Real Mileage vs Mapped

Norry1

Guru
Location
Warwick
When you map a ride on a PC, eg with MapmyRide - does this take into account the extra distance travelled due to the gradients. If so, any idea on average what difference this makes?

Martin
 

jazzkat

Fixed wheel fanatic.
As they know the gradient I would assume that the distance/gradient were already compensated for.

But then again - I have no idea:biggrin:
 

shippers

Senior Member
Location
Sunny Wakefield
mapmyride and endomondo tend to be pretty cose to the speedo distance. I think it take account of hills, but your speedo will register a slighty longer distance as you never quite ride in a straight line down the road- weaving round potholes and whatnot.
I did 70 miles on my speedo a couple of weeks back, but endomondo (satnav thing on my phone, which plots onto maps) registered 69.4, so about 1% out.
 

515mm

Well-Known Member
Location
Carmarthenshire
A damn sight more accurate than the piece of cachi* American mapping software my Mrs used to calculate/plan one of my recent training rides.

Try 62miles mapped and 76miles actually travelled. I ran out of food, so I did.:ohmy:


*Welsh really is fab isn't it!
 

Globalti

Legendary Member
Most accurate map measurement method know to Man: follow the route counting every single time you cross a grid line, even if it's a quick dive across and back. Then halve the number to get the miles. It gets more accurate the further you go.
 

yello

Legendary Member
Location
France
Norry1 said:
does this take into account the extra distance travelled due to the gradients.

Blimey, I'd never thought about that! Cool question!

I've often noted a difference between the mapped mileage and the mileage on the cycle computer after the ride but had always considered it just due to the cycle computer and/or the map not being accurate (not that I ever expected them to necessarily be the same anyway). I've never thought it might also be down to gradients!
 

fossyant

Ride It Like You Stole It!
Location
South Manchester
There is a little difference, not much, but the biggest 'shortfall' can come from sudden gradient changes like in the Peaks - the mapping software might not map the total climb...where it's a short drop, then a sharp climb - over a full ride you'll be some major feet out - will have to give a comparison soon...
 

ColinJ

Puzzle game procrastinator!
I was going to post a lot of stuff about sines and tangents of small angles being almost identical but I won't bother. Let's just say that that hills hardly make any difference to total distance covered unless you are riding up and down 20+% hills all day without any flat roads in between.

For example - the forum ride I did yesterday was fairly steep to steeply uphill for 37 km (average gradient +5%) and fairly steep to steeply downhill for 42 km (average gradient -4.5%) with about 34 km of flattish to undulating roads. The distance measured on a map using mapping software was 112.8 km. The distance actually measured by my GPS (which measures in whole kms above 100 kms) was 113 km so as near as doesn't matter the figures were the same.

I'd bet that for any sane ride in the UK the error would be well under 1% so don't worry about it! :sad:
 

Hont

Guru
Location
Bromsgrove
Wouldn't GPS have the same issue though? I noticed when I switched from a comp that measured my wheel revolutions to a GPS, the GPS had me going half a mile less on a 15 mile run.
 

ColinJ

Puzzle game procrastinator!
Hont said:
Wouldn't GPS have the same issue though? I noticed when I switched from a comp that measured my wheel revolutions to a GPS, the GPS had me going half a mile less on a 15 mile run.
That's about a 3.5% discrepancy - was the 15 miles repeatedly up and down a 25% hill or something! :biggrin:

GPS odometers should be pretty accurate though they obviously only sample your position at intervals so if you were on a very twisty turny route some of your meanderings might get smoothed out thus reducing the distance measured.

You might be able to change the GPS sampling rate. I've just checked my Garmin and its tracklog sampling rate can be set to any one of 5 values (more frequent sampling is more accurate but uses up the tracklog memory more quickly). The sampling can be done at regular distance intervals, regular time intervals or done automatically by the GPS unit whenever it calculates it to be necessary.

I think that is more likely that your cycle computer probably wasn't calibrated accurately. Tyres squash down a little when you sit on your bike thus effectively reducing the diameter of your wheels. That means they turn round more times in a given distance than if you push the bike along without you sitting on it. Ideally, you'd calibrate the computer by measuring the effective diameter of your wheel with your tyres pumped to their normal pressures and you sat on the bike. I bet most people don't do that.

The hilly part of my ride on Sunday was effectively the equivalent of riding about 40 km up a mountain 2,250 m high. Using Pythagoras' Theorem, the distance travelled along the hypotenuse (road) would actually be 40.06 km rather than 40.00 km - only 60 metres difference in 40,000 metres. As I said, for any sane ride in the UK - you can forget about the error.

If you could find a very steep hill long enough, then things would be different. 40km along a road at a nominal 25% gradient would produce an error of more than 1.2 km compared to the value read off a map.
 
OP
OP
Norry1

Norry1

Guru
Location
Warwick
I've been too lazy to do the calculations to be honest.

Yes, GPS would in theory have the same issue - but I haven't worked out if the likely error is worth worrying about. The mapping software could do the calculations - hence why I asked the question, but I doubt if they do.

Martin
 

hillrep

Veteran
Short answer: Completely agree with ColinJ - It makes no difference!

Longer answer: Completely agree with ColinJ - If you are constantly going up and down 1 in 5 hills the horizontal distance will be ~2% less than the travelled distance. For constant 1 in 10 hills the difference is 0.5% so for any real route the difference will be negligible.

Unless you have taken great care calibrating your bicycle computer it will have a much greater error. Even a change in tyre pressure can change the calibration of the computer by a few %
 

Chrisc

Über Member
Location
Huddersfield
ColinJ said:
Ideally, you'd calibrate the computer by measuring the effective diameter of your wheel with your tyres pumped to their normal pressures and you sat on the bike. I bet most people don't do that.


I did...roll out measurment.
 
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