After reading this article you will learn about the field-work in compass surveying.
1. Field-Party:
It will consist of four persons:
(i) Surveyor for taking bearings of the lines, recording field-notes and imparting instructions to other party members.
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(ii) Two chainmen for chaining the survey lines.
(iii) One flagman, cum pegman to fix stations, pegs and ranging rods as directed by the surveyor.
2. Equipment:
The equipment in compass surveying consists of a prismatic compass with tripod stand along with all the instruments as required in chain surveying.
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3. Traverse:
A series of connected survey lines of known lengths and directions is called a traverse.
It is of two types, viz:
(a) Closed, and
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(b) Unclosed or open.
(a) Closed Traverse:
A traverse is said to be closed when a complete circuit is made i.e. when it returns to the starting point forming a closed polygon as ABCDEA [fig. 5. 18(a)], or when it starts and ends at points whose positions on plan are known, as ABCDE [fig. 5.18 (b)].
The survey work in a closed traverse can be checked and adjusted. It is suitable for the survey of moderately large areas such as small towns and big villages etc., and for locating boundaries of forests, lakes etc.
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(b) Unclosed or Open Traverse:
A traverse is said to be unclosed or open when it does not form a closed polygon as ABCDEF (fig 5.19). It consists of series of lines extending in the same general direction and not returning to the starting point.
It is suitable for the survey of long narrow strips e.g., a road, railway, river, coast-line etc.
4. Surveying a Closed Traverse:
After performing reconnaissance and preparing a rough sketch as usual, the station-points (A, B, C, D, etc. fig.5.20) are marked by fixing pegs and ranging rods all visible from another and as near the boundary of the field as possible.
The compass is set up at the starting station (A) and bearings towards the back forwards stations (D and B) are taken and recorded. The length of the first line (AB) is chained and offsets on both sides of the chain line are taken in the same way as in chain surveying. Having arrived at the 2nd station (B), the length of the first line (AB) is recorded in the field-book.
The compass is then set up there, bearings towards back and forward stations (A and C) are recorded, the length of the 2nd line (BC) measured and offsets are taken as before. The work is continued in the same way until the whole circuit is complete. For rough work and rapid performance, the compass may be set up at alternate stations. The traverse may be run in the clockwise or in the anti-clockwise direction.
The positions of the objects which are out of reach of an offset are fixed by radiation. To do this, bearings of such objects are observed, and the lengths of the radial lines from the instruments are measured. More distant points such as comers of buildings may be located by the intersection of two bearings.
Field-Check on Closed Traverse:
In traversing with the compass, described above, the compass is set up at each of the successive stations and fore and back bearings of each of the lines are observed. Each bearing being observed independently of the others, the errors do not accumulate, but tend to compensate.
The fore and back bearings of a line should differ exactly by 180°. If the error between the fore and back bearings of a line exceeds the permissible error of reading (generally 15′), then they should be measured again ; and if on checking, the error is still found to exist, then either one or both the stations are affected by local attraction provided there is not other source of error. The bearing should then be corrected. For better accuracy, the survey lines should be as few in number as possible and as long as possible.
In addition to taking bearings towards the following and the preceding stations from a station of observation, bearings are also taken to some more conspicuous stations, such as towards C while at A (Fig.5.20). On arriving at C, bearing CA will be observed and if this bearing differs from the fore bearing AC as taken at A by 180°, then the accuracy of the work is ascertained and we can proceed safely.
5. Surveying an Open Traverse:
After performing preliminary steps and marking the station-points say A, B, C, D etc. (Fig. 5.21), the compass is set up over the starting station A and bearing taken towards the next station B. The compass is then set up at B and bearings taken towards A and C.
The line BC is then chained and offsets recorded as before. The work in continued is the same way till the end-station is reached from where only the back bearing towards the back station is taken.
6. Field-Notes:
The field-book for a compass survey is booked in the same way as in chain surveying, the only addition being of the bearings. Therefore bearing of the first line is written in the central column above the starting station and that of the following lines, to the right or left of the central column according as each of them turns towards the right or left of the preceding line. The back bearings are entered in the central column at the end of each line.