Surveying (Part-3) (General Engineering) (SSC JE)

In intersection method, the objects to be located are obtained at the point of intersection of radial lines drawn from two different stations.

• In this method the plotting of plane table stations are to be carried out accurately checking important and thus done by taking third sight from another station.

• The intersection method is suitable when distances of objects are large or cannot be measured properly. Thus, this method is preferred is small scale survey and for mountains region.

The compass needle is made of a slender symmetrical bar of magnetised steel or iron. It is hung from a conical jewel bearing supported on a sharp, hardened steel pivet.

Plumbing fork–It is used for the centering of the table over the station.(1) Alidade–An alidade is straight - edge, ruler having some sighting device, it is used sighting the object and drawing the line now a day's telescopic alidade are also much in use in place of plane alidade, where the point is too high or low are to be sighted, the accuracy and the range are considerably increased by providing a telescopic alidade.

(2)Trough compass–Generally it is 15 cm long and provided to plot the magnetic meridian (N-S direction) to facilitate orientation of the plane table in the magnetic meridian.

(3) Level Tube/Spirit level–Level tube is used for levelling the plane table, it is placed on the board in positions mutually at right angles and the bubbles is centered in arch position to make the board horizontal.

A series closely spaced contour lines represents a steep slope.

• A series of uniformly spaced contour lines represents a uniform slope.
• Flat ground is indicate where the contours are widely spaced.

Benchmark is established by spirit levelling.
• Benchmark is the point laid above or below the datum line with a known elevation. Bench mark is considered as check for the other level points.
Type of benchmark–
• Great trigonometrical survey benchmark
• Permanent benchmark
• Temporary benchmark
• Arbitrary benchmark

In plane table surveying the objects are simultaneously surveyed and plotted on sheet. Hence no office work is required.

The two point problem and three point problem are methods of resection and orientation.

The subtense bar is an instrument used for measuring the horizontal distance between the instrument station and a point where the subtense bar is be set up. In this method a staff or tangent rod not necessary and the theodolite required is also at the ordinary transit type.

The period of a geo-stationary satellite is the same as the period of rotation of earth which is approximately equal to 24 hours or 1 day.