Plan table surveying: Methods and applications

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Introduction to plane table surveying

Plane table surveying is a graphical method of surveying and we can plot maps directly on drawing on site. This makes this method popular. At now it is used for small or medium work where higher accuracy is not needed. Plane table surveying gives approximate data.

The principle of plane table surveying is parallelism. It simply means that we are using the Alidade, which is used to draw lines on the drawing sheet, parallel to the sightline.

Plane table surveying has the following pros and cons

Pros of plane table surveying

  • Drawing could be prepared while surveying
  • No need to maintain the field book

Cons of plane tabling

  • In the rainy season, plane tabling is not possible
  • Surveyor need to carry a lot of instruments with him
  • It is an approximate method and hence couldn’t be used for a big project
  • As in the case of plane tabling, no field data is recorded, it would be difficult to reproduce the map

Instruments used in plane table surveying

Plane table

It consists of a drawing board attached with a tripod stand. There are many types of plane tables available in the market.

  • Simple plane table
  • Johnson plane table
  • Coast survey table

Alidade

It is used to sight the object and drawing the lines on the drawing sheet. There are two different types of alidade.

  • Plain alidade
  • Telescopic alidade

Spirit level

It is used to level the plane table.

Trough compass

It contains a magnetic needle casing in a wooden or non-metallic box. It is used to locate the north-south direction.

U fork or Plumbing fork

U fork is used for centering of plane table

Methods of plane tabling

There are four well known and conventional methods, though still using to do surveying with a plane table.

  1. Radiation
  2. Intersection
  3. Traversing
  4. Resection

Each of the above is explained below

Radiation

This is most suitable when distances to be measured are small and readings should be taken from one location only.

Intersection method

This method is suitable for locating inaccessible points from the two well-known positions on the ground.

Traversing method

This method is used to connect the station of an open or closed traverse.

Resection method

Resection is the general term used for the process of determining the location of the station occupied by the instrument. There are 4 ways to employ the resection method

  1. compass method
  2. back ray method
  3. Two-point problem
  4. Three-point problem

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