The length of a line (in km) measured off a map of scale 1:R1 is approximately (R2/R1)γ of its length in the same units measured off a map at scale 1:R2. For measurements by map measurer γ may vary between 0 and 0.4 according to the sinuosity of the line, but for road length on maps of Britain it seems reasonable to assume: L 1/L2=(R2/R1)·017.