[closed] Altitude accuracy and consistency

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potstejn
Posts: 12
Joined: Wed Feb 08, 2017 2:52 pm

[closed] Altitude accuracy and consistency

Post by potstejn » Sat Sep 16, 2017 1:03 pm

I have been using the AQ app for almost a year and so far have been very impressed. Thank you.

My questions relate to the gathering and processing of altitude data. I use a Samsung GS5 with a pressure sensor and have enabled barometric measurement in the app. The device is kept in airplane mode. Calibration is done manually at the start of every hike. At the end of a hike I manually record the total trip ascent/descent as displayed on the device.

* Are these the totals derived from barometric elevation data?

* The totals can vary by as much as 10% between different devices and between the same hike measured by the app at different times. Is this acceptable variation?

* If I export the track as a .gpx, GPS-derived altitude data is exported rather than barometric data.

* When I export the statistics as .csv, I only obtain lat/long, EGM96 elevation, time, accuracy and pressure, and not the other parameters listed on the AQ website...

“Longitude” and “Latitude”, the coordinates of the location buckets in the WGS84 (GPS) datum. A location bucket is a set of one or more locations from the original track, grouped together for algorithmic reasons;
“Distance”, the cumulative distance from the start point of the buckets, in meter (m);
“Time”, the cumulative time from the start date of the buckets, in second (s);
“Elevation”, the mean elevation values computed for the buckets, based on the GPS altitude values, in meter (m), over the EGM96 (Geoid) ellipsoid;
“Speed”, the mean speed values computed for the buckets, in meters per second (m/s);
“Slope”, the mean slope values computed for the buckets, in percent (%);
“Sea-level pressure”, the calibrated, sea-level equivalent mean pressure values computed for the buckets, in hectopascal (hpa);
“Pressure”, the un-calibrated, raw mean pressure values computed for the buckets, in hectopascal (hpa);
“Barometric elevation”, the mean elevation values computed for the buckets, based on the pressure values, in meters (m), over the EGM96 (Geoid) ellipsoid;
“Accuracy”, the mean accuracy (with a 68% confidence) values computed for the buckets, in meter (m);
“Elevation (diff.)”, the mean difference values between the GPS elevations and the barometric elevations for the buckets, in meter (m);

* What does it mean that barometric altitude is automatically calibrated? I use barometric altitude because it is more accurate than GPS-derived altitude. Doesn't calibrating to GPS-derived altitude defeat the point?

* Most elevation measurements are made every 10 seconds, but sometimes there may only be a second or two between measurements?

Psyberia-Support
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Re: Altitude accuracy and consistency

Post by Psyberia-Support » Mon Sep 18, 2017 8:24 pm

Hi and thanks for your interest in this application.

I'll try to answer as clearly as possible to all your questions:
Are these the totals derived from barometric elevation data?
Yes (by default if pressure values are available), but you can change it. In the statistics settings, you can choose whether or not to use the barometric elevations. You can also check the color of the elevation profile, if it's red, it's GPS elevations, if it's brown, it's barometric elevations.
The totals can vary by as much as 10% between different devices and between the same hike measured by the app at different times. Is this acceptable variation?
It may seems a lot, but it depends on lots of factors (including the weather, the hardware, where is stored the device during the record, etc.).
If I export the track as a .gpx, GPS-derived altitude data is exported rather than barometric data.
Yes, only the GPS elevations are exported in GPX/KML files. Those formats do not support pressure value records.
When I export the statistics as .csv, I only obtain lat/long, EGM96 elevation, time, accuracy and pressure, and not the other parameters listed on the AQ website...
It depends on what you export. Track points do not have names or icons or other stuff like this, so the CSV export window won't ask you about those fields.
Concerning the fields you list, you need to export the processed data as explained here.
What does it mean that barometric altitude is automatically calibrated?
Pressure values do not mean anything in the absolute, they need to be calibrated regularly as the weather changes. On the contrary, GPS elevations stay stable in the long term and do not depends on the weather, so they can be used to calibrate pressure values. But GPS elevations are not used for the computation of the statistics themselves.
Most elevation measurements are made every 10 seconds, but sometimes there may only be a second or two between measurements?
When accuracy increases, the application will keep the new location even if the previous one as been recorded less time ago than requested by the settings.

Don't hesitate if any point isn't clear.
Best regards
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potstejn
Posts: 12
Joined: Wed Feb 08, 2017 2:52 pm

Re: Altitude accuracy and consistency

Post by potstejn » Tue Sep 19, 2017 12:18 pm

Thanks for the explanations. The only remaining challenge is exporting a track as a .csv and getting all the information. I follow the procedure as outlined below, but still can't get parameters such as barometric elevation, speed, slope, elevation difference etc.


Psyberia-Support wrote:
Mon Sep 18, 2017 8:24 pm

Concerning the fields you list, you need to export the processed data as explained here.

Thanks again.

Psyberia-Support
Site Admin
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Joined: Wed Apr 14, 2010 9:41 pm

Re: Altitude accuracy and consistency

Post by Psyberia-Support » Tue Sep 19, 2017 2:26 pm

From the details view of the track (where you've the elevation profile), be sure to select the export format "CSV file / Profile" (at the end of the list) and not "CSV file / set of waypoints". I guess it's the problem.
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potstejn
Posts: 12
Joined: Wed Feb 08, 2017 2:52 pm

Re: Altitude accuracy and consistency

Post by potstejn » Tue Sep 19, 2017 2:45 pm

I have selected csv/profile but only get the incomplete set of data fields. I have done this a few times with the same result.

Psyberia-Support
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Re: Altitude accuracy and consistency

Post by Psyberia-Support » Tue Sep 19, 2017 4:22 pm

What version do you have? Do you see the export CSV option dialog allowing you to select the fields to export?
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potstejn
Posts: 12
Joined: Wed Feb 08, 2017 2:52 pm

Re: Altitude accuracy and consistency

Post by potstejn » Tue Sep 19, 2017 7:14 pm

I have version 2.0.5.

If I click on a track displayed on the map, the next screen offer the options follow path, auto target closest, dynamic profile, details, export as, undisplay, edit color, edit details and edit profile. If I select export as from here, the only csv option is for "set of waypoints".

If I click on a track and then select "details", the submenu on the details page offers follow path, auto target closest, dynamic profile, export as, copy, cut, edit details, and delete. If I select "export as" from here, the csv/profile option is available.

In neither case can I find a csv options dialog box to select fields to export.

Psyberia-Support
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Re: Altitude accuracy and consistency

Post by Psyberia-Support » Wed Sep 20, 2017 9:33 am

So the correct method to export the processed data (including barometric elevations) in the second one, from the details.
Please double check the exported CSV file (Profile data).
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potstejn
Posts: 12
Joined: Wed Feb 08, 2017 2:52 pm

Re: Altitude accuracy and consistency

Post by potstejn » Wed Sep 20, 2017 11:36 am

Psyberia-Support wrote:
Wed Sep 20, 2017 9:33 am
Please double check the exported CSV file (Profile data).
What does this mean?

I export the file and then transfer to a Mac. When I open it there I only have the limited list of parameters.

Psyberia-Support
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Re: Altitude accuracy and consistency

Post by Psyberia-Support » Wed Sep 20, 2017 11:42 am

Here is the file I get when I export the profile data:
profile-data_20170920_134021.csv
(1.4 KiB) Downloaded 74 times
Do you get similar headers?
I use the last 2.0.6 version.
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potstejn
Posts: 12
Joined: Wed Feb 08, 2017 2:52 pm

Re: Altitude accuracy and consistency

Post by potstejn » Thu Sep 21, 2017 12:30 am

I was exporting the profile data to an external storage and then opening in Mac. The file name looked like this:

track_20170703_145438_jul_2_17_-_porter,__cascade_ 3.18.57 PM.csv

I was able to open the file but it contained only the limited parameters.

Then I exported another track and opened it on the phone with Polaris and I had all the parameters. When I transfer this file to the Mac the filename is:

profile-data_20170920_193014.csv, and all the parameters are there.

Now, whenever I export a track as .csv everything is working fine.

I have no explanation! Somehow it started working properly after I used Polaris to open the first file?

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