1. Permanent Bench mark:
These are established by different government departments like PWD, Railways, Irrigation etc., The RL of these points are determined with reference to G.T.S Benchmarks. Points on rocks, culvert, gate, pillars etc.
2. Temporary Bench mark:
These are established temporarily whenever required. These are generally chosen to close the day's work and to start the next days. Points on roofs, walls, basements etc
3. Arbitrary Bench Mark:
When the RL of some fixed Points are assumed, they are termed a arbitrary Bench mark
4. Extension of baseline:
The length of baseline is usually not greater than 10 to 20 km. As it is not a often possible to sewed a favorable sight for a longer base. They usually practice is therefore to use short base & Extend it by means. Of forming well conditioned triangles.
5. Trigonometrical levelling:
Trigonometrical levelling is the process of determining the differences of elevation of the given station from observed vertical angles and known distance.
6. Axis Signal correction :
If the height of the signed is not the same as that of height of the instrument axis above the station, a correction known as the axis signal correction or eye & objective correction is to be applied.
7. Geodetic Surveying :
In this surveying, the shape of the earth is taken into account and all the lines lying in the surface are curved lines. It is used for area greater than 250km2. It is accurate. It is conducted by great geometrical survey of India.
8. Baseline :
The Base line is laid down with great accuracy of measurement & alignment as it forms the basis for the computations of triangulation system the length
of the base line depends upon the grades of the triangulation.
9. Laplace Station :
At certain station, astronomical observations for azimuth & longitude are also made on the station is called Laplace station .
10. Triangulation :
Triangulation is nothing but the system consists of not of interconnected triangles. In this method, knowing the length of one side and three angles, the length of other two sides of each triangle can be computed.
11. Signals :
A Signal any object such as a pole target erected at a station upon which a sight is taken by a observer at another station.
12. Satellite Station :
A subsidiary station is established as near the true or principal station as possible, the station so established is called a satellite station or eccentric station or false station.
13. Reduction to centre:
If the true station were occupied by computing the corrections and apply them algebraically to the observed values is generally known as reduction of centre.
14. Base net:
A series of triangles connecting the baseline to the main triangulation is called base net.
15. Bench marking :
It is a fined reference point of known elevation.
16. Types of Bench Mark:
Great Trigonometric survey Bench mark
Permanent Bench mark
Arbitrary Bench mark
Temporary Bench mark
17. Equipments used for base line measurement:
Marking stakes or tripod Straining device Supporting stakes or tripod Steel tape
18. Methods used to measure baseline
Rigid bar method
Hunter's short base method
19. Two types of Trigonometrically leveling:
Plane Trigonometrical levelling Geodetic Trigonometrical levelling
20. Apparatus used in Baseline:
21. Corrections made while calculation of true length
Correction for absolute length
Correction for temperature
Correction for pull or tension
Correction for Sag
Correction for Slope