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Adjustment of Closed traverse
When a closed traverse is plotted from the field measurements, the end station of a traverse generally does not coincide exactly with its starting station. This discrepancy is due to the errors in the field observations i.e. magnetic bearings and linear distances. Such an error of the traverse is known as a closing error or an error of closure.
When the angular and linear measurements are of equal precision, graphical adjustment of the traverse may be made. This method is based on Bowditch’s rule. Corrections are applied to lengths as well as to bearings of the lines in proportion to their lengths.
Let l = length of any leg
Σ l = total length of the traverse
Σ L = total error in latitude
Σ D = total error in the departure
δL = correction to the latitude of the leg
δD = correction to the departure of the leg
Then as per Bowditch’s rule
A curve that consists of two opposite circular arcs of the same or different radii, is known as a reverse curve. In such curves, the centres of the arcs are on the opposite sides of the curve. The two arcs turn in opposite directions with a common tangent at the junction of the two arcs.
Latitude & Departure
Latitude is the north-south component of a line; departure the east-west. North latitudes are positive, South are negative; similarly, East departures are positive, West are negative.
The branch of levelling in which the difference of elevations of two points is determined from the observed vertical angles and measured horizontal distance is called trigonometrical levelling. The vertical angles are generally observed by a theodolite and horizontal distances are either measured directly or computed trigonometrically.
There are two methods of observation in trigonometric levelling.
- Direct Method This method is useful where it is not possible to set the instrument over the station, whose elevation is to be determined. Ex: To determine the height of the tower. In this method the instrument is set on the station on the ground whose elevation is known.
- Reciprocal Method In this method the instrument is set on each of the two stations alternatively and observations are taken. The difference in elevation between two stations A and B is to be determined.
Adjustments of Theodolite
The adjustments of a theodolite are of two kinds :
- Temporary Adjustments.
- Permanent Adjustments
The adjustments which are required to be made at every instrument station before making observations are known as temporary adjustments.
The temporary adjustments of a theodolite include the following :
- Setting up and centring the theodolite over the station.
- Levelling of the theodolite.
- Elimination of the parallax.
- Adjustment of the horizontal plate level.
- Adjustment of the horizontal axis (or trunnion axis).
- Adjustment of the telescope.
- Adjustment of the telescope level.
- Adjustment of the vertical circle index.
Obstacles in chaining
Various types of obstacles generally met during chaining, may be overcome by any one of the following methods.
Obstacles to chaining are of the following types:
- Obstacles that obstruct ranging but not chaining: In this type of obstacle, the ends of the chain line are not intervisible. Such obstacles are generally met in undulating terrain where the area consists of rising grounds, intervening hills or undulations.
- Obstacles that obstruct chaining but not range: The typical types of obstructions under this category are generally large water bodies, i.e., lakes, ponds, rivers, etc. where distances between two convenient points on the survey line on either side of the obstacles are required to be determined.
- Obstacles that obstruct both ranging and chaining: In such cases prolonging the line beyond the obstacle and finding the distance across it, may be overcome by any one of the following methods.
When intermediate ranging rods are fixed along the chain line, by direct observation from either end station, the process is known as ‘Direct Ranging’.
Following steps are taken in the direct range:
- Erect ranging rods or poles vertically behind each end of the line.
- Stand about 2 m behind the ranging rod at the beginning of the line.
- Direct the assistant to hold a ranging rod vertically at arm’s length at the point where the intermediate station is to be established.
- Direct the assistant to move the rod to the right or left until the three ranging rods appear to be exactly in a straight line.
- Stoop down and check the position of the rod by sighting over their lower ends in order to avoid error due to the non-verticality of the ranging rods.
- After ascertaining that the three ranging rods are in a straight line, signal the assistant to fix the ranging rod.
Local Attraction and its detection
The North end of a freely suspended magnetic needle always points to the magnetic north, if it is not influenced by any other external forces except the earth’s magnetic field. It is a common experience that the magnetic needle gets deflected from its normal position if placed near magnetic rocks, iron ores, cables carrying current or iron electric poles.
Such a disturbing force is known as ‘Local attraction’. Magnetic bearings are, therefore, not reliable unless these are checked against the presence of local attractions at each station and their elimination.
Detection of Local Attraction
The presence of local attractions at any station may be detected by observing the fore and back bearings of the line. If the difference between fore and back bearings is 180°, both end stations are free from the local attraction.
- By calculating the included angles at the affected stations.
- By calculating the local attraction of each station and then applying the required corrections, starting from the unaffected bearing
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