To plot the plan of a given area by compass traversing.

Objective:

The primary objective of this job is to plot the plan of a given area on the ground employing compass traversing.

Apparatus:

• Prismatic compass
• Tripod stand
• Chain
• Fiber glass tape
• Ranging rods
• Wooden pegs
• Plumb bob
• Mallet

Least count of the instrument:

Least count of the prismatic compass = 30’ or 1/2 ⁰

Least count of fiber glass tape = 0.01 m

Least count of metric chain = 0.02 m

Related theory:

Traverse

A traverse is a series of connecting straight lines that join two ground stations. The endpoints of a traverse are called as traverse stations and the straight lines joining two consecutive stations are termed traverse legs. There are two types of traverses:

1. Open traverse
2. Close traverse

Compass traversing

The process of employing a compass to determine angular measurements of a traverse along with chaining or taping to measure lengths is called compass traversing. By observing the magnetic bearings of both sides, the traverse angle between two consecutive legs is calculated.

Compass traversing is widely adopted for surveying large and rough terrain with undulations. But, before conducting a compass survey, it must be confirmed that the area to be surveyed is free from external magnetic disturbance.

Field procedure of compass traversing

Compass traversing consist of the following steps:

1. Reconnaissance
2. Preparation of index sketch
3. Marking the station on the ground
4. Measurement of bearings of traverse legs

Plotting of compass traverse

A compass traverse can be plotted by following various methods:

• By parallel meridian through each station
• By considering included angles
• By considering the central meridian

Procedure:

Fieldwork

• Along the boundary of the given area, select the stations A, B, C, D & E as shown in the figure below.
• Choose traverse stations in such a manner that each station must be visible from any traverse station on sighting.
• Fix the prismatic compass on the tripod stand and set it up on station A. Level the instrument to allow the needle of the compass to swing freely.
• To sight the back station E, rotate the case and observe the whole circle bearing of line AE. Note down this reading in the table.
• Next, rotate the case to sight the forward station B and observe the whole circle bearing of line AB. Record this value in the table.
• Measure the distance of line AB with the help of a chain or tape and record this value of length as well.
• Shift the prismatic compass fitted on the tripod stand to the forward station B and continue to follow the same procedure to record whole circle bearings and lengths as done in previous steps.
• Continue to move towards forwarding stations till the work on stations C, D and E is also completed.

Graphical Plot

• To remove the error due to the local attraction, use the observations recorded in the table to obtain the corrected whole circle bearings.
• Draw a line on the drawing sheet to show the reference direction of the magnetic meridian.
• Mark the starting point A.
• Place the circular protractor on the sheet such that its center is at starting point A and the zero line coincides with the reference direction.
• Mark the correct bearing of line AB on paper with a protractor.
• Draw the line AB according to scale and plot the position.
• To plot the direction of BC, place the center of the protractor at B and change its orientation by rotating in such a way that it’s zero direction becomes parallel to the reference direction as before. This will be achieved when the line BA cuts the protractor at the corrected bearing of BA.
• Mark the BC bearing and plot C like B was plotted.
• Continue the process till all stations are plotted.

The figure ABCDEA’ is now plotted on paper. This figure doesn’t truly represent the actual figure because it gives two positions for A but in reality, only one point exists on the ground.

This apparent displacement of A is due to the accumulation of error in surveying and plotting around the traverse.

The total error in the figure is shown by the line AA’ and it may be distributed back in the figure graphically by the following steps:

• A’ should be at a and must be moved the distance AA’ in the direction shown. This movement will allow to move the plotted position of the other points proportionally along the parallel directions.
• Parallel to the direction of closing error, draw lines through the other plotted points.
• Draw a straight line and scale off the lengths of the traverse legs along with it. The scale of this line can be different from the original scale and it is suitable to choose a smaller scale for this construction.
• Set out perpendiculars at each point along the line. Pick the length of the closing error on the plot using a pair of dividers. Mark it on the perpendicular erected at A’ and join aA.
• The intersection of aA with the perpendiculars represents the amount of adjustment required for each station.
• The amount of error at E, being eE, is measured from the diagram and shifted to the line drawn through E on the plot parallel to the closing error, giving the adjusted position e. In the same way, other errors at each station are transferred to the plot.
• Join the positions of the adjusted points which gives the new figure AbcdeA that is graphically adjusted and represents the actual layout on the ground.

Observations & Calculations:

Results & Discussions:

The plan of the given area on the ground has been successfully plotted through compass traversing.

Precautions:

• All the equipment should be handled with caution.
• Safety measures like safety vests, protective footwear, and helmet should be worn in the field.
• The center of the prismatic compass should coincide with the station point.
• If the reflection of light with a prismatic lens occurs, use the colored lens of the compass.