What is the purpose of a planimeter in measurement?

The purpose of a planimeter is specifically to measure the area of plane figures. It is an instrument used in various fields, including cartography, engineering, and environmental science, for accurately determining the area enclosed within boundaries, regardless of the shape of the figure. The way a planimeter works involves tracing the outline of the figure, which then allows the device to compute the area based on the movement of its arm.

This capability is particularly valuable in aquatic pest control, as accurately measuring the surface area of water bodies can be essential for assessing the proliferation of aquatic plants or pests, determining treatment areas, and planning management strategies. The other choices do not correctly represent the function of a planimeter, as it does not calculate volume, measure perimeters specifically for circles, or calculate distances in water bodies.