Altitude Autocalibration Barometer

[locuscycling]

I don't know i should write here in ,,Problems'' or in ''Wishlist''. It  is  not a  ''crucial thing''    but maybe some place for improwements in ''algorythm for Baromether autocalibration''.  

I tried several times and only once when the wheather  was ''super excellent'' the attitude   from GPS+autocalibration was ok ( see attachement  1)  in recordered track.But wheather like this we have on 1 for 100 days i think. (If wheather is excellent then pressure is  high and this same attitude lower.)

Otherwise it is always about 40-50 meters to high. Even today, when the wheather ( near Turnov) was also good it was still +30-50 m to high.( see attachement 2)

I also recordered today this same route with TwoNav on  WinCe Gps Unit  ( without Autocalibration ) -screen

 Higher track is recorded by Locus with autocalibration , lower one Track from TwoNav.
Then i compared some points ( for example POLANA JAKUSZYCKA 886 M) with SRTM  and:  Locus shows as i wrote before little 30-40 m to high attitude and Two Nav little to low ---Maybe some compromise

( Sorry i use Xperia Active with built in Baromether)

[tommi]

Hm, I never use autocalibrate for the pressure sensor because then I get a typically bad base value for altitude due to the uncertainty of the GPS altitude calculation.
I use manual calibration (use the known altitude value (either really known or taken from SRTM) and calibrate the pressure sensor with this information). This gives much better results, I would assume <10m deviance in few hours but of course dependent on the weather conditions.
The manual calibration has two problems:
a) it depends on weather conditions, changing pressure creates deviance from the correct altitude
b) and I think this is much worse and happens to me quite often: one forgets to do the manual calibration before starting track recording

[Maksym]

First - did you select GPS altitude correction (by default in Android altitude has about 30 m shift)?
Second - you must "recalibrate" altitude when GPS get right height. Automatic calibration use first height data from GPS, usually this data is incorrect. When you have good GPS signal you must turn it off and than on - altitude will recalibrate.

But, of course, most accurate is manual calibration by entering sea level preassure. You can get this preassure from METAR data of the nearest airport.

[locuscycling]

Second - you must "recalibrate" altitude when GPS get right height. Automatic calibration use first height data from GPS, usually this data is incorrect. When you have good GPS signaBl you must turn it off and than on - altitude will recalibrate.

Thx Maksym.I will try ,,second''. I didn't know it is necessary.

@Tommi-When it will be function  ,,SLOPE''  in Locus( i hope )   then you will have to use Barometric Attitude, otherwise measure will be incorrect.

[tommi]

First - did you select GPS altitude correction (by default in Android altitude has about 30 m shift)?
Second - you must "recalibrate" altitude when GPS get right height. Automatic calibration use first height data from GPS, usually this data is incorrect. When you have good GPS signal you must turn it off and than on - altitude will recalibrate.

Yes, I know I can tweak  the GPS altitude.

But, of course, most accurate is manual calibration by entering sea level preassure. You can get this preassure from METAR data of the nearest airport.

I always use manual calibration by entering the current altitude, not the sea level pressure. Locus then calculates a sea level pressure and works with it further to calculate the current altitude.

[tommi]

@Tommi-When it will be function  ,,SLOPE''  in Locus( i hope )   then you will have to use Barometric Attitude, otherwise measure will be incorrect.

I'm desperately waiting for SLOPE, see my dashboard viewtopic.php?f=41&t=2958.
I think the accuracy depends on the algorithm Menion will implement but of course your right that the gradient of the altitude curve is sensible to inaccurate especially jittering values.

Could you please change the name of the topic here: I think we all agree that we are talking about altitude, not attitude.
Thanks,
Tommi

[Maksym]

I always use manual calibration by entering the current altitude, not the sea level pressure. Locus then calculates a sea level pressure and works with it further to calculate the current altitude.

How do you know your current altitude? SRTM sometimes has incorrect altitude. You can use it in flat field without trees, buildings, etc. In other case you can't get precise actual height. Entering current sea level preassure you always get correct altitude.

[tommi]

Quite often I start recording at a location where the altitude is known, e.g. at my house, at other public locations like train stations etc etc. I agree that I do not always have this information.
I personally never have the information of sea level pressure. Especially when being abroad I usually have no data connection at all to request METAR data.
But anyway, Locus offers both methods and both are useful and give better results than taking GPS altitude as a reference.