The plane table surveying is a type of surveying in which plotting of the plan and field observations can be done simultaneously and plane tabling surveying is generally adapted for surveys in which high precision is not required.
The geometrical conditions of the site are manuscript in the map sheet using the plane table in case of plane table surveying and on the map after that topographic details are alidades.
Equipment in Plane Table Surveying:
There is some general equipment used for the conducting plane table surveying such as;
Arrangements of Equipment Used in Plane Table Surveying:
There are four arrangements of equipment in plane table surveying as given below;
Fixing of plane table
Leveling of plane table
Centering of plane table
The orientation of the plane table.
Fixing of Plane Table:
To the tripod stand, fix the plane table, and by using paper clips arrange the drawing sheet on the plane table, and from first to last the sheet should be in one position.
Leveling of Plane Table:
By using the spirit level the plane table should be leveled and eye estimation can be fine for the small works.
Centering of Plane Table:
By using plumbing fork the table should be centered and by doing this we can arrange the plotted point exactly over the ground point.
The Orientation of Plane Table:
Orientation is essential whenever we use more than one instrument station and by using compass it can be done. On the ground, the plane table is rotated in such positions that plotted lines in the drawing sheet are parallel to corresponding lines in this case.
Methods of Plane Table Surveying:
To perform plane table surveying there are four methods available as discussed below;
Radiation Method of Plane Table Surveying:
At one point plane table is located in this method and performs the whole from that point and various points are located by radiating a ray from the instrument station to each of the points.
For surveys of small areas that are likely to be commanded from a single station, the radiation method is suitable and if used in combination with other methods it is useful in large scale works.
The work can be finished very rapidly if the distances are obtained with the help of telescopic alidade and this method has a wider scope due to this.
Intersection Method of Plane Table Surveying:
For plotting the details on the maps this method is widely used and also used at subsequent stations for plotting the position of points.
In this method, by plotting two rays from two unknown stations we can locate the point.
As shown in fig P & Q are the known station and the equipment is placed on P first and then by sighting the station’s A, B, and Q plot the lines. Then shift the equipment to station Q and by sighting stations A, B and P plot the lines and the required location of the point of intersection is the intersection of A and B rays:
In a survey of the hilly areas and for checking the remote objects this method is also used for locating the distant and inaccessible objects and the rivers.
Traversing Method of Plane Table Surveying:
For laying down the survey lines of a closed or unclosed traverse this method can be used and in the usual way by taking offsets the details may be plotted.
In this method plane table is located at one point such as A and from that point sight towards B and measure the distance AB and BA is measured by shifting the plane table to point B and sight towards A:
For the survey of roads, rivers, etc the plane table traversing is also useful.
Resection Method of Plane Table Surveying:
In this method the location of the plane table is unknown and by sighting it to plotted points the location is determined.
For the location of station points, only the method of resection is employed, and by radiation or intersection, the details are located after establishing the stations.
Principles of Plane Table Surveying:
To represent the points P, Q, R, etc. on the ground small letters such as p, q, r, etc should be used on the paper to distinguish clearly.
The table should be clamped in position, and on the plotted station point alidade must be centered on the drawing sheet while taking the sights.
For orientation, the table must only be turned and when correctly oriented it should be again clamped in position.
Advantages of Plane Table Surveying:
There are the following advantages of plane table surveying such as;
In the field map of the area is plotted such as both the plotting and the fieldwork are done.
No required field notes of measurements.
By checking observations in the field the correctness of plotted work can be checked.
Direct measurements of lines and angles are avoided.
Errors are avoided which are caused by extended angular observations with angular instruments and incorrect plotting with bad protractors.
In a magnetic area where you cannot rely on the compass survey, the plane table surveying is suitable.
In less time and labor more work will be done and it is less costly.
Disadvantages of Plane Table Surveying:
There are the following disadvantages of plane table surveying such as;
The table is liable to warp due to changes in temperature.
Plotting may be difficult due to the strain on the eyes if the sun is bright.
For large-scale surveys, as compared to compass and theodolite surveys It is not very accurate.
To calculate the quantities or re-plot the survey to a different scale if required, it is inconvenient.