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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!

I like using Memory Map - a great product - but since completing our route for the TGO Challenge, I have been reflecting on the ascent figures generated by Memory Map, and the huge variation between our figures (obtained by contour counting) and those generated by the software. The digital mapping invariably overestimates by 20% to 25% or more on average terrain, but where we are doing a lot of steep ascent in open country it is almost exactly right. Why?

Clearly a track on the ground, even if it does vary a bit between two contours, will be much more level in reality than the software model suggests. For example, a route along the canal path from the top of Neptune’s Staircase in Fort William to Gairlochy shows 70m of ascent. I’ve walked along there – and, like the water in the canal, the path is flat. In fact a route along the canal itself still gives 50m of ascent – and no locks!

Where bridges occur we will in walk over them, and not down into the ravine and up the other side as the mapping software assumes (a route across the Forth rail bridge gives 43m of ascent).

Another suggested cause of error is inaccurate alignment of the map and altitude data, giving ascent and descent where there is none. This misalignment can sometimes be seen in 3D mode, where lochs extend up hillsides and streams run above the bottom of their valleys.

Given this, it is clear that any route contouring along a hillside can produce ascent figures so way out as to be pretty useless. A steep up and down route over a two or three Munros might well return a more accurate figure as the terrain mapping is much more likely to reflect reality, but in many other cases, perhaps the majority, it will be wrong.

So just how useful is the software’s version of Naismith’s Rule? If you were to factor in an inaccuracy of up to 25%, then a mountain day where the ascent is correctly rendered could come as a nasty surprise!

Alan & I decided to apply a bit of common sense, and worked out the ascents for our TGO route manually. It looks a lot less tiring now!

Pop over to the Gallery for some new TGO pics, which celebrate the quiet dignity of Challenge social occasions.

After an afternoon wading in freezing water putting plants in the new 'wildlife pond' ("It'll toughen you up for May" says Miss Whiplash, project supervisor) I was dismayed and apprehensive when she suggested that I might like to watch UKTV Gardens. What further onerous gardening project was I being softened up for?

But no! She meant it as a treat!

Extraordinarily (for I can see no connection with gardening) UKTV Gardens is showing Cameron McNeish's "Wilderness Walks" series.

Tonight at 7.30pm - Letterewe

The rest of the series is on at odd times (Tues 15th has Knoydart at 7.30am and 2.30pm, Weds 16th has The Cairngorms at 2.30pm). Best thing is to log on to UKTV Gardens listing for more info.

I wonder what else is hidden away in the listings for the specialist channels?