Etrex 30 Barometric Altimeter data and Strava

[Mike Conway]

Having some trouble with getting Strava to recognise the barometric data from my Etrex 30. The website converts any data received from the Etrex 30 and shows a GPS elevation data as the total amount of climbing for any given ride, i.e. not the barometric elevation reading I'm getting from my Etrex display.

To rule out anything I might be doing wrong with the device I have the Setup / Alimeter page settings as follows:

Auto Calibration - Off
Barometer Mode - Variable Elevation
Pressure Treding - Save When Power Off
Plot Type - Barometric Pressure

Next, I also tried uploading direct from the Etrex 30 to the Garmin Connect website and then exporting that as a TCX file (I made sure the Elevation Corrections on Connect were set to 'OFF'). I uploaded this TCX file to Strava, but still the data came out as 200m lower than that of my riding group who had Garmin Edge 800 devices.

My data: http://app.strava.com/activities/94046010
Data from same ride on a Garmin 800: http://app.strava.com/activities/94042901

Note that the 800 user did actually climb about 50m more than me as we started from different places. Note my Etrex 'total elevation' reading on it's display screen was 1008m / Strava it resolved to 822m

How can I fix this?

[frankly frankie]

The website converts any data received from the Etrex 30 and shows a GPS elevation data as the total amount of climbing for any given ride, i.e. not the barometric elevation reading I'm getting from my Etrex display.

To rule out anything I might be doing wrong with the device I have the Setup / Alimeter page settings as follows:
Auto Calibration - Off
Barometer Mode - Variable Elevation
Pressure Treding - Save When Power Off
Plot Type - Barometric Pressure

I would say you need to set Plot Type to Elevation/Distance.  In combination with Barometer Mode - Variable Elevation, the E30 will then definitely not record GPS elevation at all - it will only record barometric altitude.   (But in the GPX it uses the same tags either way - so Strava wouldn't be able to tell the difference anyway.)

[fuaran]

If Strava knows that the elevation came from a device with a barometric altimeter, then it will use the recorded elevation data. Otherwise, it calculates elevation from its own DEM (probably based on NASA SRTM). It doesn't use the recorded GPS elevation.
I suspect the Etrex 30 is not properly supported by Strava, so it doesn't know that it using a barometric altimeter. As frankie says, there is no way to tell from the GPX file. So it defaults to SRTM, which can be rather inaccurate.
This page has some details: https://strava.zendesk.com/entries/20965883-Elevation-for-Your-Activity

Also, the frequency of trackpoints recorded may make a difference. ie if it records 1 point per second, the total ascent/descent may be greater. I'm not sure if this affects the Strava calculations.

[Mike Conway]

Thanks. I've also confirmed directly with Strava support that they do not include the Etrex 30 as one of their 'supported devices' , meaning unless you have an Edge series GPS, you won't be able to include your barometric data in any uploads to Strava. 

[Bianchi Boy]

I have just had a mail from Garmin support and Connect 'was not designed' to support on the trial devices. I have a follow up mail but suspect that this means that all the elevation data is thrown away and replaced with DEM data. This is probably why my elevation data does not match between Connect and device.

Is there a service out there that does allow me to keep my rides with the elevation data that is generated in eTrex 30. I bought it over the 20 because of this feature and now it appears that I cannot keep the records.

BB

[Ningishzidda]

If you have your eTrex 30 on the handlebars directly in front of your mush, when you exhale heavily the draught of CO2 enriched Nitrogen will cause a slight depression around the eTrex 30.
This will screw-up the barometric pressure transducer’s instantaneous log value.

Best to put the eTrex 30 in a small Stevenson Screen on the rear rack of the bike. 

[frankly frankie]

Is there a service out there that does allow me to keep my rides with the elevation data that is generated in eTrex 30. I bought it over the 20 because of this feature and now it appears that I cannot keep the records.

You mean in a cloudy-crowdy sort of way?
Obviously you can save the GPX on your desktop and there are various utilities that allow you to view the elevation data.

Generally regardless of device, GPX trackpoint data structure is just the same - position, elevation, time.  I notice the E30 does actually identify itself in the GPX file header (my older GPSs don't) - but does anything actually acknowledge and use this information?

[Ningishzidda]

IIRC, Once you’ve saved the TCX file, you change the suffix from TCX to MLS or something like that. The MLS ?? is imported into MS Excel, not opened by the normal method.
It appears as a spreadsheet and you can work on it like any other spreadsheet calculating gradient from dist and evev’ gain/loss.

[Ningishzidda]

I rode about four dozen DIY and MidMesh 100s a few years back and saved the TCX for each.

The result of my workings was a best-fit curve through approx. 800 data points for each recording showing speed vs gradient. There were some ridiculous outliers which were discluded from the project.

Happily, I noticed the curve was getting further away from the graph’s origin with time.

A bit of statistical nonsense to keep my up till after midnight. 

[Bianchi Boy]

Is there a service out there that does allow me to keep my rides with the elevation data that is generated in eTrex 30. I bought it over the 20 because of this feature and now it appears that I cannot keep the records.

You mean in a cloudy-crowdy sort of way?
Obviously you can save the GPX on your desktop and there are various utilities that allow you to view the elevation data.

Generally regardless of device, GPX trackpoint data structure is just the same - position, elevation, time.  I notice the E30 does actually identify itself in the GPX file header (my older GPSs don't) - but does anything actually acknowledge and use this information?

Ah so the E30 identifies it's self in the header. I really want a way of using the elevation data without Connect or Strava deciding that it will replace it with DEM or other map derived data. My ride on Sunday had 811m with barometer, 1000ish with out map corrections and 1200m with map corrections from Garmin connect.

There must be a way of getting this logged and stored withot resorting to Excel and all that stuff.

BB

[Euan Uzami]

Ah so the E30 identifies it's self in the header. I really want a way of using the elevation data without Connect or Strava deciding that it will replace it with DEM or other map derived data. My ride on Sunday had 811m with barometer, 1000ish with out map corrections and 1200m with map corrections from Garmin connect.

There must be a way of getting this logged and stored withot resorting to Excel and all that stuff.

BB

Presumably the reason strava replaces your data with map-derived data is so that it's "fair" with respect to other users -  so all users get the same elevation for a given segment no matter what unit they've got. I know that doesn't help you much if you don't care about comparing yourself with other users, but "social" is where the money is nowadays...

[andrew_s]

If the Etrex 30 is identified in the header, is is not possible to just edit the header so it shows Edge 800?

[Ningishzidda]

Strava is a means by which the coroner can write “Death by misadventure” on the report, in order for the motorist to be acquitted from responsibility. 

[Toady]

Is there a service out there that does allow me to keep my rides with the elevation data that is generated in eTrex 30. I bought it over the 20 because of this feature and now it appears that I cannot keep the records.

BB

You could try www.ridewithgps.com, I think that uses the elevation data embedded in the uploaded file.  And also gives the opportunity to re-fetch elevation data, which presumably gets digital elevation data from a map set somewhere.  I'm no expert,  but it's worth checking out.  But if you do, read this ridewithgps discussion first, as it's very relevant:
https://ridewithgps.zendesk.com/entries/21703537-Why-does-my-GPS-unit-s-elevation-gain-loss-differ-from-ridewithgps-

It also has whacky things like segments that I think is similar to Strava (I've never actually seen strava so I don't know). 

Or you could keep the records on a local disk and use an appropriate tool to view them. 

[Kim]

Is there a service out there that does allow me to keep my rides with the elevation data that is generated in eTrex 30. I bought it over the 20 because of this feature and now it appears that I cannot keep the records.

Yes, it's called a file system.

Whatever you do with webby services (which are by nature transient), if you keep a copy of the raw GPX files from the device on disk, you'll always have the data.

[Bianchi Boy]

I had a reply about the elevation from Garmin and this has just confused me more. E30 is not supported by Connect and so have to read the track into BaseCamp to get the everation values. The value is very close to the values in Connect so it looks like it applies map correction and the numbers are still about 25% higher than was recorded on the ride.

I extracted the file and read it into Excel and the values were very close to web site and BaseCamp.

So how does the E30 calculate the elevation and are the values stored in the GPX file - or does the GPX file store the gps values?

I just have no data that appears consistent at the moment and this is really quite confusing.

I have ordered a new cycling computer from Sigma for £30 that has a barometric pressure meter so I hop to get two values that match somewhere.

Yours in a slightly confused state.

BB

[Ningishzidda]

The eTrex 30 would be pretty damned useless if the recorded elevation data was from the topography of the map in use.

They would get a fair few complaints from microlite flyers, glider and aircraft pilots and hot air balloonists.

It MUST be from the internal transducer. If it ain't, throw it away.

[Bianchi Boy]

The eTrex 30 would be pretty damned useless if the recorded elevation data was from the topography of the map in use.

They would get a fair few complaints from microlite flyers, glider and aircraft pilots and hot air balloonists.

It MUST be from the internal transducer. If it ain't, throw it away.

I think I was trying to say that the altitude reported on the E30 with barometric pressure is not the same as the data recorded in the gpx. On my ride there was a 25% difference. Device 811m on screen and the gpx file data summed to 1062m.

So it appears that the E30 reports one value through the screens and records another on the gpx? Which I just must be wrong - but it is where the data analysis has lead me.

BB

[Kim]

If you're not recording one trackpoint per second (which is almost certainly the rate at which the eTrex updates its cumulative totals), the discrepancy is probably simple sampling error.

[frankly frankie]

I think even if you record 1 per second it's still averaging the last several fixes, just the same as it does in 'auto'.

I think I was trying to say that the altitude reported on the E30 with barometric pressure is not the same as the data recorded in the gpx. On my ride there was a 25% difference. Device 811m on screen and the gpx file data summed to 1062m.

So it appears that the E30 reports one value through the screens and records another on the gpx? Which I just must be wrong - but it is where the data analysis has lead me.

It is wrong.  The data in the GPX is barometrically-derived (assuming you don't have the barometer set to operate in 'pressure' mode).

[Bianchi Boy]

I have just had a mail from Garmin support and the values in the GPX file are those from the barometer on the E30. Al they would say about the difference in the values shown on the device and stored in the file was that they use different algorithms to determine the total climbing. There is one other item of information that the barometer is accurate to about three meters and that the unit removes small differences.

Still in the position that two results from the same device differ by 25%. But at least if I load the device into Connect and disable the elevation corrections it is using the barometer values from the device. I will be getting a Sigma computer with a barometer in it next week and will see how close the results are.

Ahhhhhh those AAA guys who sorted out the rules and validated the climbing values for the calender events have my admiration.

BB

[Mike Conway]

The problem with Strava seems to be that the Etrex 30 doesn't identify itself - I received this response from their really good customer support people:

Hi Mike,

I downloaded the original file, which happened to be a TCX(attached). If you export a GPX, you will see different information.

This information is at the bottom of the file you attached as well: <Creator xsi:type="Device_t">
<Name>Unknown</Name>
<UnitId>0</UnitId>
<ProductID>1</ProductID>
<Version>
<VersionMajor>0</VersionMajor>
<VersionMinor>0</VersionMinor>
<BuildMajor>0</BuildMajor>
<BuildMinor>0</BuildMinor>
</Version>
</Creator>

The problem is that even if your file contains elevation data, if it doesn't include where it came from (device tag), we will not know whether or not to trust it. I'm not sure what our plans are for this particular device but I do know we hope to improve the handling of elevation data across all platforms.

Best,
Ashley
Strava Support Team

[Kim]

Looking at some raw GPXes from my eTrex30, they start with:

Code: [Select]

<gpx creator="eTrex 30" version="1.1" xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd http://www.garmin.com/xmlschemas/GpxExtensions/v3 http://www8.garmin.com/xmlschemas/GpxExtensionsv3.xsd http://www.garmin.com/xmlschemas/WaypointExtension/v1 http://www8.garmin.com/xmlschemas/WaypointExtensionv1.xsd http://www.garmin.com/xmlschemas/TrackPointExtension/v1 http://www.garmin.com/xmlschemas/TrackPointExtensionv1.xsd">
Which would seem like enough for them to go on?

Presumably therefore you need to copy the GPX directly from the device, rather than allowing Basecamp or whatever to mung it.

[Feanor]

Dunno about Strava, but Garmin Connect uses a browser plugin to detect connected Garmin devices, and probes them for files and uploads them.
The plug-in can identify the connected device type, even if no tracklogs are present.
So simply editing the Device ID in a GPX header may not be enough to fool it.

But...
There's an option on Garmin Connect to manually upload a file, not from the device.
This does seem to pick up the device type from the XML schema, and looks like it would be vulnerable to a bit of manual editing.

[Bianchi Boy]

Dunno about Strava, but Garmin Connect uses a browser plugin to detect connected Garmin devices, and probes them for files and uploads them.
The plug-in can identify the connected device type, even if no tracklogs are present.
So simply editing the Device ID in a GPX header may not be enough to fool it.

But...
There's an option on Garmin Connect to manually upload a file, not from the device.
This does seem to pick up the device type from the XML schema, and looks like it would be vulnerable to a bit of manual editing.

Ahhhhh So does anyone have an Edge 705 / 800 where they can show us the header data. We could then try putting this into the eTrex 30 data and then seeing what it did in Connect / Strada.