Field book and obstacles in chain survey

Field book

The field book is an oblong book (about 20 cm $\times$ 12 cm) in which measurements are recorded in the field. It must be kept neat, and entries must not be erased. The records are made from the bottom of the page to the top, so that the page follows the direction in which the line is chained.

Types of field book

1. Single-line field book: A single red line in the middle of the page represents the chain line. The chainages are written on the line, and the offsets are written on both sides (left and right) of it.
2. Double-line field book: Two red lines run along the middle of the page, with a space between them. The chainages are written between these lines, and the left and right offsets are written outside them, with the details drawn.

 Single line type
 | left offset | chainage | right offset |
 |   12 (fence)|   60     |              |
 |             |   45     |  5 (tree)    |
 |             |   0  A   |              |

Reference sketch of stations

For each station, a small sketch (location sketch) is drawn showing its position, with distances to two or more permanent objects such as corners of buildings, trees or poles.

Need:

• The station can be found again if the peg is lost.
• Stations can be re-established for checking or later work.
• It records the name and location of the stations and lines.

Sample page of a detailing book

Chain line AB (booked upwards):

How offsets are measured and recorded

• The tape is stretched from the chain line to the object at right angles to the line (the perpendicular is set by cross-staff or optical square).
• The chainage at the foot of the offset is read on the chain.
• In the book, the chainage is written on the chain line and the offset length is written on the left or right side, with a short note (fence, tree, building) and a sketch.
• For oblique offsets, the two tie lengths are recorded together with the chainages from which they were taken.

Obstacles in chain survey and their remedies

Obstacles are of three types.

1. Chaining free, vision obstructed (rising ground). Use reciprocal ranging: assistants stand at M1 and N1, each directing the other onto the line towards the far station, until M, N, A and B are in one line.

2. Vision free, chaining obstructed (pond or river).

• Pond: erect two equal perpendiculars AC and BD at points A and B on the line, both on the same side of it, and measure CD, which equals AB.
• River: at A set out a perpendicular AC and mark D, its mid-point; at C set out CE perpendicular to AC and move along it until E, D and B are in line. Then $AB = CE$.

3. Both chaining and vision obstructed (building). Take A and B on the line before the building and erect equal perpendiculars AC and BD. Join CD and extend it beyond the building to E and F, and set out perpendiculars EG and FH equal to AC. G and H are on the original line, and the hidden length $BG = DE$.

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