Map my ride calls a 17% hill a 3.5% - why?

cojones
cojones Posts: 131
edited June 2011 in Road beginners
Hi,

On my 60 mile route yesterday (near Marlborough, Wiltshire) we went up a hill that had a 17% hill sign by the side of the road. Looking on mapmyride it classes the hill as 3.5% gradient - why is that?

Thanks,

Matt

p.s. it hurt - and I was above my theoretical max HR for most of it!!
http://www.tuneyourdi2.com

Comments

  • amaferanga
    amaferanga Posts: 6,789
    17% is the maximum gradient. And road signs are often optimistic.
    More problems but still living....
  • cojones
    cojones Posts: 131
    so the steepest part of it was 17%. That was easy...thanks amaferanga!
    http://www.tuneyourdi2.com
  • jim453
    jim453 Posts: 1,360
    Even if you zoom right in and use mmr to find a gradient over a very short distance it considerably underestimates the value.

    I know what you're all going to say but I'm not wishfull thinking. In fact I rode up porlock hill yesterday and was interested to see later that mmr makes it seem much easier than it was. Although, it was easier than I feared it might be.

    Same for countisbury hill if anyone's interested.

    Mmr is a bit rubbish for ascent.
  • pottssteve
    pottssteve Posts: 4,069
    jim453 wrote:
    Even if you zoom right in and use mmr to find a gradient over a very short distance it considerably underestimates the value.

    I know what you're all going to say but I'm not wishfull thinking. In fact I rode up porlock hill yesterday and was interested to see later that mmr makes it seem much easier than it was. Although, it was easier than I feared it might be.

    Same for countisbury hill if anyone's interested.

    Mmr is a bit rubbish for ascent.

    Agreed entirely!
    Head Hands Heart Lungs Legs
  • briantrumpet
    briantrumpet Posts: 20,350
    I think the cause is the low resolution of elevation data on Google maps. The full OS Open Data has the resolution data down to a 50m grid, which makes programs that use it (like Mapyx Quo) very memory-hungry, in a way that a web-based resource couldn't handle. The result is that all the sites that use Google maps consistently underestimate climbing amounts, as it smooths out all the lumps and bumps.

    The biggest discrepancy on my rides so far was one over Exmoor, which Bikeroutetoaster gave as 3555ft of ascent but Mapyx Quo gave as 7400ft. In the past I've compared Mapyx figures with official figures for specific events, and it always comes out very close indeed, so I'm pretty sure its figures are reliable.
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  • Garz
    Garz Posts: 1,155
    It was probably showing you the average gradient.
  • jim453
    jim453 Posts: 1,360
    Garz wrote:
    It was probably showing you the average gradient.

    No mate, like I said, it gets it wrong even if you zoom right in and take a gradient over a very small distance (a few metres) on a section where you know it is very steep.

    And like the guy above says (the science bit) map my ride has a flawed way of working stuff out. Don't undertstand it myself but he sounds convincing.

    Map my ride is rubbish for ascent info.

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