This code, as Part 3 of the Geodetic Survey series, specifies the procedures and standards for conducting high-precision levelling. It covers instrumentation, field methodologies, permissible errors, and data reduction for establishing accurate vertical control networks essential for large-scale engineering, mapping, and scientific projects.
Lays down the procedure for geodetic levelling, including instrumentation, field procedures, and computations for high-precision height determination.
Quick Reference Values
Permissible misclosure for High Precision Levelling± 4√K mm, where K is distance in km
Permissible misclosure for Precise Levelling± 8√K mm, where K is distance in km
Permissible misclosure for Secondary Levelling± 24√K mm, where K is distance in km
Maximum length of sight for precise levelling50 m
Maximum difference in sight lengths per setup (Precise)2 m
Minimum ground clearance of line of sight0.5 m
Key Formulas
e = ± C√K — Permissible closing error in mm, where K is the circuit length in km and C is a constant based on precision.
Δh = BS - FS — Difference in elevation between two points from a single instrument setup.
Correction for Curvature (Cc) = -0.0785 * D² — Where D is the distance in km.
Correction for Refraction (Cr) = +0.0112 * D² — Where D is the distance in km.
Practical Notes
Balancing backsight and foresight distances at each instrument station is the most critical field practice to minimize errors from instrument misalignment (collimation) and earth's curvature/refraction.
Always perform a 'two-peg test' to check the instrument's collimation error before starting a high-precision levelling survey.
Ensure the levelling staff is held perfectly vertical using a staff bubble, as any tilt introduces significant error, especially on long sights.