PRISMATIC COMPASS SURVEY
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
Comments
Post a Comment