Question

List any four methods of plotting a compass traverse survey & explain any one in brief. Also state the meaning of adjustment of closing error of traverse.

Answer 1

Methods of plotting compass traverse are:

1. By parallel meridian through each station.

2. By included angle.

3. By paper protractor.

4. By rectangular co-ordinates.

5. Plotting by tangents.

(1) By Parallel Meridians through each station: (Fig.4.20) Having fixed the position of the starting station A suitably on the paper, a line representing the magnetic meridian is drawn through it. The bearing of the line AB is plotted with an ordinary protractor and its length is marked off with the scale, thus fixing the position of the station B. Through B a meridian is drawn, the bearing of BC is set off and its length measured off with the scale. The process is repeated at each station until all the lines are drawn. If the traverse is a closed one, the last line should end on the starting station A; if it does not, the discrepancy is referred to as the “closing error".

(2) By Included Angles: Fig.4.21) In this method the meridian is drawn through the starting point A and the bearing of the line AB plotted and its length laid off with the scale, thus fixing the point B. At B the included angle ABC as calculated, from the bearings of AB and BC, is plotted with a protractor and the length of BC is measured off with the scale. The operation is repeated at each of the succeeding stations.

 

(3) By Paper Protractor: (Fig.4.22). This method consists of plotting the bearings of all the lines at any point in the centre of the paper with reference to the meridian by using a large circular paper protractor, and then transferring these directions to their proper positions by drawing parallel lines with the help of a parallel ruler. Having marked the point O in the centre of the paper, draw a line through O to represent the meridian. Place the protractor with the 0 and 180° graduations coinciding with the line. At O plot the bearings of all the lines with reference to the meridian. Having settled the position of the starting point A, draw a line AB through it parallel to its bearing marked at O with the help of a parallel ruler and measure its length with the scale, thus fixing the point B as in fig.4.23. Proceed similarly until all the lines are drawn. This method is a compass traverse.

 

(4) By Rectangular Co-ordinates: (Fig.4.24) In this method each of the points of the traverse is plotted by its co-ordinates with reference to two lines drawn through some Convenient point at right angles to each other. These lines are known as the axes of co-ordinates and their point of intersection is called the origin of co-ordinates. One of the axes OX called the X-axis represents the north and south line, (true, magnetic or arbitrary) and the other OY known as the Y-axis is a line at right angles there to, and represents the east and west line. Any point may be plotted by measuring with a scale X or Y co-ordinate along the X or Y axis and laying off the other co-ordinate on the line drawn at right angles at this point. The advantage of this method is that each point is plotted independently with reference to the meridian and the line at right angles to it through a common origin and not with regard to the preceding one. Consequently, if any point is wrongly plotted, the position of any of the succeeding points is not thereby affected. The errors of plotting cannot, therefore, accumulate. Also the position of each point can be checked by scaling the distance between the point and the preceding one and by comparing it with the length measured in the field. 

 

(5) Plotting by Tangents: In this method the angles between the various lines are plotted by geometrical construction with the help of a table of natural tangents. Having fixed the position of the starting point, a line representing the meridian is drawn through it (always pointing to the top of the paper) as in fig.4.25. To plot the bearing of the first line AB, a length ABI of 20 cm is marked off on the meridian the bearing of the line AB (cm) is then laid off on this perpendicular. The line joining the points A and B2 determines the direction of the first line AB. On this line is scaled off the length of AB, thus fixing the position of the point B. The line AB is then produced to C1 making BC1 equal to 20 cm. At C1 a perpendicular is erected and the distance C1 and C2 equal 20 X tangent of deflection angle at B (cm) is scaled off on the perpendicular .The line connecting the points B and C2 gives the direction of the line BC. To mark the point C, the length of BC is marked off with the scale on BC2. Other lines are similarly plotted, marked of with the scale on BC2. Other lines are similarly plotted. If there is no room for a 20 cm base, a shorter base of 10 cm may be used.

 

Adjustment of closing error of traverse:

 

Explanation : 1. To distribute the closing error AA1 (Fig. a), draw one horizontal line of length equal to perimeter of traverse with some reduced scale. 2. Now mark the survey stations on it proportionally (Fig. b) and transfer closing error of same length using roller scale to point a. 3. Join the point A and A1 with straight line. Also draw parallel lines at point b, c, d and e. 4. Transfer B1b, C1c, D1d and E1e to point B1, C1, D1 and E1 respectively in compass traverse. 5. Finally join new points to get corrected traverse ABCDEA after graphical adjustment of closing error.