Plane table surveying

Advance surveying

1. By: - Nishu Kumar

Plane Table Surveying
The plane table surveying is a very quick method of surveying where
field observations and plotting of the plan proceed simultaneously. The
necessity of transferring the field data to office and preparation of map
is completely 'avoided. The plane table-alidade combination is an
extremely useful and a versatile instrument. It can perform all the usual
survey functions with the exception of field astronomy.
EQUIPMENTS REQUIRED
The following equipments are required for plane table survey.
I. Plain table
2. Alidade for sighting
3. Plumbing fork and plumb bob
4. Spirit level
5. Compass
6. Plane table sheet of suitable drafting media
7. Tripod
The plane table
It consists of a small drawing board mounted on light tripod in such a
way that the boards can be rotated about the vertical axis and can be

2. By: - Nishu Kumar

clamped in any position. I.S. 2539-1963 specifies three types of boards
with dimensions given below:

Designation Dimensions
A B C
Large 750 600 15
Medium 600 500 15
Small 500 400 15


Alidade (sight rule)
This may be
(i) Plain or
(ii) Telescopic.

3. By: - Nishu Kumar

In plain alidade the sight rules are made of wood or metal but the sight
vane is made of metal only. The front or object vane and the rear or
sight vane are of folding type. It is possible to suitably clamp them in
vertical position. The front vane has a thread across its length and the
rear vane has a fine slit to view through it. The two vanes can be linked
through a thread when required for sighting at high elevations or
depressions. The stretched thread, object vane thread and sight vane
vertical slit are coplanar and this plane is parallel to the edge of the
sight rule and normal to the plane table board when leveled. The
bottom surface (If the sight rule is truly plane and has beveled edges).


Plumbing fork

4. By: - Nishu Kumar

This is a U-shaped piece of metal with parallel arms 'of equal length, a
plumb bob being attached to the free end of the lower arm. The point
of the upper arm and the plumb line are in the same vertical line; the
plumbing fork is used for Centering the table Here the upper end of the
plumbing fork is placed over the plotted point and the plane table is so
adjusted that the plumb bob is on the station point below. Transferring
on the ground point Here the plane table is centered over the
underground point by means of the plumb bob while the upper arm of
the fork gives the point to be plotted on the drawing sheet.


Spirit level
A small spirit level either of the tubular variety or of the circular type is
used to check that the plane table when in use is level. This can be
ascertained by placing the level in two directions at right angles to each
other and observing that the bubble is central in both cases.

5. By: - Nishu Kumar



Compass
Usually a trough compass is used. The longer sides of the trough are
parallel and flat so that either side can be used as a ruler or laid down
to coincide with a straight line drawn on the paper.

6. By: - Nishu Kumar


Plane table sheet
It should be of superior quality and should be moisture proof and non-
hygroscopic. The dimensions of the sheet should remain stable under
variable conditions of temperature and humidity. It should be capable
of withstanding repeated erasures. It should also be stiff and tough and
suitable or longtime archival, quality storage.
TRIPOD
Tripod is also required for plain table survey. It is in one of the
important plane table survey instruments without it is impossible to do
plain table survey.

7. By: - Nishu Kumar

Tripod is used to fix the drawing board. It consists of a screw at the
bottom which is used to set and level the drawing board on it
comfortably.


Setting out of plane table
Levelling The plane table should be levelled. For small work it is done
through eye estimation. For more accurate work, spirit level is to be
used.
Centering The table should be so placed that the plotted point a is
exactly over the ground point A. This is done through plumbing U-fork,
Orientation This is done by rotating the plane table such that plotted
lines in the plane table sheet are all parallel to the corresponding lines
on the ground. This is essential when more than one instrument station
is to be used. Orientation is done by (i) Trough compass, (ii) Back
sighting.

8. By: - Nishu Kumar

Orientation by trough compass This is an approximate but quick
method of orienting the plane table. The usual method is to place the
trough compass on the plane table sheet and to rotate the plane table
till the needle floats centrally, this is the direction of magnetic north
and a line pencil line is ruled against the long side of the box. At any
other station, where the table is to be oriented, the compass is placed
against this line and the table is turned till the needle freely floats in the
middle. The table is then said to be oriented.
Orientation by back sighting This is a more accurate method and
two cases arise depending on whether it is possible to set the plane
table on a point already plotted on the sheet by way of observation
from previous station or not. In the first case orientation is done by
back sighting. Suppose the line ab has been plotted on the plane table
corresponding to the ground line AB. After shifting the plane table from
A to B, orientation will be done by (i) placing the point b exactly over
the Station B with the help of U fork. (ii) By rotating the plane table
such that from station B, alidade which is placed on ba sights the pole
at A. When this is achieved, the line ba coincides with the ground line
BA and orientation is achieved. The table is then clamped in position.

Methods of plane table surveying
There are four methods of plane table work:
(i) Radiation,
(ii) Intersection,
(iii) Traversing and

9. By: - Nishu Kumar

(iv) Resection.
RADIATION
Here plane table is placed over station point P and alidades are
pointed-towards A,B,C,D,E and F. The lengths are measured and
point a, b, c, d, e and f are plotted


INTERSECTION
One of the great advantages of plane table: is the ease with
which a point can be located by intersection. Here P and Q are
known stations and the plane table is first placed over station P
and alidade points towards PQ. The line PQ is measured and PQ is
plotted on the sheet (Fig. 16.7). To locate points A and B rays are
drawn from p towards A and B. The instrument is then shifted to

10. By: - Nishu Kumar

Q and it is so placed that q is over Q and pq corresponds to the
line PQ. In such a case the plane table is properly oriented at Q.
With the alidade pivoted at q, rays are drawn toward A and B on
the plane table sheet. The intersection of corresponding two rays
defines the map position of A and B. The difference in elevation
between A and P and that between A and Q can be obtained if
the vertical angle to A has been measured from each set up. The
product of the distance AP scaled from the map-and the tangent
of the vertical angle at P is the difference in elevation between
the alidade at P and A. The difference in elevation between the
alidade at Q and the point A can be determined in the same
manner

TRAVERSING
A traverse consists of a series of straight lines connected together. In a
plane table traverse, the angles are directly plotted without measuring
them. Here initial station A is occupied and AB is sighted and measured.

11. By: - Nishu Kumar

Then station B is occupied and BA sighted. The distance BA is measured
and the average of AB and BA used in laying out ab. The next point C is
observed with the blade touching b, distance BC and CB measured and
average value plotted as be. In similar fashion, succeeding points can be
occupied and traverse lines plotted. Whenever possible, check sights
should be taken over previously occupied points. Small discrepancies
are adjusted but if a plotted point is missed by an appreciable distance,
some or all measurements must be repeated.

Principles of plane table survey:

The main principle of plane table surveying is based on parallelism.
According to this principle, all the rays drawn through the various
details should pass through the survey station. In this regard, the
position of the plane table is kept identical in each of the stations. This
is done by orienting the plane table in the direction of magnetic north
in each station.

12. By: - Nishu Kumar

(1) To distinguish clearly, small letters like p, q, r, etc. should be
used on the paper to represent the corresponding points P, Q, R,
etc. on the ground.

(2) While taking the sights, the table should be clamped in position,
and the alidade must be centered on the plotted station point on
the paper or drawing sheet.

(3) The table must only be turned for orientation, and when it is
correctly oriented, it should be again clamped in position.

Merits and Demerits of plane table

Advantages of Plane Table Survey:

(1) Map of the area is plotted in the field, i.e., both the field work
and the plotting are done simultaneously.

(2) Field notes of the measurements are not required, and thus the
errors in booking are eliminated.

(3) The correctness of plotted work can be checked by check
observations in the field.

(4) Office work is only finishing up of the drawing.

(5) Direct measurements of lines and angles are avoided as they
are obtained graphically.

13. By: - Nishu Kumar

(6) Errors due to extended angular observations with angular
instruments and incorrect plotting with bad protractors are
avoided.

(7) This is suitable in a magnetic area where you can’t rely on the
compass survey.

(8) More work will be done in less time and labor.

(9) It is less costly than a theodolite survey.

(10) It is most suitable for preparing small-scale maps.

(11) Contours and irregular objects may be represented accurately.

(12) The reduced levels of points other than the station points of
known elevations may be found out with the help of tangent
clinometers.

(13) The survey can be completed very rapidly.

(14) Much skill is not required to prepare a map.

(15) The danger of omitting the necessary measurement is
eliminated as the survey is plotted in the field.

Disadvantages of plane table survey:

(1) Plane tabling is not suitable for work in wet climates, in places
where high winds predominate, and in wooded country.

14. By: - Nishu Kumar

(2) If the area to be surveyed is large, frequent changes in the size
of drawing sheets are required.

(3) Due to changes in temperature, the table is liable to warp.

(4) If the sun is bright, plotting may be difficult due to the strain on
the eyes.

(5) It is not very accurate for large-scale surveys as compared to
compass and theodolite surveys.

(5) The instruments and its accessories are heavy and cumber-
some, and they are likely to be lost.

(7) Since the field notes are not maintained, it is inconvenient to
calculate the quantities or re-plot the survey to a different scale if
required.

(8) Although the plane table is advantageous in open country, it is
inferior to the compass in the thickly wooded lands.

(9) The plane tabling is not intended for accurate work.

Telescopic Alidade
It consists of a telescope mounted on a column fixed to the ruler. The
line of sight through the telescope is kept parallel to the bevelled edge
of the ruler. The telescope is provided with a level tube and vertical
graduation arc. If horizontal sight is required bubble in the level tube is

15. By: - Nishu Kumar

kept at the centre. If inclined sights are required vertical graduation
helps in noting the inclination of the line of sight. By providing
telescope the range and the accuracy of line of sight is increased.


Telescopic alidade