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Friday, January 25, 2008

More on Mapping

There have been a few emails going around on this subject, which has set me off again (sorry, dog with a bone).

The big problem seems to be with roads, tracks and paths. Any road on the map is merely a graphic, scaling at maybe 30 m wide on a 1:50,000, and, crucially, unconnected to the height data. This is why, on Memory Map, the occupants of the cottage near Braemar at NO127909 are apparently faced with the problem of the opposite side of the road being 16 metres (52 ft) higher than it is outside the house (must be a right bugger backing the car out of the garage!).

Thus any route plotted along this road, if it deviates slightly from one side to the other, will record 16 metres of additional ascent at this point alone - and along its length these errors will add up to a great deal.

Zoom in on Memory Map and pick any land rover track contouring around a hill, between contours. Using a paper map, we will say that the ascent here is zero, and so it is. But check the height on the left and right hand sides of the track, and you will find that it often apparently slopes by as much as 3m or 4m. The same applies to canals - sloping water!

When we plot a route we might stay within the confines of a road or track, but it is simply not possible to get an accurate ascent figure whilst the software is incapable of recognising that a track or road is (mostly) flat from side to side.

That said, our 'paper' method does not take account of undulations along the track that do not break the contour line. However, I reckon that any underestimation that this might produce is relatively insignificant.

One for the boffins to work on, but for now ..... count those contours!!!

I'll go back to sleep now.

p.s Don't forget to visit the Wildcamp Petition - and sign it! Follow the story with Weird Darren, the author, and the other bloggers (see blogroll). Thanks!

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4 Comments:

Blogger Grumpy Orig: said...

"Digital Challengers" beware !!!!!!

January 27, 2008 at 11:33 AM  
Blogger Phil said...

Geoff sent me this link to his comments on a thread at OM, which adds a bit more thought to the ascent conundrum. You might also find it worthwhile to follow Weird Darren's exploration of the phenomenon and possible solutions.

Toodle pip

Phil the contour counter.

February 11, 2008 at 7:56 PM  
Anonymous Anonymous said...

I think the most important thing here is to only use one method. If you use the MM system and find the day hard but OK when calculated with contour counting the difference is only going to be psychological so use the one that suits you best. I have used MM for three years and think it gives a truer estimate of height gain than contour counting but when planning a route it can be frightening after years of getting "reasonable" height gain estimate by contour counting.

February 16, 2008 at 12:17 PM  
Blogger Phil said...

That's right, Anon, most of the time.

I have noticed, though, that a cross country route with lots of ascent almost always matches the "contour counting" method. Thus this route will appear quite a lot easier than it is in reality if the software's result is blindly accepted as true. This is not too important in summer (an hour or two longer on the hill than expected, and maybe a bit tired) but in winter it can be the difference between being safely off and in the pub or stumbling around in the dark with a head torch.

Nevertheless, as you imply, add a dose of common sense and MM is a great tool.

February 17, 2008 at 11:08 AM  

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