Surveying (CE3G) - Plane Table Surveying.ppt

Survey

1. SURVEYING
PLANE TABLE SURVEYING
BY S N NANAWARE
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PLANE TABLE SURVEYING
 Plane Tabling is a graphical method of surveying in which the field
work and plotting are done simultaneously.
 Useful to fill in details between stations fixed by triangulation or
theodolite traversing.
 Particularly adapted for small scale or medium scale mapping in
which great accuracy in detail is not required.
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The plane table consists essentially of:
1.A drawing board mounted on a tripod and
2.A straight edge called an alidade.

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THE DRAWING BOARD
 Made of well-seasoned wood such as teak or pine.
 Size varies from 40cm x 30cm x 75cm x 60cm or 50cm to 60 m
square.
 It is mounted on a tripod in such a manner that it can be
leveled, and revolved about a vertical axis and clamped in any
position.
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PLAIN ALIDADE
 Consists of a metal (brass or gunmetal) or boxwood straight edge or
ruler about 50 cm long.
 The beveled (ruling or working) edge of the alidade is called the
fiducial edge.
 It consists of two vanes at the ends, the vanes are hinged and can be
folded when the alidade is not in use.
 One of the sight vanes is provided with a narrow slit and the other
with a central vertical wire or hair.
 One of the vanes known as sight vane is provided with a narrow slit
with three holes, one at the top, one at the bottom and one in the
middle.
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PLAIN ALIDADE
 The other vane which is known as object vane, is open and carried a
hair or a fine thread or a thin wire stretched between the top and
bottom of the slit.
 With the help of the slit, a definite line of sight may be established
parallel to the ruling edge of the alidade.
 The length of the ruling edge should be equal to the smaller
side of the plane table.
 A plane alidade can be used only when the elevations of the of the
objects are low.
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7. PLAIN ALIDADE
Sight Vane
Object Vane
Fiducial Edge
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PLAIN ALIDADE
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Ahmed Rasheed Mirza

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TELESCOPIC ALIDADE
 The alidade which is fitted with a telescope is known as a telescopic
alidade.
 It is used to take inclined sights.
 It increases the range and accuracy of the sights.
 It consists of a small telescope with a level tube.
 A graduated scale is mounted on the horizontal axis.
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 One side of the metal ruler is used as the working edge along which
lines are drawn.
 The angles of elevation or depression can be read on the vertical
circle.

10. TELESCOPIC ALIDADE
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 It consists of a small metal tube which contains a small
bubble.
 The spirit level may also be circular but its base must be
flat so that it can be laid on the table.
 The table is truly level when the bubble remains central all
over the table.
SPIRIT LEVEL
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12. SPIRIT LEVEL
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THE MAGNETIC COMPASS
 A box compass consists of a magnetic needle pivoted at its
centre freely.
 It is used for orienting the plane table to magnetic north.
 The edges of the box compass are straight and the bottom
is perfectly flat.
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14. THE MAGNETIC COMPASS
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 The plumbing fork consists of a hair pin-shaped brass
frame, having two equal arms.
 One end has a pointer while a plumb bob is attached the
other end.
 It is used in large scale survey for accurate centering of the
station location on the table over its ground position.
PLUMBING FORK
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PLUMBING FORK
•It is also used for transferring the location of
the instrument station on the sheet on to the
ground.
•The fork is placed with its upper arm lying on
the top of the table and the lower arm below
it. The table is said to be centered when the
plumb bob hangs freely over ground mark.
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17. PLUMBING FORK
Plumbing Fork
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i)It is most suitable for preparing small-scale maps.
ii)It is most rapid.
iii)The field book is not necessary as plotting is done in the field
concurrently with the field work, and hence the mistakes in booking
the field notes are avoided.
iv)The surveyor can compare the plotted work with the actual features
of the area surveyed and thus can ascertain if it represents them
properly.
v)It is particularly advantageous in magnetic areas where compass
survey is not reliable.
vi)It is less costly than a theodolite survey.
vii)No great skill is required to prepare a satisfactory map.
ADVANTAGES OF PLANE TABLING
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i)It is not suitable for work in a wet climate.
ii)It is heavy, cumbersome and awkward to carry.
iii)There are several accessories to be carried, and, therefore, they are
likely to be lost.
iv)It is not intended for accurate work.
v)If the survey is to be re-plotted to a different scale or quantities are
to be computed, it is a great inconvenience in absence of the field
notes.
DISADVANTAGES OF PLANE
TABLING
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 The table should be set up at a convenient height. (say
about 1m). The legs of the tripod should be spread well apart,
and firmly fixed into the ground.
 The table should be so placed over the station on the
ground that the point plotted on the sheet corresponding to
the station occupied should be exactly over the station on the
ground. This operation is known as the centering of the table.
This may be done using a plumbing fork or U frame.
SETTING UP THE PLANE TABLE
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SETTING UP THE PLANE TABLE
 In this operation, the table top is made truly horizontal. For
rough and small scale work, leveling can be done by eye
estimation whereas for accurate and large scale work,
leveling achieved with an ordinary spirit level. The leveling is
specially important in hilly terrain where some of the control
points are situated at higher level and some other at lower
level. The disleveling of the plane table, throws the location
of the point considerably out of its true location.
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 The operation of keeping the table at each of the
successive stations parallel to the position which it occupied
at the first station is known as orientation. It is necessary
when the instrument has to be set up at more than one
station.
There are two methods of orienting the table:
1.Orientation by the Magnetic Needle
2.Orientation by Backsighting
ORIENTING THE PLANE TABLE
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ORIENTING BY MAGNETIC NEEDLE
 This method is used when it is not possible to bisect the
previous station from the new station. This method is not
much reliable and prone to errors due to variations of
magnetic field.
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ORIENTING BY BACKSIGHTING
 In this method the table is orientated by back sighting
through the ray which is drawn from the previous station.
This is the most accurate and reliable method of orientation
of plane table.
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There are four methods of surveying with the
plane table:
1.Radiation Method
2.Intersection Method
3.Traversing Method
4.Resection Method
METHODS OF PLANE TABLING
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 In this method the objects are located by radiating lines
from the point, and measuring the distance with chain or tape
with suitable scale. It is chiefly used for locating the details
from the station, which have been established previously by
other methods triangulation, or traversing.
RADIATION METHOD
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27. A
C
B
F
E
a
b
c
d
e
f
P
RADIATION METHOD
D
27
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Ahmed Rasheed Mirza

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 In this method the point is fixed on the plane by the
intersection of the rays drawn from the two instrument
stations. The line joining the stations is called Base line. The
method requires only the linear measurements of this line.
INTERSECTION METHOD
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29. INTERSECTION METHOD
A
B
C
D
p
F
P Q
BASE LINE
d
cba
f
e
q
E
29
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TRAVERSING METHOD
 This is similar to that of Compass Survey or Transit
Traversing. It is used for running survey lines between
stations, which have been previously fixed by other methods
of survey, to locate the topographic details. It is also suitable
for the survey of roads, rivers, etc.
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31. A
B
C
D
a
b
cd
e
E
a
a
b
a
b
c
a
b
cd
TRAVERSING METHOD
31
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Ahmed Rasheed Mirza

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RESECTION METHOD
 This method is used for establishing the instrument
stations only. After fixing the stations, details are located
either by radiation or intersection.
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33. A C
RESECTION METHOD
a
c
B
a
b
c
33
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Ahmed Rasheed Mirza

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 Following precautionary measures should be taken while
performing field work:
o The table must be accurately oriented once the table is
shifted.
o The alidade should be correctly centred on the station
point on paper.
o The expansion and contraction of paper should be taken
care off.
ERRORS IN PLANE TABLING
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o The table should accurately centred.
o The rays should be accurately drawn through the station
points.
o Table should be sufficiently clamped.
o The board should be horizontal.
o The objects should be accurately sighted.
ERRORS IN PLANE TABLING
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