PRISMATIC COMPASS SURVEY

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PRISMATIC COMPASS SURVEY
We have seen how chain surveying is based on the measurement of horizontal distances. Even if where measurement of angles becomes necessary we resort to measurement of tie lines. Suppose we have to construct a quadrilateral ABCD. If we know the angles A, B, C and D and any two of the sides we can make this figure. Another way we can draw this figure is by knowing the angles of each line AB, BC, CD, and DA and AAC make with a north south line. In the later case we will need to know the length of only one line. The prismatic compass surveying is based on principles of measuring the angles that different lines make with respect to N-S line. Only one line is measured to maintain the scale relationships between the ground and map details.

Prismatic compass
To determine the angle each line makes with a north –south line we make use of prismatic compass

Figure 1           Prismatic Compass
                           

Prismatic compass is like any other ordinary magnetic compass except that it has a prism, an eye and an object vane with a hair line at the other. The eye vane the centre of the compass and the thin wire fixed vertically in the object vane make a straight line. Both the prism and the sight vane are hinged so that they can be folded horizontally. A magnetized needle is pivoted at the centre. One end of this magnetized needle is colored or made distinct.

The colored end points to the magnetic north. Usually there are two arrows of graduations on the disk.
The inner row has its North point 0o       360o     and south point 180˚. In the outer row the 0˚ or 360˚are marked at the south point and figures are given in reversed images the needle moves to point the Magnetic north, the disk also moves along with it so that when we through the prism we read the angles that the object sighted makes with the magnetic north to facilitate the sighting of the luminous objects the prism is fitted with a pair of coloured glasses which can be fixed on the line of sight

In addition to these, a sliding plane mirror can be adjusted to any angle and thus the bearing of the objects which lie too far below or high up the plane of the compass can be conveniently read.

The angles read with the help of prismatic compass are called bearings. A bearing is an angular measurement with respect to a reference direction. If the reverence direction is a magnetic north the bearing are called magnetic bearing (MB). If it is true north the true bearing the bearing is called true bearing (TB) In case of prismatic compass survey we deal with magnetic bearing (MB) only


PROCEDURE
The first step in prismatic compass survey as in all other lower order is to prepare a sketch showing the object s for which the bearing has to be recorded. Then a base line is selected. The base line should be such that its two ends are inter visible and easily accessible. The object to be located should also be clearly from both ends.

Figure 2           Sketch Map of the Surveyed Area


In figure 2 above, AB is the base line and CD, EF and G are the point for which bearing are to be noted. To start with, the compass is at A. The compass must be in the horizontal position to enable the magnetic needle to swing freely. Lift the prism and allow the needle to come to rest. Fix a ranging pole at point B look through the prism and make the hairline of the object vane intersect the ranging pole. Read the bearing of AB and record it on the field book. The next step is to measure the length of AB. It should be also recorded field book Now place the compass at point B.








Figure 3           The Field Book

After leveling read the bearing from B. This bearing is called Back bearing where as the bearing from A is called Forward Bearing (FB). The difference between the two bearings should always be 180˚ 

Read the bearing of other bearing objects also and note them in the field book. In many cases the object selected for representation on a map are so close to the traverse lines that the recording of the bearing is not necessary. We can use those objects as offset lines meet the line of sight to the object to the as in chain surveying.

The distances at which the offset lines meet the line of sight can be recorded in the same way as in the case of chain surveying.  Compass survey is also of two types like chain surveying.

Open traverse
Is one in which a surveyor does not come back to the starting point as in case of route survey.
The surveyor starts from point A, reads the forward bearing of B and moves on to point B after measuring distance of line AB He then takes the BB of A,. And the FB of C and repeats the process of measuring the line BC and moving to point  C and then to point D. while measuring the distances of lines he also takes the offsets whenever necessary.

Closed traverse
In a closed traverse the surveyor comes back to the starting point. if the starts from A he comes back to A after passing through other points as shown in figure 4.











Figure              4          Closed Traverses


After field work is over, the surveyor comes to the laboratory to plot the data recorded in the field book
The first thing that he does is to mark a magnetic north – south line at the starting point. From that point and with reference to N – S line he mark the respective bearing recorded in the field book. For example, in figure 5 our starting point is A.

If the FB is 10˚ then he will mark this and draw a line AB to its proper length. he then move to point B and repeat the process for other lines until we come back to point A. If bearing are taken and recorded correctly, and there are no mistakes in plotting the line BA and EA will meet at a common point A.

But more often then not they would not meet and may deviate as shown in figure 6

SOURCES OF ERRORS AND METHODS OF CORRECTION

The errors in prismatic compass surveying may be due to one of several of the following reasons
1.                              Presence of iron in the area surveyed
2.                              Defects in the compass
3.                              Errors in observation
4.                              Wrong recording in the field book
5.                              Wrong plotting

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