Which tool is essential for measuring angles in land navigation?

In land navigation, the compass is an essential tool for measuring angles because it allows users to determine direction relative to the Earth's magnetic field. The compass enables navigators to take bearings, which are essential for plotting a course on a map. It provides a method to convert magnetic heading into map navigation, ensuring that the individual can accurately orient themselves in the terrain by aligning the compass with map features.

A protractor, while it is indeed used for measuring angles, is less practical for real-time navigation in the field. It is typically used in a more static environment, such as when working with maps at a table. A scale ruler assists with measuring distances on a map, while a caliper is used for measuring physical dimensions and is not relevant to navigation. Thus, while all the tools have their applications, the compass uniquely serves the purpose of measuring and conveying directional angles during land navigation.