In excavation, which of the following is not a method to establish levels?

The correct choice is radars because they are primarily used for detecting objects and measuring distances rather than establishing levels in excavation and grading. Establishing levels is critical in excavation work to ensure that the site is graded correctly for drainage and stability.

String levels, transits, and lasers are all commonly used methods in this context. String levels involve using a taut string to create a horizontal reference line, making it straightforward to establish level measurements. Transits are optical instruments that allow precise alignment and leveling over longer distances. Lasers provide a more modern and accurate approach, projecting a level line of light that can be easily read at various points on the site. These methods are essential for achieving the necessary accuracy in excavation tasks and ensuring that the ground is prepared correctly for subsequent construction phases.