Depth-survey instrument
Lower the bob, clamp at depth, and read deviation from true vertical — one continuous datum from the surface to the floor of the shaft.
How it reads
There is no grid of widgets. Every reading hangs off a single plumb-line, so the operator always knows how deep the bob is and how far the structure has left true.
Fix the reel at the surface and let the bob find true vertical. The line becomes the reference for everything below.
Pause the sonde at each station. Instruments read offset on both axes, twist, and line tension against the datum.
Commit offset, tension, and time to the survey log. Green is true, amber is drift inside the band, red lost the return.
At the bottom the bob settles and the full profile of deviation reads back up the line, station by station.
What it carries
A continuous plumb-line runs the height of the survey; stations clamp to it so depth and offset are never lost.
A horizontal datum with a needle reads how far each station sits off vertical, against a documented tolerance band.
Depth, axis, offset, tension, and committed state — a record an inspector can sign, not telemetry filler.
Aldgate clock tower · sounding rig PB-4 · 5 stations stamped