Exercise 2 solution

Split each reference into easting (first three digits) and northing (last three).

218447: easting 218 = line 21 + 8 tenths; northing 447 = line 44 + 7 tenths.
        Near the right-hand side of square 2144, and high in it.
603092: easting 603 = line 60 + 3 tenths; northing 092 = line 09 + 2 tenths.
        In square 6009, left of centre and low down.
470135: easting 470 = line 47 + 0 tenths; northing 135 = line 13 + 5 tenths.
        On the left edge of square 4713, halfway up.

Distance between the first two squares' left and bottom lines:
  218447 is in square 21 / 44 and 603092 is in square 60 / 09.
  Difference in eastings: 60 - 21 = 39 lines = 39 km
  Difference in northings: 44 - 09 = 35 lines = 35 km
  So the lines are 39 km apart east-west and 35 km apart north-south.
  (The straight-line distance is about sqrt(39^2 + 35^2) = about 52 km.)

Note: both references must come from the same map and 100 km letter square
for this comparison to mean anything.