CHAIN TRIANGULATION
Chain surveying is that type of surveying in which only linear measurements are made in the field.
he principle of chain survey or Chain Triangulation, as is sometimes called, is to provide a skeleton or framework consistin of a number of connected triangles, as triangle is the only simple figure can be plotted from the length of its side measured in the field.
SURVEY STATION
A survey station is a prominent point on the chain line and can be either at the beginning of the chain line or at the end.
A survey station may be marked on the ground by driving pegs if the ground is soft. However. on roads and streets etc.
SURVEY LINES
The lines joining the main survey stations are called main survey lines. The biggest of the main survey line is called the base line and the various survey .stations are plotted with reference to this. If the area to be surveyed has more than three straight boundaries.
Check lines :- Check lines or proof lines are the lines which are run in the field to check the accuracy of the work. The length of the check line measured in the fteld must agree with its length on the plan. A check line may be laid by joining the apex
of the triangle to any point on the opposite side or by joining two points on any two sides of a triangle.
Each triangle must have a check lines.
Tie lines. A tie line is a line which joins subsidiary or tie stations on !he main line. The .main object of running a tie line is to take the details of nearby objects but it also serves the purpose of a check line. The accuracy in the location of the objects depends upon the accuracy in laying the tie line.
CONDITONS TO BE FULFILLED BY SURVEY LINES OR SURVEY STATIONS
The survey stations should be so selected that a good system of lines is obtained fulfilling the following conditions:-
- Survey stations must be mutually visible.
- Survey lines must be as few as possible so that the framework can be plotted coveniently.
- The framework must bave one or two base lines. If one base line is used. it must run along the length and througb the middle of the area. If two base lines are used, they must intersect in the form of letter X.
- The lines musr run through level ground as possible.
- The main lines should form well-conditioned triangles.
- Each triangle or portion of skeleton must be provided wilh sufficient check lines.
- All the lines from which offsets are taken should be placed close to !he corresponding
surface fearures so as to get shan offsets.
- As far as possible, the main survey lines should not pass lhrough obstacles.
INSTRUMENTS FOR SETIING OUT RIGHT ANGLES
There are several types of instruments used to set out a right angle to a chain line,the most common being (1) cross staff (2) optical square (3) prism square (4) site square
A) Cross Staff
The sintplest instrument used for setting om right angles is a cross staff. It consistsof either a frame or box with two pairs of venical slits and is mounted on a pole shod for fixing in the ground. The common forms of cross staff are (a) open cross staff (b)French cross staff (c) adjustable cross staff.
1. Open Cross Staff.
It shows an open cross staff. It is provided with two pairs of vertical slits giving two lines of sights at right angles to each other.The cross staff is set up at a point on the line from ehich the right angle is to run, and is then turned until one line of sight passes through the ranging pole at the end of the survey line however, it is to be used to take offsets, it is held vertically on the chain line at a point where the foot of the offsets is likely to occur.
It is then turned so that one line of sight passes through the ranging rod fixed at the end of the survey line. Looking through the other pair of slits, it is seen if the point to which· the offset is to be taken is bisected. If not, the cross staff is moved backward or forward till the line of sight· also passes through the point.
2. French Cross Staff
If consists of a hollow octagonal box. Vertical sighting slits are cut in !he middle of each face, such that the lines between the centres of opposite slits make angles of 45° with each other. It is possible, therefore, to set out angles of either 45° or 90 o with this instrument.
3. Adjustable Cross Staff.
The adjustable cross staff consists of two cylinders of equal diameter placed one on top of the other. Both·are. provided with sigbting slits. The upper box carries a vernier and can be rotated relatively to the lower by a circular rack and pirtion arrangement actuated by a milled headed screw. The lower box is graduated to degrees and sub-divisions. It is, therefore, possible to set out any angle with the help of this instrument.
B) OPTICAL SQUARE
Optical square is somewhat more convenient and accurate instrument than the Cross
staff for setting out a line at rigbt angles to another line.
1) Hold the instrument in hand at any intennediate point C on AB, sigbt a pole held at A and direct an assistant to fix a ranging rod at a. such that the images of the ranging rods at a and A coincide in the instrument.
2) Tum round to face B and sigbt the ranging rod at a. If the image of the ranging rod at B coincides with the image of ranging rod at a, the instrument is in adjusnnent.
3) If not, direct the assistant to move to a new position b so that both the iruages coincide. Mark a point d on the ground ntid-way between a and b. Fix a ranging rod at d.
4) Tum the adjustable mirror till the iruage of the ranging rod at d coincide with the iruage of the ranging rod at B. Repeat the test till correct.
C) PRISM SQUARE
The prism square shown in works on the same principle as that of optical square. It is a more modem and precise insuument and is used in a similar manner. It has the merit that no adjusnnent is required since the angle between the reflecting surfaces (i.e. 45°) cannot vary.
Fig:- Prism Square
D) SITE SQUARE
A site square. designed for setting out straigbt lines and offset lines at 90°consists of a cylindrical metal case containing two telescopes set at 90° to each other, a fine sening screw near the base, a circular spirit levei at the top and a knurled ring at the base. It is used in conjunction with a datum rod screwed into the base of the instrument.
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