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OS GB 50K Mapping Errors

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Now that my mapping software is on the Walkhighlands site I've been able to look again at some tracks from my GPS on 1:25,000 and compare with the 1:50,000. It's been interesting.

Here's an example from a walk we did today1. If you're a map nerd we're looking at TL 5264 and TL 5364. Here's our track plotted on a 1:25,000 OS map2. It's the purple line running diagonally from bottom left to top right.

White Fen Drove at 1:25,000

As you can see we walked up White Fen Droveway, turned right onto a byway, then left to walk under the pylons up to the point where we crossed a track and appear to start yomping across a field (it's actually the National Trust's newly acquired White Fen which is why it's not on the map yet).

Now let's look at that plotted on a 1:50,000 OS map.

White Fen Drove at 1:50,000

It's immediately obvious that the byway has been drawn on the map in the wrong place! It's quite significantly wrong too: twice the distance from the droveway and it now ends where the pylons met the cross track as opposed to you having to walk under the pylons which must be about 100m from the correct position.

Similarly later on the same walk we appear to be walking on the wrong side of the lode (a a small Fen river/canal).

This confirms something we've suspected for a while which is that 1:50,000 maps cannot be relied on if accuracy is important: if you had entered the end of that byway as a waypoint based on 1:50,000 mapping your GPS would have taken you to a point 100m away from the correct place. It doesn't matter much here in the Fens but on "interesting" terrain in Scotland it could be the difference between a safe point and a very dangerous one.

So if you are going route planning and waypoint positioning is critical then don't do it using 1:50,000 maps!

  1. Beth walked from home to Lode with Jake and I met her there where we met up with Gareth and Suzanne and their dog Bess for a six mile loop through White Fen, full track here.
  2. These maps are Crown Copyright but I'm allowed to reproduce segments here under the Copyright, Designs and Patents Act 1988 which allows under s30 (1) Fair dealing with a work for the purpose of criticism or review.

Tags: maps, walks Written 17/01/10

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