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 computed. However it is not accurate method since the atmospheric pressure depends upon season and temperature also.
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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 computed.
However it is not accurate method since the atmospheric pressure depends upon season and temperature also. It may be used in exploratory surveys. Hypsometric Levelling This is based on the principle that boiling point of water decreases with the elevation of the place. Hence the elevation difference between two points may be found by noting the difference in boiling point of water in the two places.
This method is also useful only for exploratory survey. Direct Levelling It is common form of levelling in all engineering projects. In this method horizontal sight is taken on a graduated staff and the difference in the elevation of line of sight and ground at which staff is held are found. Knowing the height of line of sight from the instrument station the difference in the elevations of instrument station and the ground on which staff is held can be found.
This method is thoroughly explained in next article. Indirect Methods In this method instruments are used to measure the vertical angles. Distance between the instrument and staff is measured by various methods. Then using trigonometric relations, the difference in elevation can be computed. This is considered beyond the scope of this book. One can find details of such methods in books on surveying and levelling.
Terms Used In Direct Method of Levelling 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 should not be confused with the height of telescope from the ground where the instrument is set. It is always the first reading after the instrument is set in a place. It is also known as plus sight, since this reading is to be added to RL of the point Benchmark or change point to get plane of collimation.
The intension of taking these readings is to find the reduced levels of the points where staff is held.
This is also a minus sight. This is a point on which both fore sights and back sights are taken. After taking fore sight on this point instrument is set at some other convenient point and back sight is taken on the staff held at the same point. The two readings help in establishing the new plane of collimation with respect to the earlier datum. Since there is time gap between taking the two sights on the change point, it is advisable to select change point on a well defined point.
On this phenomenon method of barometric leveling is built. Definition of barometric leveling — glossary Levelliny method is thoroughly explained in next article. It is, in addition to temperature and pressure also takes into account the absolute humidity and latitude of the measuring point, that is, in practice it seems to be not in use. Recent Comments France Haule. Barometric stage — the height at which we must ascend lfvelling the pressure drop by 1 mm Hg That is, we took and simplified Babinet formula to expression and calculate h for different values of temperature and pressure. Taking the average pressure of mmHg and using barometric table we get In this method horizontal sight is taken on a graduated staff and the difference in the elevation of line of sight and ground at which staff is held are found.
Definition of barometric leveling
Barometric Leveling Analysis