Unit:-4
Chain Survey
Pinciple of chain survey
The principle of chain surveying is trangulation. Trangulation means the area to be surveyed is divided into number of triangles which should be well cnditioned, THere is no angular mesurement in this surveying.
Suitability of chain surveying
Chain survey is recommended when:-
1.The ground durface is horizontal.
2.When small area is to be surveyed.
4,The survey area has less details.
5.Formation of well conditional triangle is easy.
Unsuitability of chain survey
1.The survey area is not plane.
2.The survey area is very large.
3.The survey area is crowed with many details.
5.Formation of well conditional triangle is no possible due to obstacles.
Types of triangle
1.Ideal triangle
The triangle is said to be ideal, if all the angles of it are equal i.e 60. Ideal triangle jis not psddible in feild due to undulations, many details etc.
2.Well conditioned triangle
The triangle is said to be well conditioned if no angle of ot is less than 30 or more than f120, well conditioned triangle is requried for survying.
3.ILL-conditioned triangle
The triangle is said to be ill conditioned triangle, if any one angle of ot is either less than 30 or more than 120.
Survey station and survey lines
1.Main station
The ends of the survey lines that comands the boundry of survey is called main station.
2.Main line
The line joining the main station are called main lines.
3.Subsidiary station / tie station
Any points selected on main survey line where it is necessary to ru auxiliary lines to locate interior details such as tress, buildingd, fence etc is called subsidiary / tie station. Subsidiary station are requried when the details are far from the main line.
4.Subsidiary line / tie lines
The line joining the subsidiary / tie station are called subsidiary / tie line
5.Base line
The line on which framework of the survey is built is called base line. It is the most important survey line. Generally the longest main survey line is called base line.
6.Check line
The line joining the apex of triangle to some fixed point on its base line is called check line. It is used to check the accuracy of the triangle. Sometimes it is also used to take interior details.
7.Offsets
The lateral measurement taken from an object to chain line is called offsets. Offsets are taken to locate objects with refrence to chain line offsets are of teo types:-4
1.Perpendicular offsets
2.Oblique offset
1.Perpendicular offset
When the lateral measurement are taken perpendicular 90 to the chain line, it is called perpendicular offset.
They can be taken in the following ways:-
i)By swinging the tape from object to the chain lone. The int of minium reading will be the base of perpendicular line (offset)
ii)By setting right angled triangle in the ratio 3:4:5.
iii)By setting right angle with the help of builder square or tri-square.
2)Oblique offset
Any offset that is not perpendicular (90) to te chain line is called oblique offset. Generally when object are far from chain line oblique offsets are taken.
Reconnaissance survey
Before starting the survey work, the survoyer should walk over the area to be surveyed in order to examine the ground and determine the possible arrangement of framework of the survey. During reconaissance survey , intervivibility of station, possible area for station etc are checked. It is also ensured if the well conditional triangle can be formed or not
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