Measurement and Instrumentation
This domain tests instrument operation, error sources, and adjustment. Expect questions on EDM corrections, curvature and refraction, traverses, least squares, levels, and GNSS measurement.
Core rules
- Systematic errors repeat and can be corrected, such as prism constant and atmospheric ppm. Random errors are treated statistically. Blunders must be found and removed, not averaged away.
- EDM atmospheric correction is applied in parts per million based on temperature and pressure.
- Combined curvature and refraction correction in feet is about 0.0206 D squared, where D is in thousands of feet. The rod appears high, so the correction is subtracted.
- Traverse precision is the ratio of linear misclosure to perimeter. A 0.15-ft misclosure on 3,000 ft is 1:20,000.
- Bowditch and transit rules distribute misclosure; least squares distributes by weights and gives residuals and redundancy.
- Reciprocal leveling cancels refraction and curvature over long sights.
Worked example
A 2,000-ft shot is taken with a level. D = 2.0 thousand ft, so correction = 0.0206 x 4 = 0.082 ft. Because curvature and refraction make the distant rod appear higher than it is, subtract 0.082 ft from the rod reading. If the reading was 5.000 ft, the corrected reading is 4.918 ft.
Traps
- Using the wrong sign for prism constant.
- Forgetting to apply the scale factor to traverse distances.
- Averaging a set that contains a blunder.
- Using D in feet instead of thousands of feet in the curvature formula.
- Ignoring instrument and reflector centering errors on short sights.