Field and office work; length of offsets

Field work

1. Reconnaissance of the area and preparation of an index sketch.
2. Selection and marking of stations and lines so that triangles are well-conditioned.
3. Ranging and chaining of the lines.
4. Taking and measuring offsets (perpendicular and oblique) to all details.
5. Booking of all measurements in the field book.
6. Check lines and ties measured to test accuracy.

Office work

1. Check the field book, compute any corrections.
2. Choose a suitable scale and arrange the layout on the drawing sheet.
3. Plot the base line first; then plot triangles by intersecting arcs using the compass and scale.
4. Plot the offsets, and draw the details (buildings, roads, boundaries).
5. Check by scaling the check lines and compare with the field values.
6. Ink and colour the plan, add the title, north line, scale and a legend.
7. Compute the areas (triangles, offsets by trapezoidal or Simpson's rule).

Factors governing the length of offsets

1. Scale of the plan. The error in plotting is at best 0.25 mm; at a smaller scale, a longer offset can be accepted.
2. Accuracy required. High accuracy needs short offsets.
3. Method of setting the perpendicular. A tape can give a good perpendicular up to about 15 m, a cross-staff up to 15 to 20 m, an optical square up to 20 m.
4. Nature of the ground and details. Offsets to well-defined permanent details must be short; rough details allow longer offsets.
5. Obstacles and the ground. Offsets over broken or undulating ground are made shorter.
6. Importance of the object. Important features are surveyed by short offsets or ties.