What is the purpose of a turning point (TP) in differential leveling?

The purpose of a turning point (TP) in differential leveling is to temporarily transfer an elevation. This process involves using the TP as a reference point when leveling between two locations that are too far apart to establish a direct line of sight with the leveling instrument. By taking a reading at the TP, surveyors can ensure that the elevation can be accurately continued from one setup of the instrument to the next, allowing for consistent measurements across the survey area. This method is crucial for maintaining accuracy in leveling work, particularly when there are obstacles or distance that would prevent direct measurements from one benchmark to another.

The other options do not accurately describe the function of a turning point. Marking the endpoint of a distance measurement and recording ground angles are not relevant to the primary function of a TP, which focuses exclusively on elevation transfer. Setting a base station for GPS measurements also does not relate to the purpose of a TP in the context of traditional leveling techniques.