Lesson previewWhy Plotting Matters1 section shown
Preview lesson
Why Plotting Matters
What you will learn
Explain why timed systematic observations are needed before a radar contact can support a relative-motion, CPA or TCPA forecast.
Plotting converts separate range-and-bearing observations into a time-based relative-motion forecast. One contact position is evidence of where an echo was, not where it will pass.
A radar observation supplies a target range, bearing and time under the current set-up. A sequence of observations made at known intervals shows how that position is changing relative to own ship. From that sequence, a manual or automatic plot can forecast the closest point of approach (CPA) and the time to it (TCPA) if the observed motion continues.
COLREG Rule 7 requires proper use of radar when fitted and operational, including radar plotting or equivalent systematic observation. MCA guidance warns against assumptions from scanty radar information. A single point, one digital CPA or an unverified trail is not a systematic observation history.
CPA and TCPA support early risk appraisal; they are not universal safe-passing limits. Conditions, vessel manoeuvrability, traffic, visibility and available sea room shape the required margin. If information is doubtful, collision risk is treated as existing while the operator gathers better evidence and preserves time and space.
Timed relative-plot lab
Build the forecast one evidence step at a time
Select each stage of one fictional north-up exercise. The picture, construction, arithmetic and stop boundary stay together.
1 · Picture
One observation is a position, not a forecast
At 1200 the target is plotted at 6.08 M and 080.5°T. Its relative position is 6 M east and 1 M north of own ship.
2 · Construction
Mark the time beside the point. Do not draw a motion line, CPA or TCPA from one observation.
3 · Arithmetic
There is no timed displacement yet, so relative speed cannot be calculated.
4 · Stop boundary
The target may be closing, opening, crossing or manoeuvring. A single plot cannot distinguish those possibilities.
Move from one observation to a checked forecast without turning a provisional plot into certainty.
Worked example
A target remains at 045°T while its measured range falls from 6.0 M at 1200 to 4.8 M at 1206.
- 1The target closed 1.2 M in six minutes, and six minutes is 0.1 hour.
- 2The observed relative closing speed is 1.2 ÷ 0.1 = 12 kn.
- 3If the same straight relative track continued to own ship, 4.8 ÷ 12 = 0.4 hour, or 24 minutes, gives a nominal zero-range time of 1230.
Sense check: This is an urgency forecast, not proof of impact: remeasure, assess the full encounter and act early enough for the result to be obvious and monitored.
| Evidence available | Supportable conclusion | Not yet supportable |
|---|---|---|
| One range, bearing and time | Relative position at that observation | Relative course, speed, CPA or TCPA |
| Two accurate timed positions | Provisional relative track and relative speed | A stable future forecast without confirmation |
| Three coherent timed positions | A more credible current trend | Guaranteed continuation or target intention |
| Fresh observations after action | Whether the new relative picture is improving | That one successful-looking sweep ends the risk |
Turn detections into a timed evidence chain
A new contact appears ahead of the beam while visibility and traffic make an early assessment important.
1. Record
Take range, bearing and exact time without losing the target or wider lookout.
2. Repeat
Use a suitable interval and range scale, retaining the same target and measurement reference.
3. Forecast
Construct relative movement, CPA and TCPA, then state the assumptions and remaining uncertainty.
4. Refresh
Take another observation and continue through any manoeuvre until the encounter is past and clear.
Sense check: If another competent person cannot reconstruct the trend from the recorded values and times, the evidence chain is incomplete.
Common mistake or limitation
- Reading motion or intention from a single echo position.
- Treating a displayed CPA/TCPA as a guaranteed outcome or universal safe limit.
- Starting systematic observations too late to leave time for clear action and verification.
Recap
- Plotting is a time-based forecast built from repeated observations.
- CPA and TCPA remain conditional on the motion and inputs continuing.
- Uncertainty requires more caution, not more decimal places.
Optional quick check
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