UKH

Hi  I have plotted a route for the Carnedds  and come out with differences in the  route distance and ascent

Using map Quo and doing a track route it is air distance 17, 111.2 m, Total DTM distance 17.344.9m, Total ascent 1,150.4

Transfering GPX from map quo to Os maps ( plotting with straight lines-using shorter lines to follow the curature of the footpaths) 18.67 km, Total ascent 1369.59m

plotting directly into OS maps 17.40 km ascent 1130.70m ..

I have also measured point to point on Map quo and manually documented and it is 17.14 Km

Trail magazine using memory map 17. 3 km and ascent 1100m

I am wondering if it the software rounding each plot up & the use of straight lines for plotting.

Has any one else come across similar issues

You get a lot of differences with different systems (often the cause of complaint over trail races), depending on the density of the tracking points and the underlying contour map that is used. None of this differences above are going to make you choose to do or not do the route. Just pick the biggest numbers when it comes to bragging about it. 

There is no 'right' answer*, just lots of different ones depending on what datasets they use and what assumptions/simplifications they make.

See: https://users.math.yale.edu/~bbm3/web_pdfs/howLongIsTheCoastOfBritain.pdf

It's a rathole best avoided (for all practical purposes).

* Mainly because the concept of 'distance-on-the-ground' is not well-defined in the real world.

You need to make sure the mapping tools are not manipulating the points when you load the GPX file (you should use a single GPX file, and use the identical points in different programs). Whilst point thinning can be useful when importing recorded tracks, and converting to routes (WheresThePath was rather good at this), you need to use exactly the same points if you want to compare distances.

Then you need to consider if the tool is using 2D or 3D path lengths, and, if the latter, what terrain database it is using. Finally, you might even need to consider what projection it is using when calculating route segment lengths; converting WGS84 lat/long to OSGB36 GRs and then using those to compute segment lengths is likely to give different values if you convert to a different projection.

Ascent & descent are notoriously difficult to measure accurately, due to the digital terrain model used, and its resolution & accuracy. Traditional contour counting is also inaccurate, especially if comparing with captured GPS logs (imagine walking over sand dunes just less than 5m tall, none of which would break contours, but would be 5m ascent/descent every dune...

Oh, and, for recorded tracks, there's always the problem of noise; random error in each position can increase the segment length. The higher the frequency of point recording, the greater the significance this noise becomes (since the noise magnitude becomes comparable to the actual segment length). The fractal geometry of nature, and all that.

Pick the one that gives the easiest answer?

I find this pleasingly reassuring; I'll stick to counting contours and squares and maybe adding a bit on for the wiggly bits.

There is nothing quite like studying a map and working out a route. It is a pleasure.

I do use digital mapping for measurement to make sure I am not biting off too much and to get tracks for races.  Accuracy and precision tools these things are not and to add to this every time we run, my wife's watch displays more distance and a so a faster speed even though we were within 5 yards of each other. 

Can you make the 18.67km and 17.4km ones which you have on OS Maps publicly viewable?

If you compare the GPX exported from each you will likely find that the OS version has more points.  This is almost certainly why you have a difference.

Exactly. When I used to do fell running and mountain marathons I used to just estimate horizontal distance from km squares and count (uphill only) contours and apply a self-calibrated version of Naithsmith's Rule (mins-per-KM-plus-mins-per-contour-line-with-tweeks-for-ground-conditions) to estimate time-over-ground. I found it pretty accurate most of the time (<5% error on segments <1 hour). Key skill for score (fixed time, variable distance) events.

Oh, and these tools are presenting spurious precision; specifying distances to one tenth of a metre? When the route has been plotted from OS mapping? IIRC, OS mapping has an rms error of 7 m, then there's the accuracy with which you can select a point using a mouse or finger (OS 1:25k mapping has 625ppkm, so a single pixel is 1.6m), as well as the straight line segment approximation. They are obviously doing some rounding, after the line segment (square root) calculations, so they would be better rounding to some sensible precision for presentation; 10m would seem reasonable, given the accuracy of the input data. 

Hah! See the 1967 paper about the length of Britain's coast, by one B.B. Mandelbrot, linked in Marek's post, just above mine...

People with shorter legs really do walk further.

More so on the West coast of GB than any other coastline in the world if that paper is to be believed, makes you proud to be British.

BTW: Just checking - does everyone know what the "B" in Benoit B. Mandelbrot stands for? 
It stands for Benoit B. Mandelbrot.

Or perhaps not. It is generally considered a joke*, but not necessarily of B.B.M's making.

* and not even a good one since recursion and fractal geometry are not the same thing.

It's Benoit B Mandelbrots all the way down...

Well I have just opened the OSMAP app and it shows the distances in km and to the nearest 100m.  Not sure where OP is seeing OS distances expressed in meters and to nearest 1/10 meter. 

Aha, you got me!  My mistake - I just thought it was funny because I hadn't subjected my little joke to the correct mathematical analysis.  Always the best way to appreciate comedy.

Actually the "B" doesn't stand for anything at all - Benoit B. Mandelbrot didn't have a middle name.  He added the initial while he was working at IBM for reasons that aren't clear (but it seems likely he felt it gave his name more 'presence' as an author).  Unlike James T. Kirk he never got around to inventing a name to go with it later.

They come as a set.

You can enhance the "counting squares" algorithm for score class mountain marathons with a piece of string (calibrated to be long enough for a suitably ambitious schedule). This worked well until friends overlooked the different scales of the OMM maps and bit off a LOT more than they could chew

Yep, had one of those too, but just for the start-of-day-can-we-get-those-high-scoring-checkpoint planning. A knot at hour intervals also provided some guidance as to whether we were on schedule. All good tricks.

For bragging about afterwards, you obviously choose the one that gives the furthest distance.

When trying to rope in partners, go for the shortest.

I like this

With the transfer, some how there was a section that went back on itself. It had not been plotted in Map Quo and bot sure why it had occurred. Each distance across the software was different.