Surveying Instruments, Field Procedures, and Safety
What this domain tests
Instrument behavior and field discipline: knowing the error budget of each tool, checking it in the field, and running a crew that goes home unharmed.
Total stations and EDMs
- Accuracy is specified as ±(a mm + b ppm), where ppm converts directly to millimetres per kilometre.
- The prism constant must match the prism in use; a wrong constant shifts every distance by that amount.
- Atmospheric correction refines the EDM; record temperature and pressure if the instrument does not.
- Observe both faces to detect collimation and vertical-index error; keep backsights long enough to control angular error.
Levels and GNSS
Check an automatic level with a two-peg collimation test and balance sight lengths at turning points. For RTK, control PDOP, watch for multipath near buildings and water, and confirm a fixed solution against a known point before collecting.
Field procedures
Record instrument height and target height explicitly; check into a second control point at the end of the set-up; close traverses; keep field notes that a stranger could follow.
Safety
Call 811 before any excavation. Set traffic control per the MUTCD, wear high-visibility PPE, never enter an unprotected trench, use fall protection, and treat railroad and highway work as permitted, flagged operations.
Worked example
A total station is rated ±(3 mm + 3 ppm) and you measure 1,200 m. The constant term is 3 mm; the scale term is 3 ppm × 1.2 km = 3.6 mm. Combined in quadrature or arithmetically, the expected error is about 6.6 mm, call it ±7 mm. A 30 m shot under the same specification carries roughly 3.1 mm - the constant term dominates short distances, which is why short backsights are so damaging.
Traps that cost marks
- Entering target height into the instrument-height field.
- Applying the prism constant with the wrong sign.
- Misapplying ppm as a fixed offset rather than per kilometre.
- Trusting a float RTK solution or a fix taken under canopy without an independent check.