Factors for choosing stations; offset types

Factors in choosing stations of a chain survey

1. Stations should be intervisible and the lines should be easy to range and chain.
2. The framework should have well-conditioned triangles (angles between $30^\circ$ and $120^\circ$).
3. The base (main) line should be the longest and on level, firm ground, running through the middle of the area, so that offsets stay short.
4. The number of stations should be minimum, for less work and fewer errors.
5. Lines should pass over level ground, avoiding steep slopes and obstacles such as ponds and buildings.
6. Provide check lines and tie lines to test and to fix details.
7. The lines should run close to the details so that offsets are short (preferably not more than 15 m).
8. Stations should be on firm, permanent places and tied to permanent objects for recovery.
9. Boundary lines and detail features may be used as survey lines.

Types of offsets

1. Perpendicular offsets: measured at right angles to the chain line, using a cross-staff, optical square or the tape (3-4-5 method).
2. Oblique offsets: measured at an angle to the chain line, usually from two points on the line (tie), when perpendicular is not possible.

By length: short offsets (up to about 15 m for ordinary work) and long offsets (for rough details).

   A------+--------+-----B
          |         \ 
      perpendicular   oblique