Obstacles; field sketch for detail survey

Obstacles in chain surveying are of three types: (1) chaining free but vision obstructed, (2) vision free but chaining obstructed, and (3) both obstructed.

1. Chaining free, vision obstructed

Rising ground between two stations A and B. Use reciprocal ranging. Two assistants at M1 and N1 (each sees the other and one end station) direct each other alternately onto the lines to the far stations, until M, N, A and B are in one line. The line is then chained over the hill.

        M1 ...... N1
   A ___/  hill     \___ B

2. Vision free, chaining obstructed

(a) Pond. Erect equal perpendiculars AC and BD at points A and B on the line, both on the same side of it. Measure CD; then $AB = CD$.

(b) River (width AB). At A set out a perpendicular AC and mark its mid-point D; at C set out CE perpendicular to AC and move along CE until E, D and B are in line. Then triangles ABD and CED are congruent, so $AB = CE$.

          B   (opposite bank)
          :
   river  :
          A ---- D ---- C
                        |
                        E

(c) Distance across by angles. Set out a perpendicular AC at A and measure the angle ACB; then $AB = AC \tan\angle ACB$.

3. Both chaining and vision obstructed

A building on the chain line. At A and B on the line before the building, erect equal perpendiculars AC and BD, long enough to clear it. Join C and D, and extend CD beyond the building to E and F. At E and F erect perpendiculars EG and FH equal to AC. The line G-H is the prolongation of AB, and the hidden distance $BG = DE$.

    C ------------ D ------------------ E ------------ F
    |              |                    |              |
 ---A--------------B----[ building ]----G--------------H--->

Field sketch for detail survey by linear measurements

The chain lines (framework) and objects are drawn freehand in the field book as a key plan. A detail survey by chain or tape is made by:

1. Running the main chain lines AB and BC and a check line.
2. Taking perpendicular or oblique offsets to buildings, roads, trees and fences, and booking them.
3. Drawing a sketch of each object beside the chain line, with its dimensions.

        C
       /|\    [House] offset 8 m
      / | \ 
     /  |  \     (o) tree
    A---D---B ----> road edge

Later, the triangles are plotted by arcs and the details by offsets.