Unit 2

COMPASS SURVEYING AND
PLANE TABLE SURVEYING
UNIT-2
D.PARTHIBAN-AP/CIVIL

SYLLABUS
• Prismatic compass - Surveyor's compass -
Bearing - Systems and conversions - Local
attraction – Magnetic declination - Dip -
Traversing - Plotting - adjustment of error -
Plane table instruments and accessories -
Merits and demerits - Methods - Radiation -
Intersection - Resection - Traversing

WHAT IS A COMPASS SURVEYING?
• Compass surveying is the branch of surveying in which the position of an
object is located using angular measurements determined by a compass and
linear measurements using a chain or tape.
• Compass surveying is used in following circumstances:
– If the surveying area is large, chain surveying is not adopted for
surveying rathercompass surveying is employed.
– If the plot for surveying has numerous obstacles and undulations which
prevents chaining.
– If there is a time limit for surveying, compass surveying is usually
adopted
• Compass surveying is not used in places which contain iron core, power
lines etc which usually attracts magnets due to their natural properties and
electromagneticpropertiesrespectively.
• Compass surveying is done by using traversing.
• A traverse is formed by connecting the points in the plot by means of a
series of straight lines.

Magnetic compass
• Magnetic compass is used to find out the magnetic bearing
of survey lines.
• The bearings may either measured in Whole Circle Bearing
(W.C.B) system or in Quadrantal Bearing (Q.B) system
based on the type of compass used.
• The basic principle of magnetic compass is if a strip of steel
or iron is magnetized and pivoted exactly at centre so that it
can swing freely, then it will establish itself in the magnetic
meridian at the place of arrangement.
• Major types of magnetic compass are:
– Prismatic compass
– Surveyor’s compass
– Level compass

Prismatic compass
• Prismatic compass is a portable
magnetic compass which can be
either used as a hand instrument
or can be fitted on a tripod.
• It contains a prism which is used
for accurate measurement of
readings.
• The greatest advantage of this
compass is both sighting and
reading can be done
simultaneously without changing
the position.

Major parts of a Prismatic Compass are
Magnetic needle
Graduated ring
Adjustable mirror
Sliding arrangement for mirror
Object vane
Eye vane
Metal box
Glass cover
Horse hair

Surveyor’s compass
• Surveyor’s compass consists
of a circular brass box
containing a magnetic needle
which swings freely over a
brass circle which is divided
into 360 degrees.
• The horizontal angle is
measured using a pair of
sights located on north –
south axis of the compass.
• They are usually mounted
over a tripod and leveled
using a ball and socket
mechanism.

Adjustments of compass
• Two types of adjustments:
– Temporary adjustment
– Permanent adjustment

Temporary adjustments
• Centering: it is the process of fixing the compass exactly
over the station. Centering is usually done by adjusting the
tripod legs. Also a plumb-bob is used to judge the accurate
centering of instruments over the station.
• Leveling: the instrument has to be leveled if it is used as in
hand or mounted over a tripod. If it is used as in hand, the
graduated disc should swing freely and appears to be
completely level in reference to the top edge of the case. If
the tripod is used, they usually have a ball and socket
arrangement for leveling purpose.
• Focusing the prism: Prism can be slide up or down for
focusing to make the readings clear and readable.

• Permanent adjustments
• They are done only in the circumstances
where the internal parts of the prism is
disturbed or damaged. They are
Adjustments in levels
Adjustment of pivot point
Adjustment of sight vanes
Adjustment of needle

Advantages & Disadvantages of
Compass surveying
• Advantages
 They are portable and light weight.
 They have fewer settings to fix it on a station
 The error in direction produced in a single survey line does not
affect other lines.
 It is suitable to retrace old surveys.
• Disadvantages
 It is less precise compared to other advanced methods of
surveying.
 It is easily subjected to various errors such as errors adjoining to
magnetic meridian, local attraction etc.
 Imperfect sighting of the ranging rods and inaccurate leveling
also causes error.

Errors in compass survey
• Errors can be arising due to various reasons
during the process of surveying, they are
classified as:
Instrumental errors
Personal errors
Natural errors

Instrumental errors
 As the name suggests they are arise due to the wrong adjustments of the
instruments. Some other reasons are:
 If the plane of sight not being vertical, it causes error in sighting and reading.
 If the magnetic needle is not perfectly straight or if it is sluggish, readingsmay
not be accurate.
Personal errors
They arise mainly due to the carelessness of the surveyor. They are:
 Inaccurate leveling
 Inaccurate reading
 Inaccurate centering
Natural errors
Natural errors are occurring due to the various natural causes which
affect the working of compass. It has nothing to do with the surveyor and to
minimize them, some corrections in calculations applied. They are:
 Local attraction
 Proximity to the magnetic storms
 declination

BEARINGS
Bearing is very important to get the
direction of a survey line

What is Whole Circle Bearing (W.C.B)?
• The horizontal angle made by a line with the
magnetic north in the clockwise direction is
the whole circle bearing of the line.
• This system is also known as the azimuthal
system.
• In this system, only the north direction is used
as reference meridian.

Whole Circle Bearing Example
• To understand the whole circle bearing measurement clearly, please
carefully observe the following image illustrations.
In the above images, the vertical line N is indicatingthe north line.
In Fig 1, line X is in the first quadrant.
It is at an angle of 46° with the north line in the clockwise direction.
Therefore, whole circle bearing (W.C.B) of line X is 46°.
On the other hand, the line Y in Fig 2, is making 226° angle with the north line in
the clockwise direction.
So, whole circle bearing (W.C.B) of line Y is 226°.D.PARTHIBAN-AP/CIVIL

Quadrantal Bearing (Reduced Bearing)
• The horizontal angle made by a line with the magnetic
north or south (whichever is closer from the line) in the
eastward or westward direction is the Quadrantal
Bearing or Reduced Bearing of the line.
• In quadrantal bearing or reduced bearing, both north
and south are considered as reference meridians.
• Depending upon the position of a survey line, the
direction of the reference meridian to the line can be
either clockwise or anticlockwise.
• In the expression of the reduced bearing value of a
line, quadrant has to be mentioned in which the line
lies.

Difference Between Whole Circle Bearing and Quadrantal
Bearing

PLANTABLE SURVEYING

What is Plane Table Surveying?
• The plane table surveying is the fast method of surveying.
• In this type of surveying plotting of the plan and field
observations can be done simultaneously.
• In case of plane table surveying Geometrical conditions of site
are manuscript in the map sheet using plane table and alidade
after that topographic details are arranged on the map.

Equipment Used in Plane Table Survey
General equipment used for conducting
planetable survey are
 Plane table
 Alidade for sighting (telescopicor
simple)
 Plumb bob and plumb fork
 Compass
 Spirit level
 Chain
 Ranging rods
 Tripod
 Drawing sheet and drawing tools
 Paper clips or screws

Plane table survey equipment is
arranged in 4 steps as follows
Fixing of Plane Table
Fix the plane table to the tripod stand. Arrange the drawing sheet on the
plane table using paper clips or thumb screws. The sheet should be in one position
from first to last.
Leveling of Plane Table
Plane table should be leveled using spirit level. For small works, eye
estimation can be ok.
Centering of Plane Table
The table should be centered by using plumbing fork. By which we can
arrange the plotted point exactly over the ground point.
Orientation of Plane Table
Whenever we are using more than one instrument station, orientation is
essential. It can be done by using compass or back sighting. In this case, the plane
table is rotated such that plotted lines in the drawing sheet are parallel to
corresponding lines on the ground.

Methods of Plane Table Surveying
• Generally there are four methods are available to
perform plane table surveying. They are
 Radiation
 Intersection
 Traversing
 Resection

Radiation
• In this method, plane table
is located at one point “o”
as shown in fig. and
perform the whole from that
point.
• From point O, sight the
points A,B,C,D and E using
alidade, locate and plot the
points as a,b,c,d and e in the
drawing sheet.

Intersection
• In this method we can locatethe
point by plottingtwo rays from two
known stations. As shown in figure,
P and Q are the known station.
• First the equipment is placed on P
and plot the lines by sighting the
stationsA, B and Q. then shift the
equipment to station Q and plot the
lines by sighting stationsA, B and P.
• Finally, the intersection ofA and B
rays is the required locationof point
of intersection.

Traversing
• Traversing is the connection of series of straight
lines.
• In case of traversing, plane table is located at one
point for suppose A as shown below.
• From that point sight towards B and measure the
distance AB.
• Then shift the plane table to point B and sight
towards A and measure BA.
• Average distance of AB and Ba are plotted to
scale in drawing sheet.
• Then Sight the point C from B and measure BC
and repeat the same procedure until last point.
• Conduct some checks at some points.
• Finally traverse lines are plotted on the drawing
sheet.

Resection
• Resection is a method of plane table surveying
in which location of plane table is unknown
and it is determined by sighting it to known
points or plotted points.
• It is also called method of orientation and it
can be conductedby two field conditions as
follows.
The three-point problem
The two-point problem

The Three Point Problem
• In this condition, three points and their positions in the
field are known.
• Plane table is placed at apposition from where all the
three points are visible.
• So, by sighting those three points we can locate the
point where equipment is located.
• This can be achieved by many methods as follows.
 Tracing method
 Lehmann method
 Analytical methods
 Graphical method

Tracing Method in Plane Table
Surveying
• In tracing method, plane table is located
at a point from where three points are
visible.
• The table is oriented with respect to the
plotted lines of those three points.
• Place the tracing paper on the drawing
sheet and again sight the three points
and plot the radiating lines.
• The tracing paper is then moved above
the drawing sheet until the three
radiating lines pass through
corresponding points previously plotted
on the map.
• Finally, the position of plane table is
marked.

Lehmann Method
• In this method, Plane table is
located at a point P and sight the
station A, B and C and plot the
rays Aa, Bb, and Cc.
• The rays form small triangle
which is called triangle of error.
• Another point P1 is chosen to
reduce the error and sight the
point A from P1 similarly to B
and C.
• which will give another triangle
of error.
• Repeat this procedure until error
becomes zero.

Analytical Methods
There are many analytical methods are
developed in three-point problem condition.
In this method, from station P A, B and C are
sighted and note the values of angles and lengths.
From these values determine the position of
unknown points by using analytical formulae.
Graphical Method
In graphical method also, angles and lengths
are determined and represented it on a graph and
determines the location of plane table.

The Two-Point Problem
• In the two-point problem, two points are
sighted from other point corresponding to the
points given in plane table sheet.
• Here two cases are to be discussed.

Case 1: when the points can be occupied by the plane table
• As shown in fig. A and B are the
two points corresponding to the
points a and b.
• Now, plane table is located at B
and oriented by sighting A.
• sight C from B and bx is plotted on
the sheet.
• Then shift the plane table to C,
oriented by backsighting B along
xb.
• Then alidadeis placed over a and
sight station A, then line Aa cuts
the line bx at somewhere which is
located as point c at station C.

Case2: When the plane table cannot occupy the controlling
stations
• In this case, an auxiliary point D is
considered nearer to C.
• Locate the plane table at D according to
the line ab parallel to AB.
• Then sight the station A and B
corresponding to a and b.
• the rays drawn are intersected at some
point which is marked as d. then sight
towards C by placing alidade at d.
• mark the distance Dc as c1.
• Shift the table to C and backsight to D
with reference to c1.

• Then sight A corresponding to
a, the ray drawn is intersects
the previously drawn ray from
D in c2.
• From c2 sight B draw a ray
which intersects db and marked
the intersection as b1.
• The table is oriented till ab
comes in line with P. From P
sight and draw rays Aa and Ba.
• The intersection of these two
rays will give the Location of
Point C. D.PARTHIBAN-AP/CIVIL

Unit 2

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