# Plane Table Surveying- Principle, Types, Two-point Problem

Surveying is the art of determining the relative positions of the points above, on, or below the earth’s surface by the means of direct or indirect measurements. And Plane Table surveying is a graphical method of doing it.

In this method, observations and plotting are done simultaneously, which eliminates recording of the observations and chances of missing out the observations.

In-plane table surveying, instruments like alidade, Sprit level, Compass, Plumbing fork with a plumb bob, the plane table is used. No optical and automatic instruments are used in this surveying. Therefore, the plane table surveying is feasible for small-scale works where a high degree of precision is not required.

## Principle of Plane Table Surveying

Plane table Surveying works on the principle that all lines are parallel to the corresponding line on the ground of the plane table is properly oriented in position.

Here orientation refers to the process of determining the position or location of unknown points with the help of a known point.

## There are four methods of conducting plane table surveying listed as below:

• Intersection Method
• Traversing Method
• Resection Method
• By Compass
• By Back sighting
• By solving Two-point Problem
• By solving Three- point Problem,

## Procedure of plane Table Surveying

• Plane Table Surveying also holds 5 basic principles of surveying. Therefore, major control points are established first, and even before that, a Reconnaissance survey is done to fix the control points, check the intervisibility between adjacent points, and referencing the major control points.
• Setting up of Plane Table
1. Firstly, the plane table is set perfectly horizontal using sprit level and paper is placed over it and fixed it with thumb pins.
2. The next thing to be done is orienting it in the north direction. The plane table is provided with a ball and socket joint so that it allows movement about the horizontal axis and horizontal axis.
3. For centering, one end of the plumbing fork is placed on the station mark of the paper then the plane table is moved to a vertically align point on the paper and point on the ground. But while plotting the first station one appropriate point is chosen.
• Then, ranging rods are placed on the adjacent points, which are sighted using alidade. A faint line is drawn along the sighted point and distance is measured using tape and plotted with an appropriate scale. Repeat the step for another adjacent point.
• After that, change the station and repeat the process.
• When all the stations are plotted, the closing error is calculated, which should be within.

Where n is number of sides.

• The closing error is divided graphically to all sides, using Bowditch rule.
• Only after error proportioned, detailing done. For detaining plane table is placed over the station. Centering and leveling are done accordingly. Orientation is done with help of other known stations.
• The points to be surveyed are sighted with alidade and tape is used to measure distance. The point is marked simultaneously with an appropriate scale.

## Two-point Problem

A two-point problem is one of the methods of resection. Resection is the process of determining positions of an unknown point with other known points.

In a two-point problem, the position of the plane table is determined with two known points. Let us say, P is the point whose position is to be determined, A&B be the known points and Q be an arbitrary point. As shown in the figure below.

• While choosing point Q the triangles formed APQ and BPQ should be well-conditioned.
• Point Q is roughly oriented, and A and B are plotted in it. Similarly, alidade is sighted to P and name it p1.
• Now orient the plane table back sighting to Q produces the ray from A to intersect which is named p. Also, draw the ray PB to intersect at line bp at b1.
• Now, b and b1 points might not coincide due to the presence of angular error in it.
• To correct it, fix the point R along with the AB and rotate the table till the ab line is sighted to R. which is the correct orientation of A and B.
• Again, the resector line aA and bB are drawn to get position p of point P.

Happy Learning – Civil Concept

Contributed by,

Civil Engineer – Rukshana Shrestha

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