Memory Map – A User Guide

4 Responses

  1. Simon says:

    Hi Chris & Stuart,
    This is because the “Eastings” that are drawn on an OS map (I assume you are using Ordnance Survey) are not actually lines of constant Latitude. If this seems a little surprising at first, then get out a paper map (assuming that you’ve still got some around) and have a look at the borders of the map – these mark both the latitude and longitude against the OS grid. For example, one sheet of OL 2 (Yorkshire Dales) has Lat 54 deg 18 min at Northing 4896 at the top left, and at Northing 4894 at the top right. You can see that there’s a Northing difference of 0002.

    So why is this? It’s because of the map projection used by the Ordnance Survey, which is Transverse Mercator. The distortion Chris noted increases the further east or west of the “true origin” of the projection, which is at 2 deg West. This true origin is the same across the whole of the OS grid and minimises the average distortion by being approximately midway between the east and west-most parts of the area mapped by the OS.

    None of this matters particularly to walkers because the track legs we use are relatively short and our compasses (and sometimes our skill sets) are not overly accurate. Aviators and mariners use much longer legs and use maps with different projections as a result (mariners use Mercator, and I guess aviators do also).

  2. chris Power says:

    Hi Stuart Could I ask a quick Memory Map V5 question? When using the track feature to do a bearing,when you draw a track along an Easting it reads 088 instead of 090 and 178 instead of 180 when running it down a Northing. This is with mode/ units/ magnetic variation set to zero. What am I doing wrong! Many thanks Chris Power

    • lonewalker says:

      Chris, bloody good question! I’ve never noticed it – mine is slightly different. Shows 89 for easting and 179 for northing, also with MV set to 0.
      If I set MV to 1.0 W I get 90 and 180
      If I set MV to 2.0 W (which is about what we should be at the moment I think) I get 91 and 181
      Don’t think that helps you very much, but interesting nonetheless 🙂

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