Plane Table Survey

 The plane table surveying is the fast method of surveying. In this type of surveying plotting of the plan and field observations can be done simultaneously. In the case of plane table surveying Geometrical conditions of the site are manuscript in the map sheet using plane table and alidade after that topographic details are arranged on the map.

Equipment Used in Plane Table Survey

General equipment used for conducting plane table survey are

• Plane table
• Alidade for sighting (telescopic or simple)
• Plumb bob and plumb fork
• Compass
• Spirit level
• Chain
• Ranging rods
• Tripod
• Drawing sheet and drawing tools
• Paper clips or screws

Plane table survey equipment is arranged in 4 steps as follows

Fixing of Plane Table

Fix the plane table to the tripod stand. Arrange the drawing sheet on the plane table using paper clips or thumb screws. The sheet should be in one position from first to last.

Leveling of Plane Table

Plane table should be leveled using spirit level. For small works, eye estimation can be ok.

Centering of Plane Table

The table should be centered by using plumbing fork. By which we can arrange the plotted point exactly over the ground point.

Orientation of Plane Table

Whenever we are using more than one instrument station, orientation is essential. It can be done by using compass or back sighting. In this case, the plane table is rotated such that plotted lines in the drawing sheet are parallel to corresponding lines on the ground.

Methods of Plane Table Surveying

Generally there are four methods are available to perform plane table surveying. They are

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

Radiation

In this method, plane table is located at one point “o” as shown in fig. and perform the whole from that point. From point O, sight the points A,B,C,D and E using alidade, locate and plot the points as a,b,c,d and e in the drawing sheet.

Intersection

In this method we can locate the point by plotting two rays from two known stations. As shown in figure, P and Q are the known station. First the equipment is placed on P and plot the lines by sighting the stations A, B and Q. then shift the equipment to station Q and plot the lines by sighting stations A, B and P. Finally, the intersection of A and B rays is the required location of point of intersection.

Traversing

Traversing is the connection of series of straight lines. In the case of traversing, the plane table is located at one point for supposing A as shown below. From that point sight towards B and measure the distance AB. Then shift the plane table to point B and sight towards A and measure BA. The average distance of AB and Ba are plotted to scale in the drawing sheet. Then Sight the point C from B and measure BC and repeat the same procedure until the last point. Conduct some checks at some points. Finally, traverse lines are plotted on the drawing sheet.

Resection

Resection is a method of plane table surveying in which the location of the plane table is unknown and it is determined by sighting it to known points or plotted points. It is also called the method of orientation and it can be conducted by two field conditions as follows.

• The three-point problem
• The two-point problem

The Three-Point Problem

In this condition, three points and their positions in the field are known. The plane table is placed at an apposition from where all the three points are visible. So, by sighting those three points we can locate the point where equipment is located. This can be achieved by many methods as follows.

• Tracing method
• Lehmann method
• Analytical methods
• Graphical method

Tracing Method in Plane Table Surveying

In the tracing method, the plane table is located at a point from where three points are visible. The table is oriented with respect to the plotted lines of those three points. Place the tracing paper on the drawing sheet and again sight the three points and plot the radiating lines. The tracing paper is then moved above the drawing sheet until the three radiating lines pass through corresponding points previously plotted on the map. Finally, the position of the plane table is marked.

Lehmann Method

In this method, Plane table is located at a point P and sight the station A, B and C and plot the rays Aa, Bb, and Cc. The rays form small triangle which is called triangle of error. Another point P1 is chosen to reduce the error and sight the point A from P1 similarly to B and C. which will give another triangle of error. Repeat this procedure until error becomes zero.

Analytical Methods

There are many analytical methods are developed in three-point problem condition. In this method, from station P A, B and C are sighted and note the values of angles and lengths. From these values determine the position of unknown points by using analytical formulae.

Graphical Method

In graphical method also, angles and lengths are determined and represented it on a graph and determines the location of plane table.

The Two-Point Problem

In the two-point problem, two points are sighted from other point corresponding to the points given in plane table sheet. Here two cases are to be discussed. Case 1: when the points can be occupied by the plane table As shown in fig. A and B are the two points corresponding to the points a and b. Now, plane table is located at B and oriented by sighting A. sight C from B and bx is plotted on the sheet. Then shift the plane table to C, oriented by backsighting B along xb. Then alidade is placed over a and sight station A, then line Aa cuts the line bx at somewhere which is located as point c at station C.

Case2: When the plane table cannot occupy the controlling stations In this case, an auxiliary point D is considered nearer to C. Locate the plane table at D according to the line ab parallel to AB. Then sight the station A and B corresponding to a and b. the rays drawn are intersected at some point which is marked as d. then sight towards C by placing alidade at d. mark the distance Dc as c1. Shift the table to C and backsight to D with reference to c1. Then sight A corresponding to a, the ray drawn is intersects the previously drawn ray from D in c2. From c2 sight B draw a ray which intersects db and marked the intersection as b1. The table is oriented till ab comes in line with P. From P sight and draw rays Aa and Ba. The intersection of these two rays will give the Location of Point C.