Surveying Fundamentals and Chain Surveying
Core concepts
Surveying fixes the relative positions of points on or near the earth's surface. Know the primary divisions: plane surveying (earth treated as flat, valid for small areas) versus geodetic surveying (curvature accounted for). Understand the principles of working from whole to part, of two independent checks, and of locating a point by two measurements from fixed references.
Chain surveying suits open, fairly level ground with simple detail. Learn the instruments: chain/tapes, arrows, ranging rods, offset rods, cross-staff, optical square and plumb bob. Master the terms: base line, tie line, check line, main station, subsidiary station, offset (perpendicular or oblique), ranging (direct and indirect), and the survey stations' numbering.
Traps in questions
- Confusing check line (joins two already-fixed points, verifies work) with tie line (joins a station to a nearby fixed detail).
- Mixing up ranging: direct ranging needs intervisibility; indirect ranging is used over a hill or across a valley.
- Chain corrections: too long/short chain, temperature, pull, sag and slope. Remember correction is applied opposite in sign to the error.
- Offsets beyond the chain length limits (roughly 15 m with tape, 30 m with chain) must be avoided.
Worked example / rule of thumb
A 20 m chain is found to be 0.10 m too long. Measured distance = 150 m. True distance = measured × (actual/standard) = 150 × (20.10/20) = 150.75 m. Rule: for a long chain, true = measured × L'/L; for a short chain the value reduces.
Cross-staff checks: set the base line, take perpendicular offsets, and confirm with a check line whose measured length should match the computed length.