The instrument which enables a surveyor to find the direction of a base line is

The instrument which enables a surveyor to find the direction of a base line is the "magnetic compass." A magnetic compass is an instrument that uses a magnetized needle to indicate the direction of the Earth's magnetic field. By aligning the compass with the direction of the magnetic field, the surveyor can determine the orientation of a base line, which is a fixed reference line used in surveying. In surveying, a base line is a line of known length and orientation that is used as a reference for making other measurements. It is typically established by measuring between two points with a high degree of accuracy using a tape measure or other measuring instrument. Once the base line has been established, the surveyor can use a magnetic compass to determine its orientation relative to the Earth's magnetic field. Using the magnetic compass, the surveyor can measure the angle between the base line and a reference direction, such as true north or magnetic north. This information can be used to orient other measurements taken during the survey, such as angles between points or distances between points. In this way, the magnetic compass is a critical tool for surveyors in determining the direction and orientation of a base line.