Plane Table Survey. Its Principle, Types , and its Method . Advantage , disadvantage of Plane Table Surveying | Temporary adjustment of plane table

 

What is Plane Table Surveying?

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 case of plane table surveying Geometrical conditions of 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

Principle Of Plane Table Surveying:-

Plane table surveying is based on the principle that lines drawn plotting always lie parallel to the corresponding lines actually present on the ground.

For example, let us consider four ground stations A, B, C, and D which on joining provides a rectangular ABCD. This has been plotted on a sheet of paper at a scale by plane table surveying. Here, the sides AB, BC, CD and DA are plotted in such a way that they are parallel to the sides actually available on the ground. 

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 Radiation Intersection Traversing 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 case of traversing, plane table is located at one point for suppose 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. Average distance of AB and Ba are plotted to scale in drawing sheet. Then Sight the point C from B and measure BC and repeat the same procedure until last point. Conduct some checks at some points. Finally traverse lines are plotted on the drawing sheet. Advantages of Plane Table Surveying: Some of the advantages of plane table surveying are: (i) It is one of the most rapid method of surveying. (ii) Field-notes are not required, and thus the possibility of mistakes in booking is eliminated. (iii) Measuring of lines and angles is mostly dispensed with since they are obtained graphically. (iv) Since the map is plotted in the field, there is no chance of omitting necessary measurement. (v) The surveyor is fully confident about the true representation of the area since he can always compare his work with the actual features on the ground and cannot, therefore, over-look any essential detail. (vi)The surveyor can check the accuracy of his work more frequently and from any position he may desire, thus eliminating all error at the spot. (vii) It is particularly suitable for filling in details in hilly areas and in magnetic areas where chain and compass surveys are not suitable. (viii) Contours and other irregular objects may be accurately represented on the map since the tract is in view. (ix) It is less costly than theodolite survey. (x) No great skill is required in making a satisfactory map and the work can be entrusted even to a subordinate. Disadvantages of Plane Table Surveying: (i) It is unsuitable for work in a wet climate, and is difficult in high wind. (ii) It is not useful for large scale surveys and accurate work. (iii) It is not suitable for surveying a densely wooded area. (iv) The instrument is heavy and cumbersome and the various accessories, being loose, are likely to be lost. (v) The absence of field-notes is sometimes inconvenient if the survey has to be replotted to a different scale. (vi) Only day time can be availed of for the field and plotting work whereas in other methods of surveying, day time can be used for field- work and night or even hot and rainy days can be utilized for plotting. Situations where Plane Table Survey is used:- The plane table surveying is one of the fastest and easiest methods of surveying. Plotting of plans and field observations  can be done at the same time in plane table surveying. It is useful for the following cases:  - It is best fitted for small-scale surveying i.e. any types of fields. - It is also used in surveying industrial areas where compass survey fails to perform. - It is often used to fill in details between stations fixed by triangulation method. Telescopic Alidade:- An alidade used with a plane table, consisting of a telescope mounted on a straight edge ruler, fitted with a level bubble, scale and vernier to measure angles and calibrated to measure distance. Telescopic alidade is an instrument used in topographic surveying, along with a topographic drawing board, for plotting relief, contours, and ground features on map. The alidade consists of a ruler for drawing straight lines on drawing board and an upright that supports on the horizontal axis a telescope with stadia hairs and a vertical circles with degree readings for determining angles of slope.