Author Topic: Total elevation readings on Edge 705  (Read 1093 times)

Total elevation readings on Edge 705
« on: October 17, 2010, 09:21:15 am »
I've noticed that the elevation gain readings on my Edge 705 when I download the data show a significantly higher reading than when I enable elevation corrections.

For instance yesterday i did the "Taste of the Test" Permanent.  Device showed 2211m ascent, corrected elevation showed 1570m.  This seems a huge difference.

Anything I need to be doing to get accurate readings?
Does anyone know what the "proper" ascent is for the route?

Thanks

Re: Total elevation readings on Edge 705
« Reply #1 on: October 17, 2010, 09:53:21 am »
The error on GPS elevation readings is just too big to make it reliable.

I've seen 3 GPSes, all the same model, all on the same ride give wildly different readings: 1400m, 1800m and 2200m climbing.

The problem is that any errors in individual elevation readings (i.e. too high) generate extra climbing. They don't average out over the length of the ride as a corresponding erroneously low elevation will also generate extra climbing as the device thinks you've gone down further than you have and will dutifully climb back up to the right elevation. It's these spikes, and general flutter, that add to the real climbing figures.

I did the TotT calendar event earlier this year, my legs would say that the figure near 1500m is about right. There's quite a bit of flat on that ride. I'll check what my GPS (also an Edge 705) said about it but the tcx file is on my work laptop (at work).
"Yes please" said Squirrel "biscuits are our favourite things."

Re: Total elevation readings on Edge 705
« Reply #2 on: October 17, 2010, 10:32:39 am »
Thanks, I will have to accept that my legs feel tired due to lack of fitness rather than climbing!

Will make a point of using elevation correction to get best figures.

Is there anything you can do to the unit to ensure more accurate readings?

red marley

Re: Total elevation readings on Edge 705
« Reply #3 on: October 17, 2010, 10:48:15 am »
There is another important issue that is not 'accuracy' as such, but rather the frequency at which elevation is recorded. For example, I did a couple of DIY 200s last month - one from London to Norfolk and the other the exact reverse of the journey. For the trip north I set the GPS track recording to 'high' resolution. This gave a total distance of 225km and total height gain of 1694m. For the return journey, I set the GPS track recording to 'medium'. The total recorded distance was again 225km, but elevation gain was 1215m. This is because as a proportion of the total distance travelled in the horizontal and vertical planes, the vertical profile tends to be much more sinuous than the planimetric profile.

In other words, there isn't a meaningfully 'true' elevation gain on a ride. Rather, we need to have an approximately agreed resolution at which we measure elevation before climbing figures can be compared. This is why I like AUK's 'contour counting' as a standardised way of describing climb. An alternative is to plot your route over a standard Digital Elevation Model (and I'd strongly recommend Ordnance Survey's, now free, 50m DEM) and use software to calculate gain.

Re: Total elevation readings on Edge 705
« Reply #4 on: October 17, 2010, 11:06:21 am »
This is why I like AUK's 'contour counting' as a standardised way of describing climb. An alternative is to plot your route over a standard Digital Elevation Model (and I'd strongly recommend Ordnance Survey's, now free, 50m DEM) and use software to calculate gain.

Not knowing how to do either, I just looked here - Audax Altitude Award

I've never calibrated the altimeter on my Hcx vista, which probably explains why the numbers I get in sporttracks are so variable, even with smoothing. I've found this using Google's map. Is there a free OS equivalent?

Google Maps Find Altitude


Re: Total elevation readings on Edge 705
« Reply #5 on: October 17, 2010, 11:20:03 am »
I'd strongly recommend Ordnance Survey's, now free, 50m DEM) and use software to calculate gain.
So with this data should I be able to write an application that will take the track points and look up their elevation. Do you have any pointers ?

NB the Google Maps Elevation API will permit this

red marley

Re: Total elevation readings on Edge 705
« Reply #6 on: October 17, 2010, 12:51:24 pm »
I'd strongly recommend Ordnance Survey's, now free, 50m DEM) and use software to calculate gain.
So with this data should I be able to write an application that will take the track points and look up their elevation. Do you have any pointers ?

Yes.

I've been meaning to create a web-service option for querying the OS 50m DEM for a while now. I don't know if anyone else has created a similar web service, but it wouldn't surprise me. Technically not that difficult. The main problem is the size of the dataset and the fact that the distribution of DEMs is one huge file containing many hundreds of separate DEM tiles. I will work on this later this year, but things workwise are a bit busy for me at the moment.

Re: Total elevation readings on Edge 705
« Reply #7 on: October 17, 2010, 01:04:28 pm »
I'd strongly recommend Ordnance Survey's, now free, 50m DEM) and use software to calculate gain.
So with this data should I be able to write an application that will take the track points and look up their elevation. Do you have any pointers ?

Yes.

I've been meaning to create a web-service option for querying the OS 50m DEM for a while now. I don't know if anyone else has created a similar web service, but it wouldn't surprise me. Technically not that difficult. The main problem is the size of the dataset and the fact that the distribution of DEMs is one huge file containing many hundreds of separate DEM tiles. I will work on this later this year, but things workwise are a bit busy for me at the moment.
I'll be keen to give you a hand, right now I am at the stage of learning about this sort of information.

Re: Total elevation readings on Edge 705
« Reply #8 on: October 18, 2010, 10:18:55 am »
For instance yesterday i did the "Taste of the Test" Permanent.  Device showed 2211m ascent, corrected elevation showed 1570m.  This seems a huge difference.

GTC claims that the Taste Of The Test (Calendar) was 2591m climbing. Ha!
"Yes please" said Squirrel "biscuits are our favourite things."

Re: Total elevation readings on Edge 705
« Reply #9 on: October 18, 2010, 04:53:47 pm »

GTC claims that the Taste Of The Test (Calendar) was 2591m climbing. Ha!

I quite like the sound of this - 2591m climbing would explain more easily how I felt afterwards!