Posted by Introduction to plane table surveying
Plane table surveying is a graphical method of surveying and we can plot maps directly drawing on site. This makes this method popular. 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.
- Radiation
- Intersection
- Traversing
- 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
- compass method
- back ray method
- Two-point problem
- Three-point problem
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