Surveying often requires precise measurements, especially when calculating the area of irregularly shaped plots. A planimeter is an essential tool in such scenarios. This instrument is specifically designed to compute the area of any given plan or map by simply tracing its outline. The simplicity and effectiveness of the planimeter make it indispensable in surveying. In this article, we’ll explore its parts, how it works, and its usage in detail.
Parts of a Planimeter
Understanding the parts of a planimeter is crucial to mastering its use. Below are the key components and their roles:
- Tracing Arm
The tracing arm is responsible for maintaining the position of the tracing point. It is connected to the hinge, enabling controlled movement. - Tracing Point
The tracing point is a movable needle connected to the tracing arm. This needle is used to trace the boundary of the area to be measured. - Anchor Arm
The anchor arm helps stabilize the anchor point, or needle, on the plan. One end is connected to the weight and needle point, while the other is linked to the integrating unit. - Weight and Needle Point (Anchor)
Also known as the anchor, this is a heavy block with a fine needle at its base. It acts as a fixed point for the instrument during measurement. - Clamp
The clamp secures the tracing arm at a standard length, ensuring consistent measurements. - Hinge
The hinge connects the tracing and anchor arms to the integrating unit. It allows these arms to rotate freely around their axes for flexibility in tracing. - Tangent Screw
The tangent screw is used to extend the tracing arm to the desired length for accurate measurements. - Index
This is the section of the planimeter where all measuring components, such as the wheel and dial, are located. - Wheel
The wheel is fixed within the integrating unit and plays a key role in measuring the tracing length. It is also used to set the scale to zero before starting. - Dial
The dial, or scale, is calibrated to zero at the initial setup using the setting wheel. It records measurements during tracing. - Vernier
The Vernier scale, attached to the wheel, is a drum-like component divided into 100 parts, ensuring precise measurements.
How to Use a Planimeter in Surveying
Using a planimeter involves a straightforward process, allowing surveyors to calculate the area of any shape quickly and accurately.
Step 1: Setting Up the Planimeter
- Begin by fixing the anchor point. For small plans, place the anchor outside the boundary; for larger plans, place it inside.
- Position the tracing point on the boundary of the area to be measured using the tracing arm.
Step 2: Measuring the Area
- Mark the initial tracing point and record the Vernier reading (A) as the initial value.
- Carefully move the tracing needle along the outline of the area in a clockwise direction. Continue until the needle returns to the starting point.
- Record the final Vernier reading (B) once tracing is complete.
Step 3: Calculating the Area
The area of the traced plan is calculated using the formula:
Area = M (B – A + 10N + C)
Where:
- A = Initial Vernier reading
- B = Final Vernier reading
- N = Number of completed revolutions of the wheel during tracing
- Positive if the dial rotates clockwise
- Negative if the dial rotates counter-clockwise
- M and C = Constants provided with the planimeter
- C is used only if the anchor point is inside the plan.
Conclusion
A planimeter is a simple yet powerful tool for accurately calculating areas of any shape. By understanding its components and learning to operate it correctly, surveyors can achieve precise results with minimal effort. Whether dealing with small or large areas, the planimeter remains a reliable instrument, making it a valuable addition to any surveyor’s toolkit.