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…
Advantages are 1. Possibility of omitting measurement is eliminated. 2. The surveyor can compare the plotted work in the field then and there only. 3. Irregular objects are plotted more accurately, since they are seen while plotting. 4. Booking errors are eliminated. 5. Local attractions do not influence the plotting. 6. No great skill is…
The errors may be grouped into the instrumental and personal errors. Instrumental Errors 1. The surface of plane table not perfectly plane. 2. Bevelled edge of alidade not straight. 3. Sight vanes of alidade not perfectly perpendicular to the base. 4. Plane table clamp being loose. 5. Magnetic compass being sluggish. 6. Drawing sheet being…
The following four methods are available for carrying out plane table survey: 1. Radiation 2. Intersection 3. Traversing 4. Resection. The first two methods are employed for locating details while the other two methods are used for locating position of plane table station on drawing sheet. Radiation After setting the plane table on a station,…
The most commonly used plane table It consists of a well-seasoned wooden table Top mounted on a tripod. The table top can rotate about vertical axis freely. Whenever necessary table can be clamped in the desired orientation. The table can be levelled by adjusting tripod legs. The following accessories are required to carry out plane table survey:…
A freely suspended and properly balanced magnetic needle is expected to show magnetic meridian.However, local objects like electric wires and objects of steel attract magnetic needle towards themselves.Thus, needle is forced to show slightly different direction. This disturbance is called local attraction. The list of materials which cause local attraction are: (i) magnetic rock or iron ore, (ii) steel structures, iron…
The magnetic meridian and the true meridian may not coincide with each other in a place. The horizontal angle between these two meridians is known as magnetic declination. The magnetic north at a place may be towards east or west of true north . If it is towards east, it is known as eastern or +ve declination. Western…