This Article Covers What Is Plane Table Surveying, Instruments Of Plane Table Surveying And Methods Of Plane Table Surveying.
Plane Table Surveying
Plane table is a graphical method of surveying in which the field works and the plotting done simultaneously.
It is most suitable for the filling in of the details between stations previously filled by triangulation or theodolite survey.
It particularly adopted for small scale or medium scale mapping , in which great accuracy in in details not required as for topographical survey.
Plane Table Surveying Instruments
The Plane Table Essentially consist of:
- A drawing board, mounted on a tripod.
- A straight edge called Alidade.
The Drawing Board
Made of well seasoned wood, 40cm x 30cm to 75cm x 60 cm Or 50 cm to 60 cm square, mounted on a tripod, such that can be leveled and revolved about vertical axis and clamped in any position. Board fitted with a ball and socked arrangement.
Consist of metal ruler about 50 cm long having straight and smooth ruling edge (working edge) also called Fiducial edge.
Accessories Of Plane Table Surveying
- Trough compass or circular compass.
- A Plumbing fork for or U frame, with a plumb bob for centering the table.
- A Water proof cover to protect the sheet from rain.
The Points to kept in mind while surveying with plane table
- The small letters a,b,c used to represent the stations on the paper.
- The table will remain stationary while the alidade rotated to slight the station.
- While taking the reading the alidade must be centered.
- The board turned only when the table is to oriented.
Setting Up The Plane Table
It includes the following operations:
- The table fixed at a convenient height (Say about 1m and approximately leaved, legs should spread well apart and firmly fixed)
Orientation Of Plane Table
The preparation of keeping the table at each of the successive stations parallel to position which it occupied at the first station known as Orientation.
When table properly oriented the lines on the paper are parallel to the lines on the ground, which they represent.
There are two methods of orienting the table:-
Orientation By Magnetic Needle
- To orient the table at any subsequent station, the rough compass(or circular compass) placed along the line representing the magnetic meridian which has drawn on the paper at first station and the board is then turned until the ends of needle opposite the zeros of scale.
- The board then clamped in position.
- This method sufficiently accurate provided there is no local attraction.
- Suitable for rough small scale.
Orientation By Back Sighting
- This is most accurate method and always preferred.
- In this method, the fiducial edge of alidade laid along a ray drawn from previous station to the present station and the plane table then rotated till the line of sight of alidade bisects exactly the ranging rod placed at previous station.
- The plane table then clamped and said to oriented. In this method, the level of the plane table has to be maintained identical in both the stations.
Methods Of Plane Table Surveying
There are 4 methods of Surveying with Plane Table
Radiation Method Of Plane Table Survey
In this method the point is located on plan by drawing a ray from the plane table station to the point, and plotting to scale along the ray, the distance measured from the station to the point.
The method suitable for the survey of small areas which can commanded from a single station.
Chiefly used for locating details from stations, which have previously established by other methods such Triangulation or Traversing.
- Select Point P on ground such that all the points to be located are visible.
- Set up the table at point P and after leveling it clam it.
- Select a point on sheet such it exactly above the P, this done with the help of U-frame.
- Mark the direction of the magnetic meridian with trough compass.
- Centering the alidade at p sight various points all around and draws the rays along the fiducial edge.
- Measured the distance PA, PB, PC and so and plot on the sheet according to scale.
Intersection Method Of Plane Table
In this method the point fixed on plan by Intersection of rays drawn from Two Instrument Station.
The line joining these stations called Base Line.
The method requires only the linear measurement of this line.
The method commonly used for locating:-
- Distant and inaccessible points
- Broken boundaries
- Checking distant objects
- Select two points P and Q in commanding position, line joining P and Q called Base line.
- Select a point p on sheet above station P, center and level it and also mark the magnetic north.
- With the alidade centered with p draw rays to Q and other points say A,B,C etc.
- Measure distance between P and Q accurately and mark the location of q on sheet.
- Shift the table and fix it at Q such that q is exactly above the station Q.
- Place alidade along qp and orient the table by back-sighting and clamp it.
- With the alidade placed along q draw rays to the objects again.
- The point of intersection will give the location of all these points.
Traversing Method Of Plane Table
This method is similar to the compass traversing.
It used for running survey lines between stations which have previously fixed by other methods of surveying to locate the topographical details.
It is also suitable for survey of Rivers or Roads etc.
Resection Method Of Plane Table
This method used for establishing the instrument stations only.
After fixing the stations, Details are located by either radiation or intersection.
The characteristics feature of Resection that, the point plotted on plan the station occupied by the Plane Table.
- Select a base line PQ on ground, measure it accurately with steel tape.
- Setup table at P, Center it.
- Placing alidade at line pq Orient table by running ray until Q sighted, clamped it.
- With alidade touching p, sight R , which is to be located by intersection.
- Shift the instrument and setup at R, orient the table by taking back sight on P, clamp it.
- With alidade centered at q, sight the station Q and draw a ray.
- The point of intersection of this ray and that previously drawn from p gives the required point r.
This method called Back Ray method.
Advantages Of Plane Table Survey
- It is most suitable for preparing small scale survey.
- Is most rapid.
- The field book not required.
- The surveyor can compared the plotted work to the actual features.
- There is no chance of omitting the necessary measurements, as the map prepared in the field.
- It is less costly than the theodolite.
Disadvantages Of Plane Table Survey
- It is not suitable for work in wet climate.
- It is heavy and Is difficult to carry.
- There are many accessories to carry and may be lost.
- It not intended for accurate work.
- If the map to re-plotted it very inconvenience in the absence of field notes.
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