What is a planimeter used for?

A planimeter is a tool used specifically for measuring the area of a two-dimensional shape by tracing its perimeter. This device allows users to determine the area enclosed by a geometric figure through a mechanical process that integrates the length of the perimeter. When the operator moves the planimeter along the edges of the shape, it automatically calculates the area based on the path traced.

The focus is on the ability of the planimeter to quantify area measurements, making it particularly useful in fields such as surveying and cartography, where understanding the dimensions of land and bodies of water is essential. Its precision and ability to work with irregular shapes set it apart from other measurement tools. While the other choices mention aspects like measuring liquid volume, calculating GPA rates, and measuring area of land, they do not pertain to the specific function of a planimeter. Thus, the primary purpose of a planimeter is correctly captured through the answer relating to measuring area by tracing perimeter.