Exercise 2 solution

Variation: 2 degrees west of grid north.

(a) The compass reads 128 degrees magnetic.
    Magnetic to grid: subtract 2. Grid bearing = 126 degrees.
    Draw the line on the map at 126 degrees from the landmark's side.

(b) You are standing somewhere along that line, looking at the landmark. The line from the landmark
    back to you is the back bearing: 126 + 180 = 306 degrees grid.
    (On the compass the equivalent is 128 + 180 = 308 degrees magnetic.)
    So you are on the line running from the landmark towards 306 degrees grid.

(c) Drift from a small error: sideways error is about distance x tan(error angle).
    3 degrees over 800 m:  800 x tan(3 degrees) = 800 x 0.0524 = about 42 m off line.
    5 degrees over 500 m:  500 x 0.0875 = about 44 m off line.
    Rule of thumb: 1 degree of error drifts you about 17 m sideways per 1,000 m walked,
    so 2 degrees gives the 35 m quoted in Chapter 4.

(d) Why use short legs and fixed marks: each time you pick a mark on the line and walk to it, any
    error is reset. A long single leg lets a small error grow.