Field sketch and mapping by linear measurement

Field sketch

A neat, freehand key plan (index sketch) of the area is prepared first. It shows the boundary, main details, the stations (A, B, C, D) and the survey lines with their names. The lines are numbered in the order in which they will be chained.

   Key plan (not to scale)
        C
       /|\    [House]
      / | \ 
     /  |  \     Pond  (o)
    A---D---B ---->  road
   (CD = check line)

Complete process of mapping by linear measurement only

Field work

1. Reconnaissance and selection of stations A, B, C ... so that the triangles are well-conditioned (angles between $30^\circ$ and $120^\circ$).
2. Mark the stations with pegs and draw the key plan.
3. Fix the base line (the longest line, on level ground) and the check lines.
4. Chain the lines. Measure each side of every triangle (AB, BC, CA) and the check line CD.
5. Take offsets to the details: perpendicular offsets by tape with a cross-staff or optical square; oblique offsets (ties) for hidden or distant objects.
6. Book all measurements in the field book (from the bottom upwards).

Office work

1. Choose a scale (for example 1:500) and compute the length of each line on paper.
2. Plot the base line AB first. Then plot C by intersecting arcs of radii AC and BC (to scale) drawn from A and B.
3. Plot the check line CD and compare its scaled length with the field value to detect errors.
4. Plot the offsets from the chain lines using the chainages and offset lengths, and join the plotted points to get the boundaries and details.
5. Ink the plan and add the north line, scale, title and legend.

Thus, the whole map is made from linear measurements only, since a triangle is fixed by the lengths of its sides, and details are fixed by offsets and ties.