ControlPoint Manual — versioned with the code; every worked example below is a REAL corpus job and its numbers are pinned by the test suite.
The .obs observation file
What it does
Every ControlPoint module reads and writes ONE observation format: .obs (CPOBS). Total-station setups, GNSS vectors (with their full 3×3 covariance), raw level runs, and reduced height differences (DH records — level or tape) all live in the same plain-text file. Third-party formats (.dat L/E records, GSI, LandXML) are import/export bridges, not parallel formats. Export your levelling job as .obs and every leg carries its type, spike offsets, use flag and fieldbook:line audit key.
Worked example
DH from=2HPX01N15 to=TAPE1 dh=-5.10440 dist=5 kind=tape src=M_16032023.dat:13 DH from=TAPE1 to=BM6 dh=1.54314 dist=21 src=M_16032023.dat:14
Exporting the champion job and re-importing it reproduces the identical adjustment (pinned: same error factor to three decimals, tape typing preserved).
Theory in one paragraph
An observation file is an audit document: every record says what was measured, between which marks, how well, and where the booking lives. Keeping one format across techniques is what makes a future combined adjustment (levels + tapes + total station + GNSS in one solve) a file-format non-event instead of a migration.