Forerunner can do what this thing can do....

         

        https://buy.garmin.com/shop/shop.do?cID=145&pID=310

         

        See how they run...

        Trent


        Good Bad & The Monkey

          Beetle, that device relies on a barometric altimeter to measure elevation, not on GPS data.  That is the difference.

            We only get the accuracy that the government wants to give us and the cost of our "expensive" gps devices is nowhere near the costs of a system that can calculate the accuracy needed for 3d modeling from what the government gives us.

             

            We have tens of thousands of dollars into our survey equipment, repeaters and base stations that can get us to within a centemeter of vertical accuracy for our terrain surveys. Some days that accuracy is not possible as it depends on how many satellites we can lock onto.

             

            The pain that hurts the worse is the imagined pain. One of the most difficult arts of racing is learning to ignore the imagined pain and just live with the present pain (which is always bearable.) - Jeff

             

            2014 Goals:

             

            Stay healthy

            Enjoy life

             


            Outside Lane

              Beetle, that device relies on a barometric altimeter to measure elevation, not on GPS data.  That is the difference.

               

              I know the unit has a barometric altimeter - but it is a GPS device.  I just wanted to point out that there were GPS devices available from Garmin that can measure elevation.  No problem....  

              See how they run...

              Trent


              Good Bad & The Monkey

                Sorry, I was not clear.  It is a measuring device that includes several technologies, including a GPS transponder and a barometric altimeter, among others.  Calling the complete device a "GPS device" is as accurate as calling it a "barometric altimeter" or a "compass". 


                Imminent Catastrophe

                  It's not the government (SA was turned off years ago), it's just triangulation error, like Trent said. You can't get the angular separation in the z-axis (elevation) that you do in xy plane. Simple geometry.

                  "Able to function despite imminent catastrophe"

                   "To obtain the air that angels breathe you must come to Tahoe"--Mark Twain

                  "The most common question from potential entrants is 'I do not know if I can do this' to which I usually answer, 'that's the whole point'.--Paul Charteris, Tarawera Ultramarathon RD.

                   

                  √ Javelina Jundred Jalloween 2015

                  Cruel Jewel 50 mile May 2016

                  Western States 100 June 2016


                  Outside Lane

                    Sorry, I was not clear.  It is a measuring device that includes several technologies, including a GPS transponder and a barometric altimeter, among others.  Calling the complete device a "GPS device" is as accurate as calling it a "barometric altimeter" or a "compass". 

                     

                    Lighten up man.  If it's that important for you to win - you win. 

                    See how they run...

                    ymmv


                      Sorry, I was not clear.  It is a measuring device that includes several technologies, including a GPS transponder and a barometric altimeter, among others.  Calling the complete device a "GPS device" is as accurate as calling it a "barometric altimeter" or a "compass". 

                      GPS transponder? i thought it was only a GPS receiver.

                      heh...

                        I had assumed that we simply average out the errors better horizontally than vertically.  I may run 10 miles in distance while never going up or down more than 100-200 feet.  If the error is +/- 30 feet, that's a huge percentage of a small hill, but nothing compared to several miles.  When I've used my Garmin while hiking a mountain, the elevation tends to look a bit more realistic, as I gain a couple of thousand feet of distance up.

                        -------------------------------------
                        5K - 18:25 - 3/19/11
                        10K - 39:38 - 12/13/09
                        1/2 - 1:29:38 - 5/30/10
                        Full - 3:45:40 - 5/27/07

                        AmoresPerros


                        Options,Account, Forums

                          So, look, someone go out and run several mile of altitude change, in only 200-300 feet, and come back and report on the results. I nominate Trent.

                          It's a 5k. It hurt like hell...then I tried to pick it up. The end.


                          Patzer

                            Amores, LOL.  Though a stair climb in a tall building would almost qualify (assuming GPS signals got through the building!).

                            Current goal: an Olympic distance triathlon. Did two of them! New goal: a half-ironman?? Did that too. New goal: I'll have to get back to you.

                            ymmv


                              Amores, LOL.  Though a stair climb in a tall building would almost qualify (assuming GPS signals got through the building!).

                               This 2,063 ft tower in north dakota might be the best he can do. The description says there is a ladder to the top


                              Patzer

                                That would be one f*%^@!$ scary climb up the ladder.

                                Current goal: an Olympic distance triathlon. Did two of them! New goal: a half-ironman?? Did that too. New goal: I'll have to get back to you.

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