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I want to change the map distance units option in QGIS from metres to centimetres. I am currently using projection British National Grid EPSG 27700. I would like the print composer to print a map out on the scale 1cm:5000cm. Also how accurate is this projection relating to actual distance measured on the ground? To the nearest metre (as EPSG 27700 projection is in metres?)? Would there always be some margin of error in QGIS as it is open source software?

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    The error-margin stems from the projection or most likely the data, not the software. Also, 1:5k is always 1:5k - regardless of the units.
    – Erik
    Commented Aug 6, 2019 at 11:27

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The distance-distortion in the map projection will be independent of the software (ignoring blunders); it is instead dependent on the projection used, and is also spatially variable.

Ordnance Survey National Grid is a Transverse Mercator projection chosen that the distances on the ground are identical to distances in the grid along two meridians: two lines of true scale about 180 km to the east and west of the central meridian (2°W). Away from these lines distance distortion is spatially variable. For example:

  • In London, UK, the OSNG distance distortion is ~200PPM (or 20cm distortion per km)
  • In Birmingham, UK, the OSNG distance distortion is ~400PPM (or 40cm distortion per km)

Note that this does not take into the height effect - the distance between two points on the ground increases the higher the elevation (about 1PPM per 6.4m height change).

For general mapping purposes this distortion between ground and grid may be considered entirely negligible; for precise engineering however this distortion is very significant and must be dealt with in some manner (for example, use of alternate projections: Snake Projection or Highways England Grids).

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If you are using EPSG:27700 and all you want to do is measure distances any errors will depend on the accuracy of the data. If this is OS data the errors will be very small.

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  • Thanks for this, I notice MapInfo has a BNG EPSG projection of 1mm accuracy- does QGIS have this projection? I'm using QGIS for CAD purposes at the moment, to produce digitise objects and prepare maps at 1:1250 for planning applications, the distances have to be accurate.
    – Jonathan Hall
    Commented Aug 7, 2019 at 15:40
  • The distances will be as accurate as your data. If, for example, you have four coordinates that define the corners of a rectangular land parcel the accuracy will depend on the accuracy of the coordinates. The BNG is just a simple rectangular grid, think of it as a piece of squared graph paper.
    – nhopton
    Commented Aug 8, 2019 at 12:59

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