Ascent


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By ThinArthur on 19/06/16 at 7:50pm

TallPaul, Interested in your comments/thoughts on one of the perrennial challenges of computer mapping, namely ascent. I uploaded a walk I did in South Wales today into a couple of different mapping tools I use. As ever all gave virtually identical distance (23.7m), but equally as predictably the ascents varied enormously as follows:

Walk Lakes: 668m Anquet Maps: 965m Garmin BaseCamp: 1123m

I think I half understand the difficulties and reasons behind the variation, but would welcome any obersvations you may have.

Kind regards,

ThinArthur


By TallPaul on 19/06/16 at 9:44pm

Ah yes, that old chestnut. I wrote about my journey to solve this problem in some detail in my personal blog in a post Going Up ... and Down.

Essentially the problem is that GPSs, although very good at giving accurate 2D fixes (especially with the aid of EGNOS if your GPS supports it) are nowhere near so good at working out your elevation with errors in the order of +/- 10-20m depending of what satellites it can see[1].

The other problem is that you get jitter. So if you're walking along dead flat ground then your elevation will change.

Our solution to this is that we just ignore the elevation data your GPS gives us in a track1. Instead we rely on Ordnance Survey's Land-Form PANORAMA® database which gives us the height of every point in the UK on a 50m x 50m grid to an apparent accuracy of 1m.

So one reason for the variation in results that you're seeing is that some people rely on GPS elevation. This is a BAD THING, mainly because of the jitter problem, and is likely to result in over-estimates of ascent/descent but if you're Garmin and writing for a worldwide audience then this may be what they're reduced to doing.

The other possibility is that they're relying on a NASA elevation database which we did look at a long time ago but that's much more coarsely grained than the OS database.

As for Anquet they're a UK based company so they may well be using the same data we are however the problem we found with that (as my blog post explains) is that without doing some cunning corrections you get an over-read in the order of 50% (so in the example in my post we were originally seeing an ascent of over 1000m but after the cunning corrections we got an ascent of 635m). I note that Anquet's figure is about 50% higher than ours so I'd suggest they've probably not done what we did.

Having said all of that there is a fundamental problem which is that there is no "correct" answer to this. It's like asking how long the coastline of the UK is[2]. All I can tell you is that when we were working on this we looked at a lot of tracks and routes and we were eventually getting figures that looked plausible to us ... and that's about as good as you can get really.

Footnotes

  1. There's also another problem, which is that depending of which datum you use you get a different alleged elevation but most modern GPSs can cope with this. If you want to read more about that read my blog post GPS and Elevation. However as ascent and descent are relative, not absolute, it doesn't matter when calculating those.

  2. it's pretty much infinite if you count it down to grains of sand.

  1. A track records where you have been and when you were there. For more information click here.

By ThinArthur on 21/06/16 at 9:18pm

Thanks TallPaul that's really helpful. I new a bit about the problem with the density of contour lines and the "coastline" problem, but that's given me a much better insight. Also good to know more about your own approach to the 'solution'. Sounds like you've made a dent in some of my ascent totals though! Kind regards,

ThinArthur



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