Sun light or artificially generated electromagnetic waves consists of waves of different lengths. The spectrum of an electromagnetic wave Among these waves microwaves, infrared waves and visible light waves are useful for the distance measurement. In EDM instruments these waves are generated, modulated and then propagated. They are reflected at the point up to which…
Horizontal sight is taken as zero vertical angle. Angle of elevations are noted as +ve angles and angle of depression as –ve angles. To measure vertical angle the following procedure may be followed: 1. Complete all temporary adjustment at the required station. 2. Take up levelling of the instrument with respect to altitude level provided on the…
Theodolite is used for measuring horizontal and vertical angles. For this the theodolite should be centered on the desired station point, levelled and telescope is focussed. This process of cantering, levelling and focussing is called temporary adjustment of the instrument. 1. Set the theodolite at Q with vertical circle to the left of the line of sight…
Shows a sectional view of a typical Vernier theodolite and plate 16.1 shows photograph of such theodolite. Main parts of such a theodolite are: 1. Telescope: A telescope is mounted on a horizontal axis (trunnion axis) hence it can rotate in vertical plane. Its length varies from 100 mm 175 mm and its diameter is 38…
Theodolite is an instrument which replaced compass and level. It can measure both horizontal and vertical angles. If telescope is kept at zero reading of vertical angle it serves as an ordinary level. In this modern era of electronics equipments have come up to measure the distances to relieve surveyor from chaining long lines. Total station is…
The following are the different types of direct levelling: (i) Simple levelling (ii) Differential levelling (iii) Fly levelling (iv) Profile levelling (v) Cross sectioning (vi) Reciprocal levelling. Simple Levelling It is the method used for finding difference between the levels of two nearby points. Differential Levelling If the distance between two points A and B is large,…
The adjustments to be made at every setting of the instrument are called temporary adjustments. The following three adjustments are required for the instrument whenever set over a new point before taking a reading: (i) Setting (ii) Levelling (iii) Focussing. Setting Tripod stand is set on the ground firmly so that its top is at…
The following terms are used in direct method of levelling: (i) Plane of Collimation: It is the reduced level of plane of sight with respect to the datum selected. It is also known as ‘height of instrument’. It should not be confused with the height of telescope from the ground where the instrument is set. (ii)…
The following methods are used to determine the difference in elevation of various points: (i) Barometric levelling (ii) Hypsometric levelling (iii) Direct levelling and (iv) Indirect levelling. Barometric Levelling This method depends on the principle that atmospheric pressure depends upon the elevation of place. Barometer is used to measure the atmospheric pressure and hence elevation is…
Elevation measurements Elevation measurements involve measurements in vertical plane. It is also known as levelling. It may be defined as the art of determining the elevations of given points above or below a datum line or establishing given points of required heights above or below the datum line. OBJECT AND USES OF LEVELLING As stated…