What is metrix Chain, Chain Surveying, Chain Introduction in chain survey

As far as possible the triangles should have angles close to 60°, in no case less than 30°.

* Offsets taken may be perpendicular or oblique.

* Perpendicular offsets may be set by swinging, using cross staffs, optical square or prism square.

* Fieldwork consists of:

1. Reconnaissance survey to select stations and prepare typical key plan.

2. Drawing reference sketches

3. Line by line surveying

* Office work consists in plotting

* Obstacles to chaining may be grouped as

1. Obstacles to ranging

2. Obstacles to chaining

3. Obstacles to both chaining and ranging

By making suitable alternate measurements, the obstructed length may be calculated.

Angle and Direction Measurements

The direction of a survey line may be defined as the horizontal angle between the line and a reference

line. The reference line is known as meridian and the angle as bearing.

* A compass measures bearing of a line w.r.t. magnetic north-south direction whereas it is possible to

measure an angle between any two lines using theodolite and sextent.

* In prismatic compass the graduations are shifted by 180° and are written inverted since the angles

are read at rear end of line of sight through prism. The markings are from zero to 360°. In

surveyor’s compass there is no need for writing inverted letters since the reading is not through

prism. In this graduation disc is fixed to the box and magnetic needle is free to rotate. The angles

are marked from zero to 90° on either side of north and south and noted as

Nq°E, Nq°W, Sq°E, Sq°W, etc.

* To take magnetic bearing of any line the temporary adjustments required are centring, levelling and

focussing.

* The direction shown by a compass in an area affected from local attraction is with respect to

magnetic bearing. But at any point the line passing through the point and North-South pole of earth,

known as true meridian, may not coincide with the magnetic meridian. The horizontal angle

between the magnetic meridian and the true meridian is known as magnetic declination. It is

observed that these are small variations in declination which may be classified as secular

variation, annual variation, daily variation and irregular variation.

* The vertical angle between the horizontal and the direction shown by a perfectly balanced magnet is

known as the dip. It is 0° at the equator while 90° at the magnetic poles.

* For line AB, the bearing taken from A to B is called forebearing while taken from B to A is called

back bearing. In whole circle bearing.

BB = FB ± 180°, plus if FB is less than 180° and minus if FB is more than 180°.

In case of quadrantal bearing system, N (North) and S (South) are to be interchanged.

* Some local objects like steel structures and electric wires influence magnetic needle and attract

towards them. This disturbance is called local attraction. It is easily determined by taking for

bearing and back bearing. If local attraction exists, the difference between the two will not be

exactly 180°.

* Fieldwork consists of reconnaissance survey, making location sketches, measurement of distances

and directions. Office work consists in plotting by angle and distance method and coordinate

method. Closing errors are to be adjusted by Bowditch method, transit method, graphical method

or axis method.

* Limits of precision: It there are n number of sides is a traverse, the angular error of closure should

not exceed minutes. The linear error of closure should not exceed 1 in 300, preferably it

should be 1 in 600.

* In a closed traverse the following equations should be satisfied:

SL = l1 cos q1 + l2 cos q2 + … = 0

SD = l1 sin q1 + l2 sin q2 + … = 0.

Hence, a maximum two missing measurements may be adjusted. However, in doing so, the change

of checking accuracy of measurements is lost.