Optical levels, transit levels, and theodolites measure elevation differences between two points for site surveying and construction applications. They assist with determining and transferring level over distances that are too large to measure with straight edges and bubble levels. These instruments are typically paired with a tripod that sets them at a comfortable working height, and they are centered and leveled on the tripod to ensure accurate measurements. They contain a telescope with magnifying lenses and a crosshair sight for viewing the target point (typically a grade rod) in order to establish a reference line.