Compass Surveying and Its Necessity:
According to the method employed, surveying is classified into triangulation surveying and traverse surveying. A series of connected survey lines of known lengths and directions is called a traverse. When it is not possible to divide them into network of triangles as is done in chain triangulation, traversing method is employed. In traversing, a number of connected lines are run in the area to be surveyed, whose lengths are measured with chain or tape as usual and directions are determined with angular instruments.
A survey line can only be plotted if its length and direction both are known. The direction of the survey line may be expressed in terms of the angle at which it is inclined either to its adjacent line or to a fixed reference line called a meridian. The instruments commonly used for the measurements of angles and determining the direction of the survey lines are…
1. A Compass
2. A Theodolite
In traversing, when compass is used for making angular measurements, it is known as compass traversing or compass surveying, the compass is used to determine the direction of survey lines of the frame-work of the traverse by measuring the angles which these lines make with magnetic meridian. The process of chaining and offsetting is the same as in chain surveying and running of check lines is not necessary. The compass surveying is suitable…
1. When the survey details are required along the long narrow strips e.g. a road, a railway, a stream, etc. 2. When the area cannot be divided into triangles such as in towns.
3. When the ground is a large open track with a few isolated noticeable features.
4. When long straight lines and convenient tie lines cannot be drawn on account of obstacles.
5. When surveying is done in a jungle or in a dense forest.
6. When the country is hilly where chaining is tedious and liable to error.
The compass surveying is unsuitable in places abounding in magnetic rock or iron ore. It is less accurate than Theodolite traversing but can be performed rapidly. Therefore, it is generally suitable for rough surveys where speed and not the accuracy is main consideration such as in sketching and night marching for military purpose etc.
Field work in compass surveying Field Party
It will consist of four persons:
1. Surveyor for taking bearings of the lines and recording field-notes and imparting instructions to other party members.
2. Two chain- men for chaining the survey lines.
3. One flag- men, peg man to fix stations, pegs and ranging rods as directed by the surveyor.
The equipment in compass surveying consists of a prismatic staff, optical-square, chain etc.
: series of connected survey lines of known lengths and direction is called a traverse.
1. Closed traverse
: A traverse is said to be closed when a complete circuit is made i.e. when returns to the starting point forming a closed polygon. 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.
2. Unclosed or open traverse
: A traverse is said to be unclosed when it does not form a closed polygon. It is suitable for the survey of long narrow strips e.g. a road, railway line, coast line etc.
Surveying of an open traverse/of a road
After performing preliminary steps and making the station points, say A,B,C,D etc, the compass is set up over the starting station A and bearing taken towards the next station B. The line AB is chained and offsets on both sides recorded as already described. 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.
The fieldbook for a compass survey is booked in the same way as in chain surveying, the only edition being of the bearings. Therefore bearings of the first line is written in the first column above the starting station and that of the following lines, to the right are left of the central column according as each of them turns towards the right are left of the preceding line. The back bearings are entered in the central column at the end of the each line.
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