The Plotting Room – Visitor Guide

What the plotting room did

It turned sightings of a ship into firing data (elevation & deflection) for the coastal guns—fast, repeatable, and with built‑in checks to avoid mistakes.

Where it sat

Capel’s plotting room was two‑storey and underground, close to the gun emplacements but protected by earth and concrete. Cables connected it to observation posts (on the cliffs) and to each gun pit.

Who was inside (typical team)

• Plotting Officer – in charge of the solution and fire orders
• Recorders ("plotters") – marked positions on the plotting table and kept the log
• “Numbers” / Calculators – ran mechanical aids (range clocks, deflection drums)
• Telephonists / Signalers – maintained voice circuits to observers & guns
• Battery Commander – authorised fire and rules of engagement
• Spotters link – received “fall of shot” corrections after firing

The workflow (8 essential steps)

1. Spot & report – Observer calls “Bearing 112, Range 17,400” at each tick.
2. Lay the plot – Recorder marks each report on the table; a line through points reveals target course & speed.
3. Predict – Calculators add time of flight to project a future aim point along that track.
4. Correct – Apply wind, drift, muzzle velocity (barrel wear), height of site, and any tide/current effects.
5. Compile firing data – Convert the aim point into elevation (for range) and deflection (to aim ahead/aside).
6. Transmit to guns – Telephonist reads, gun by gun: “Gun 1: Elevation 7° 30’, Deflection 3 left, One round, Fire on my order.”
7. Fire & observe – On command, guns fire. Observers report fall of shot: “200 short, one left.”
8. Correct & repeat – Plotting room updates the solution, sends new figures, and the cycle continues until the target breaks contact or is destroyed.

Why it worked

• Prediction beats reaction – The enemy moved during the shell’s flight; prediction accounted for that.
• Common time base – Everyone worked to the same ticks; without it, data drifted and misses multiplied.
• Redundancy – Duplicate phone lines and procedures minimised a single‑point failure.
• Discipline – Messages were repeated back verbatim; only the proper rank issued orders.

Safety & control

• Strict line discipline (read‑back of every fire order).
• Separate magazines with anti‑spark tools and rubber mats.
• Logged every plot, order, and correction for review and learning.

A quick example

• 11:00:00 – Bearing 110, Range 18,200 → plotted.
• 11:00:10 – Bearing 111, Range 17,900 → course/speed estimated ~30 knots to the ENE.
• Time of flight to 17,500 yds ≈ 30–40 seconds (example). Aim point advanced to where the target will be.
• Firing data sent; salvo goes. Spotter: “100 over, 2 right.”
• New settings issued; next salvo “straddles” → walk onto a hit.

Glossary (tiny)

• Elevation – up/down barrel angle to reach range.
• Deflection – left/right lead to meet a moving target.
• Time of flight – seconds the shell is in the air; core to prediction.
• Fall of shot – where the last rounds landed relative to the target.