Surveying Instruments and Procedures
Angles and the total station
A total station combines a theodolite, an EDM, and a data collector. Understanding the error-cancelling procedures is the point of most questions. Turning a set with the telescope direct (face left) and reversed (face right) and averaging cancels horizontal collimation error and vertical-circle index error. Multiple sets, each advanced by a fraction of the circle, average out circle graduation errors. A backward/forward plunge or transit is not the same as a full set.
Distances, levels, and GNSS
EDM accuracy depends on the prism constant, atmospheric correction, and the instrument's specified accuracy (commonly ±(2 mm + 2 ppm)). The prism constant must be entered with the correct sign; a mismatch shows up as a short, consistent offset. Levels are run by the height-of-instrument method or rise-and-fall; a closed loop misclosure is distributed by distance, not uniformly, when the runs differ in length. GNSS surveying demands attention to initialization status, PDOP, multipath, and the stated accuracy of the base solution - RTK positions are relative to the base, not absolute.
Key trap: curvature and refraction
Long horizontal sights require a correction because the line of sight curves with refraction while the level surface curves away. Combined curvature and refraction is subtracted from the rod reading: 0.0206 × M² in feet, where M is the sight distance in hundreds of feet (0.0675 × K² in meters, K in km).
Worked example
A 500-ft sight: M = 5.00, so the correction is 0.0206 × 25 = 0.515 ft, subtracted from the rod reading. At 1000 ft the correction is 2.06 ft - which is why long shots are avoided for precise work.