After some quick check via an Excel table, I evaluated atmospheric influences on barometric altimeter :
1/ The biggest concern is for the air pressure trend. The difference 2 hPa makes altitude error about 17-18 metres. The pressure change about 2 hPa/h is easily and steadily happening during 12-18 hours before advent of the warm front. A static altimeter would report you are climbing 18 metres in 1 hour. Near a cold front, pressure trough or ridge, the air pressure can change even faster. Once can easily check this effect during a day trip with the same start and destination. Difference can be tens of meters. 2/ Less concern is change of free air temperature wrt the standard atmosphere, as altitude error is proportional to the altitude difference. 10 deg C temp. change makes error about 33 metres for 1000 m difference. This is more concern for the aircrafts than terrestrials. 3/ The least concern is vertical temperature profile different from standard atmosphere -00065K/m. The error grows with the square of the altitude difference, but is usually small for the difference <1000 m. It is usually biggest during foggy cold seasons. In winter isotermia in lower 2000 m it makes about 11 m / 1000m and 44 m/2000m Dne 18/02/2018 v 00:39 Robert Grant napsal(a): > While my experience agrees with you regarding accuracy and stability, > it's still better to know the local pressure setting, especially if > landing an aircraft without a radio altimeter. Setting an altimeter > based on GPS sounds quite rare to me. > > On Feb 17, 2018 3:12 PM, "Poutnik" <[email protected] > <mailto:[email protected]>> wrote: > > As being trained in past as the military meteorologist, in pre-GPS > era, I am aware of that. But the offset value is bigger than GPS > accuracy of the static value averaged. BTW, the most handy way > how to calibrate the barometric altimeter at unknown altitude is > the GPS device. While barometric altimeters have superior > short-term accuracy and stability, GPS devices have superior > long-term accuracy and stability. Fortunately, for most personal > usage, absolute altitudes are not that important, rather the > relative changes. > -- Poutnik ( The Wanderer ) My Brouter profiles https://github.com/poutnikl/Brouter-profiles/wiki -- You received this message because you are subscribed to the Google Groups "Osmand" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
