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March 2020 - Contours, round or over, how to do a rough calculation

by Nigel Williams

This is sometimes referred to as Bob's Law in mountain marathon circles, but I have no idea who Bob is or was! But it gives us a rough rule of thumb. The premise is that roughly 100m of ascent equates to 1km on the flat. Firstly, we need a way to quickly estimate how far it is to go from A to B via D compared to going over the end of the ridge A to B via C. A to B via D is approximately the combination of distances A to B (via C) 1km and C to D 0.6km = 1.6km.

Next, we count up the contours we cross going up. 8 = 120m of ascent on a Harvey map (15m contour interval). If we assume a walking speed of 4km per hour, 1.6km around the end of the ridge will take approximately 24 mins.

If we assume, according to Naismith's Rule, that we add 1 minute per 10m of ascent we get 12 minutes to add to the 15 minutes direct route over the ridge, giving a total of 27 minutes. So, in this example we might be quicker going around.

Of course, we all travel and manage hills at different speeds. Steepness and under foot conditions have a varying impact. If this example used a 10m interval for the 8 contours then it might be margin-ally quicker to go over.

On a mountain marathon the cunning route planner starting from A would have a check point at B, and then have the next check point back up on the ridge but several kilometres further along. So those that don't plan beyond the next check point risk climbing the hill twice instead of going around and then enduring the climb just once.


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