Planimeter is an instrument used in surveying to compute the area of any given plan. Planimeter only needs plan drawn on the sheet to calculate area.
Generally, it is very difficult to determine the area of irregular plot. So, by using planimeter we can easily calculate the area of any shape. The essential parts of planimeter and its working is explained below.
Parts of a Planimeter
Before we start with how to use planimeter, first let us know about the parts of a planimeter.
Following are the parts of a planimeter:
Weight and needle point
Tracing arm is an arm which manages the position of tracing point at one end with the help of hinge.
Tracing point is the movable needle point which is connected to tracing arm. This point is moved over the outline of area to be measured.
Anchor arm is used to manage the anchor position or needle point position on the plan. Its one end is connected to weight and needle point and other end to the integrating unit.
Weight and Needle Point
It is also called as anchor. A fine needle point is located at the base of heavy block. This needle point is anchored at required station on the plan.
Clamp is used to fix the tracing arm in standard length without any extension.
The tracing arm and anchor arm are connected by hinge to the integrating unit. With the help of this hinge the arms can rotate about their axes.
Tangent screw is used to extend the tracing arm up to required length.
Index is a location where all the measuring arrangements like wheel, dial are located.
Wheel is fixed in the integrating unit which helps to measure the tracing length. It is used to set zero on the scale.
Dial is nothing but scale which is to be set zero at the initial level using setting wheel.
Vernier is attached to the wheel as rounded drum with graduations on it. It is divided into 100 parts.
How to Use Planimeter in Surveying?
Planimeter is used to compute the area of given plan of any shape.
In the first step anchor point is to be fixed at one point. If the given plan area is small, then anchor point is placed outside the plan. Similarly, if the given plan area is large then it is placed inside the plan.
After placing the anchor point, place the tracing point on the outline of the given plan using tracing arm. Mark the tracing point and note down the reading on Vernier as initial reading A.
Now move the tracing needle carefully over the outline of the given plan till the first point is reached. The movement of tracing needle should be in clockwise direction. Note down the reading on Vernier after reaching the first point and it is the final reading B.
Now the area of the plan which boundary is traced by the planimeter is determined from the below formula.
Area =M (B – A + 10N + C)
Where, A = initial reading
B = final reading
N = no. of completed revolutions of wheel during one complete tracing. N is positive if dial passes index in clockwise, N is negative if dial rotates in anti-clock wise direction.
M and C = constants which values are provided on the planimeter. Constant C is used only when the anchor point is placed inside the plan.
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