Plane Table Survey

Becky January 31, 2013

Plane tabling  is a graphical method of surveying in which the field work and plotting are done simultaneously. It is most suitable for filling in the various details between the previously fixed stations.
It is generally adopted for small or medium scale mapping.


Plane Table Survey Is Adopted Or Preferred

  • For Surveys in which high precision is not required.
  • For small or medium scale mapping.


Plane table essentially consists of

1.     A drawing board mounted on a tripod and
2.     A straight edge called an "alidade"

Instruments used in plane table surveying

1.     Tripod
2.     Plane table
3.     Alidade
4.     U-Fork
5.     Trough Compass
6.     Drawing sheet
7.     Drawing pins
8.     Scales, pencils

The use of trough compass and alidade in plane table survey

Trough Compass:

For making North-South direction and orientation of table.

Alidade:

To bisect  object with the help of object vane and draw ray along bevelled edge.

Orientation in plane tabling

It is defined as an operation of keeping the plane table at each successive station parallel to the position occupied by the table at the first station.
Methods: 1) Back sighting  2) By magnetic needle

The temporary adjustment of plane table

1.     Setting up the plane table
  • Set up table on tripod
  • Legs of  tripod should be spread
  • Firmly fix the legs
  • Level the table with the  help of spirit level
  • Fix the drawing sheet 
2.      Centering
  • Place plumbing fork such that plumb is exactly over station. Mark the point on paper.
3.     Orientation
  • Draw North-South direction with the help of trough compass.

The working operations of plane tabling at each stations

Three operations are needed
1. Fixing the table to the tripod
2. Setting
  • Levelling the table
  • Centering
  • Orientation
3. Sighting the points

After selecting the stations the table is mounted on the tripod which is then centered on  the station with the help of plumbing bob and U frame. The spirit level is used to test the horizontal position of  plane table. A trough compass is used to mark the north direction for the purpose of orientation.
The working edge of an alidade is kept against a station on sheet and the object in sighted. The distance is measured from the ground station to the object and it is plotted to the chosen scale.
After all the operations are complete from the station, the table 3 is shifted to another station and orientation  is done by means of trough compass or back sighting and operations are repeated.

Methods of plane tabling

i) Radiation ii) Intersection iii) Traversing iv) Bisection


Radiation method of plane table survey


1.     Select instrument station 'O'
2.     Other two stations should be visible
3.     Set up the plane table
4.     Level it up and clamp
5.     Use U-fork and plumb bob
6.     Locate station on sheet
7.     Mark magnetic meridian by trough compass
8.     Fix a pin at station on sheet
9.     With alidate draw rays to various object
10.  Measure distance
11.  Select scale
12.  Plot all those points on paper

The method of Intersection in plane table surveying

In this method the point is fixed on plan by the intersection of the rays drawn from the two instrument stations. The line joining these stations is called the base line. This method is employed for locating  i) the details  ii) the distant inaccessible points  iii) the broken boundaries  iv) the rivers and the instrument station (points)

Let the first station of the base line XX, be X. Set up the table over this station in such way that all points to be plotted are visible. Level the table and clamp the same. Draw the north line in top corner with trough compass. Mark station on sheet. From station sight the ray to Y. Measure the distance carefully and plot the position to scale. This becomes a base line. With the alidade touching the point draw rays towards different objects. Now shift the table on the station Y. Set point y on Y. Orient the table by trough compass or back ray method, clamp the table. Draw rays to different objects. The intersection of these rays gives the position of difference objects.

Note: This method will not give accurate results when rays  drawn are too oblique.


Merits and Demerits of plane table survey

Merits:


1.     Field work and plotting are done simultaneously.
2.     Field notes not required.
3.     Can be checked at site only.
4.     Map is ready.
5.     Direct measurements can be avoided.
6.     Suitable for magnetic area.
7.     Most suitable for interior details.
8.     Rapid.
9.     Cheaper.
10. No much skill required.

Demerits

1.     Not suitable in rainy season.
2.     Shrinkage, expansion of paper takes place.
3.     In sunshine, glazing takes place.
4.     Not accurate.
5.     Heavy.
6.     So many accessories.



Becky

Posted by Becky