🕑 Reading time: 1 minute
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 SurveyGeneral equipment used for conducting plane table survey are
- Plane table
- Alidade for sighting (telescopic or simple)
- Plumb bob and plumb fork
- Spirit level
- Ranging rods
- Drawing sheet and drawing tools
- Paper clips or screws
Plane table survey equipment is arranged in 4 steps as follows
Fixing of Plane TableFix 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 TablePlane table should be leveled using spirit level. For small works, eye estimation can be ok.
Centering of Plane TableThe table should be centered by using plumbing fork. By which we can arrange the plotted point exactly over the ground point.
Orientation of Plane TableWhenever 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 SurveyingGenerally there are four methods are available to perform plane table surveying. They are
RadiationIn 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.
IntersectionIn 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.
TraversingTraversing 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.
ResectionResection 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 ProblemIn 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