What characteristic defines a Jacobs staff in surveying practice?

A Jacobs staff is distinguished by its known length, which is typically marked with a series of graduated measurements. This characteristic is crucial in surveying as it helps provide accurate reference points when measuring distances or determining elevations. The known lengths allow surveyors to perform precise calculations and reduce the risk of errors during their measurements. This staff is often used in conjunction with a leveling instrument, where its fixed lengths aid in establishing a horizontal plane or verifying levels across different points in a survey area.

While features such as adjustable height or reflective surfaces may be found in other surveying tools, they do not specifically define a Jacobs staff, underscoring the importance of its known length in the field of surveying.