Measurement and Positioning Technology
Measurement and Positioning Technology tests field methods and their error budgets: total stations, GNSS, differential leveling, datums, and the checks that make an observation usable.
Core rules
- A total station measures horizontal and vertical angles and slope distance; the instrument reduces slope distance to horizontal and vertical components using the vertical angle and prism constant.
- GNSS RTK provides coordinates in a datum such as NAD83(2011); heights may be ellipsoidal or converted to NAVD88. A 'fixed' solution with a good PDOP and redundant observations is stronger than a 'float' solution.
- Differential leveling closes a loop by comparing sum of backsights and foresights. Misclosure is distributed in proportion to distance, not simply averaged.
- Calibrate rods, prisms, and antennas; record temperature and pressure when required; check known points at the start and end of a session.
- In New Hampshire, dense canopy, steep terrain, and magnetic declination make independent checks important.
Worked example
A level loop of 2,000 feet closes with −0.08 foot. Linear misclosure is 0.08 foot. Precision = 2,000/0.08 = 25,000, or 1:25,000. If the project standard is 1:10,000, the loop passes; if the standard is 1:50,000, it fails and must be rerun. The reasoning is the ratio of loop length to misclosure, not the raw 0.08 foot alone.
Traps
Accepting an RTK float or single-epoch fix under canopy; ignoring the difference between grid and ground distances; applying a state-plane scale factor in the wrong direction; and treating a total station's displayed coordinates as independent evidence. Also watch rod plumb, prism constant, and benchmark stability.