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Keywords: Planimeter, measurement of area, zero circle method
Planimeter is an accurate instrumental method used for the computation area. Read more about Planimeter here.
For measuring the area of plots with planimeter, two methods are generally adopted.
Method 1: the area of the figure is measured in square cm and the field area is determined by multiplying the area of the figure by the square of the scale of the plot. Along with the planimeter, the manufactures supply a scale chart. In this, the positions at which the vernier is to be set on the tracing arm for different scales are given. The vernier is set to a scale of 1:1 and the area of the plot is determined. This is multiplied with the square of the scale to get the field area.
Method 2: The filed area is directly obtained from the plot in sq.m. For this, the vernier is set to the position on the tracer bar corresponding to the scale of the plot. The area of the field is directly obtained in sq.m
Procedure for finding area with planimeter
The stepwise procedure for determining the area by method two is given below:
1. Let the scale of the plot be 1:S
2. Set the vernier to the position on the tracer bar for a scale of 1:1
3. Fix the anchor point firmly on the paper outside or inside the figure according as the figure is small or large. Then move the tracing point only in the clockwise direction around the plot figure. Check whether all the points in the boundary can be reached without any difficulty. If not, change the position of the anchor point. Also note whether the total rotation of the wheel is a forward or a backward motion.
4. Mark a definite point on the outline of the figure and set the tracing point exactly on it.
5. Note the initial reading (I.R) or alternately set the dial and the wheel to zero.
6. Move the tracing point exactly around the outline always in a clockwise direction until it again reaches the starting point exactly. Note the final reading (F.R).
7. Note the number of times the zero mark of the dial passes the fixed point (index mark) in a clockwise or counter clockwise direction, while the tracing point is moved along the outline of the figure.
8. Area of the figure is given by:
A = M (FR-IR = -10N+ C)
Where M= the multiplier
FR= final reading
IR= initial reading
N= It is the number of times the zero mark of the dial guage passes the fixed index mark
C= a constant
The following points are to be noted:
1. Use+ sign when the zero mark of the dial passes the fixed index mark in clockwise direction
2. 2. The constant C is to be used only when the anchor point is fixed inside the figure.
3. When the anchor point is inside the figure, the following points are of importance:
i. In the case of clockwise rotation, the net rotation of the wheel is always forward, if the area of the figure is greater than that of the zero circle.
ii. In the case of clockwise rotation, the net rotation of the wheel is always backward, if the area of the figure is less than that of the zero circle.
Precautions in the use of planimeter
1. For accuracy, measure the area twice with different positions of the anchor point for each measurement.
2. The area should be checked roughly by estimation/ rough calculation.
3. If the area is too large, divide the area into parts and find the area of the parts and add them together.
4. The plan should be placed horizontally when measurements is done with planimeter.
5. The surface of the paper must be smooth
Zero circle or the circle of correction is another important part of this method. You can read it here.