Plane table survey

Plane table survey

Plane table

The plane tables in an instrument used

for surveying by a graphical method in

which the field work and plotting are

simultaneously.

• its main features is the topographic features to be mapped

are in full view. Hence no chance of

missing of any important detail.

• It is suitable for small and medium

scale-mapping (1:10,000 to 1:2,50,000).

Where the great accuracy is not

required. It is also used for plotting the

topographical maps in the field.

• Before commencing a plane table

survey, the instrument stations are

fixed to cover the entire area.

• These stations may be fixed by

surveying a trigonometrically framework,

establishing a network of control points

on a pattern to suitable at the scale at which

plane tabling is carried out.

• The elevations of these points are

determined with the help of leveling.

• A surveyor starts filling in details from

any of these control points, one by one,

and traverses all the control points.

• The finished maps so produced are

known as topographic maps.

• This graphical method of producing

topographic maps is known as

cartographic surveying.

• It should be noted that all the

measurements made are plotted

directly on the drawing sheet instead of

recording in the field work.

• The principle used in plane table

surveying is that an unknown point of

interest can be established by measuring

its directions from known points.

 ADVANTAGES

1. The sighting and plotting are done

simultaneously. Therefore there is no

risk of omitting necessary details.

2. The error and mistakes in plotting can

be checked by drawing the check lines.

3. Irregular objects can also be plotted

accurately as it is .

4. It is most rapid and useful for filling in

details.

5. No great skill is required.

6. It is less costly in comparison to

theodolite survey.

7. It is very advantageous in areas, where

compass survey is not reliable e.g. area

effected by magnetic fields.

 DISADVANTAGES

1. It is not suitable for, work in a wet

climate & in a densely wooded country.

2. The absence of measurements (field

notes) causes inconvenience, if the survey

is to be replotted to some different scale.

3. Plane table is heavy and awkward to

carry and the accessories are likely to

be lost.

4. It does not give very accurate results.

ACCESSORIES USED IN PLANE TABLE 

SURVEYING 

(1) Plane table board

(2) Tripod

(3) Alidade

(4) Trough compass

(5) Spirit level

(6) Plumbing fork

(7) Drawing sheet

1. Plane Table Board

• The drawing board is carefully made of

well-seasoned wood so that counteract

the effect of warping and damages due

to weathering.

The upper surface is kept smooth.

• The table at the centre of the underside,

is attached to the tripod with the help of

a screw and wing nut. By means of the

wing nut, table can be clamped in any

position.

• Plane tables are available in the

following different sizes.

Designation Size(mm × mm)

B0 1500 × 1000

B1 1000 × 700

B2 700 × 500

B3 500 × 350

2. Tripod

• An open frame type light tripod is

usually used in the simplest form of

plane tables, surveying of the board is

achieved with the tripod legs and

checking the horizontality of the board

with the help of two spirit levels fixed at

right angles to each other in a block of

wood.

3. Alidade

• An alidade is a straight –edge ruler

having some sighting device. It is used

for sighting the objects and drawing the

lines.

• Plain Alidade, it is a straight-edge ruler

about 450 mm long, made of a metal or

wood. One of the edges is beveled and

graduated. The alidade is provided with

a sight vane at each end. The sight

vanes have hinges at the lower end so

that they can be folded down on the

ruler when not in use. One of the sight

vanes is provided with a narrow slit and

is used as an eye vane. The object vane

is open and it carries a hair or thin wire

at its centre. The two sight vanes is

open and it carries a hair or thin wire at

its centre. The two sight vanes provide

a definite line a sight.

• The beveled edge of the ruller is also

known as the fiducial edge. The line of

sight of the alidade is in the same plane

as that of the fiducial edge or in a plane

parallel to it.

4. Trough Compass

• Generally it is 15 cm long and is

provided to plot the magnetic meridian

(N – S direction) to facilitate orientation

of the plane table in the magnetic

meridian.

• At the extremities of the trough

compass, there are graduated scales

with zero at the centre and marking

upto 5° on either side of the zero line.

Longer sides of the trough compass are 

parallel and plane such that they can be 

used as a ruler for drawing the line or 

for placing the compass such that it 

coincides with a line already drawn on 

the drawing sheet.

5. Level Tube/Spirit Level 

• The essential condition in plane table 

surveying is that the board should be 

level, This level tube is either tubular or 

of the circular type.

• It is placed on the board in two 

positions mutually at right angles and 

the bubble is centered in each position 

to make the board horizontal.

6. Plumbing Fork 

• A plumbing form is a U-shaped piece of 

metal frame. It is used for the centering 

of the plane table over the station.

• One end of the frame is pointed and is 

kept over the drawing sheet touching 

the plotted position of the instrument 

station while the other end of the frame 

carries a plumb bob.

• The position of the plane table is 

adjusted until the plumb bob hangs 

over the station occupied by the 

instrument.

• Use of a plumbing fork is justified only if 

the plotting is done at large scale and 

the rays being short. Whereas small–

scale mapping, which is usually done 

with a plane table, the use of plumbing 

fork is a sheer waste.

7. Drawing Sheet 

• The drawing sheet used should be of 

the best quality to withstand 

rubbing and scrubbing.

SETTING UP THE PLANE TABLE 

Following operations are included in 

setting up of the Plane Table: 

1. Centering 

• It is the operation of bringing the 

plotted station point exactly over the 

ground station. A plumbing fork is used 

for checking the centering.

• For small-scale mapping, an error in 

centering of about 30 cm is permissible.

2. Leveling 

• It is the operation in which the plane 

table is madeimade to fall in horizontal plane. 

• Level the board with the help of a spirit 

level. 

3. Orientation 

• It is the operation in which  the plane is kept parallel to the position it occupied at 

the first station.

• In such a case all the lines plotted will 

be parallel to the corresponding lines 

on the ground. If the position of board is 

different at successive stations, the 

relative positions of the plotted details 

will not remain same as the relative 

positions of the details on the ground.

• As a result, the plotted work of the 

previous stations cannot be connected 

to that of the successive stations.

• During orientation the table is rotated 

and the plotted position of the 

instrument station is also disturbed and 

shifts relative to the ground stations.

• Therefore operations of orientation and 

centering are therefore interrelated. 

Orientation of plane table can be close 

using a trough compass, back sighting 

or by resection.

METHODS OF PLANE TABLE 

ORIENTATION

1. By Trough Compass

• A trough compass is placed on the top

right side corner of the plane table in

such a way that the magnetic needle

points exactly towards the N – S

direction.

• Draw a line parallel to the edge of the

compass.

• Shift and set the plane table on the

Succeeding station. Place the trough compass

along the N – S line.

• Rotate the table till the magnetic

needle coincides with N – S line

previously drawn.

2. By Back Sighting

• This is the most accurate method of

orientation.

• In this method plane table is fixed on a

new station and the alidade is placed

against the line joining the new station

with the preceding station and the table

is rotated until the line of sight bisects

the previous station.

• To get, this let the plane table is

shifted from station A to B and let the

line ab has been plotted with the plane

table at A.

• Set the table on station B, place the

alidade along the plotted line ba and

rotate the till until the line of sight ba

bisects the station A. Clamp the board

along this line of sight. The line ba truly

represents the line BA on the ground.

3. Resection

• The method of resection will be

discussed later in this chapter in details.

METHODS OF PLANE TABLE SURVEYING

The methods of surveying with a plane 

table are

• Radiation,

• Traversing,

• Intersection and

• Resection.

1. Radiation

• In this method the instrument is setup

at a station and rays are drawn to

various stations which are to be plotted.

• The distances are cut to a suitable scale

after actual measurements.

• This method is suitable only when the

area for which surveying is done is small and all the desired stations to be plotted are clearly visible and accessible from the

instrument station.

• The scope of the method is increased

when the distances are measured with

the help of a tacheometer.

2. Traversing

• This method is similar to compass or

theodolite traversing.

• The table is set on each of the station one by one.

• A foresight is taken to the next station

and required distance is cut according

to a suitably chosen scale.

• This method is most suited when a

narrow strip of terrain is to be

surveyed, e.g. survey of roads, railways,

etc.

• This method can be used for traversing

both the open as well as close traverses.

3. Intersection

• In this method two stations are selected 

such that all the other stations to be

plotted are visible from these. 

• A line joining these two stations is 

called base line. The length of this base 

line is measured very accurately. 

• Rays are drawn from these stations to 

the station to be plotted.

• The intersection of the rays from these 

two stations gives the position of the 

station to be plotted on the drawing 

sheet. 

• Sometimes, this method is also called as 

graphical triangulation. 

• This method is most commonly used for 

plotting details. It is preferred when the 

distance between the stations is too 

large, the stations are inaccessible, or 

the ground is undulating. 

For example is of broken boundaries, 

which can be very conveniently plotted 

by this method.

4. Resection 

• This method of orientation is employed 

when the plane table occupies a 

position not yet plotted on the drawing 

sheet.

• Resection can be defined as the process 

of locating the instrument station 

occupied by the plane table by drawing 

rays from the stations whose positions 

are already plotted on the drawing 

sheet.

• The point representing the resection of 

two rays will be the station to be 

located, provided the orientation at the 

station to be plotted is correct, which is 

seldom achieved.

• This problem can be solved by any of 

the methods such as resection after 

orientation by back ray, by two points, 

or by three points. 

• This method is employed when 

surveyor feels that some important 

details can be plotted easily by choosing 

any station other than the triangulation 

stations.

• The point  of such a station is fixed on 

the drawing sheet by resection. 

Three point problem: Locating the 

position of station on the plan by 

means of observation to three 

well defined points whose positions 

have been previously plotted on the 

plan. 

It involves minimizing the error triangle 

to a single point by section from three 

known points on grand as well as sheet.

Two Point Problem 

• marking the position on the plan, of the 

station occupied by the plane table by 

means of observation to two well 

defined points whose positions have 

been plotted on plan.

• Use of ah auxiliary point near the 

station is done for orientation and 

plotting of station at desired location.

Errors in Plane Table Surveying : 

1. Instrumental errors

2. Errorinpletting

3. Error due to manipulation and sighting 

a) Non horizontality of board. 

b) Detective sighting 

c) movement of board between sights. 

d) Detective of inaccurate centering.