Safety and gas bind before the survey does. Adapt to the conditions or stop — a partial ring is always a valid outcome.
This dive is optional — it is not required for certification; the assessment is the Dive 4 capstone. Dive 5 asks a question about the method itself: if the survey ring is run at half its radius — sampling a smaller, inner patch more densely — does it report the same density as the full-radius ring at the same hub? It probes whether six stations at 2r adequately represent the ground around the hub, or whether the survey's number would shift under a different sampling geometry — a check on the method's spatial sufficiency, not on the reef. Run quickly, it is the dive that lets a student see whether the six-station survey is measuring the reef or measuring its own geometry.
The half-radius ring. Everything is the same as the full survey except one number: the swim-out is R = r instead of R = 2r. The six headings are identical (out 000°, then +60° at each stop: 120 / 180 / 240 / 300 / 000 / 060), the +60° turn rule is identical, and there are still six panoramas. Only the leg length is halved — r kicks instead of 2r. Because the diver swims half as far on every leg, the dive is short, which is why it fits as an optional third dive of Day 2.
What halving the radius does to the geometry. At R = r, the six station centers sit at distance r from the hub and are r apart from each other — but each panorama disk still has radius r. Centers r apart with radius-r disks means the disks now overlap heavily rather than sitting tangent. The ring samples a smaller, inner patch, and samples it with overlapping captures. This is deliberate: the half-radius ring is a denser look at the ground closest to the hub.
Why this is a diagnostic check, not a correction. The half-radius reading is not a more correct number that fixes the full-radius one — neither is ground truth; both are visibility-limited estimates with their own detection limits. So the two are compared, not used to adjust each other. The honest question is consistency: does the dense inner ring report a density consistent with the full ring at the same hub? Because the hub already carries full-radius readings with a measured noise floor (Dives 2–4), the test has a yardstick: if the half-radius density falls within the full-radius noise floor, the method is internally consistent at this site; if it falls well outside, the survey's reading depends on the radius chosen — which is itself a finding worth recording.
The caveat, stated plainly. The overlapping captures of the half-radius ring share water, and therefore share organisms: an animal in an overlap region is seen by more than one capture. So the six half-radius densities are not independent — they are correlated by construction, partly because they re-see the same animals. The half-radius ring's internal spread therefore understates its true sampling uncertainty, and the comparison with the full ring must be read with that in mind. The diagnostic check is suggestive, not decisive: it flags gross inconsistency rather than certifying agreement.
Performance requirements. By the end of this dive the student will have:
-
Re-run the six-station ring at half radius (R = r) at a hub with full-radius readings, a panorama at each of the six centers.
-
Produced a reconcilable two-observer count as before.
-
Computed the six half-radius densities and their site index.
-
Compared the half-radius index with the full-radius readings and their noise floor, and stated whether the two are consistent.
-
Articulated the shared-fish caveat — why the overlapping half-radius captures are not independent samples.
Equipment for this dive. As Dives 2–4.
Pre-dive briefing. Confirm the hub (one carrying full-radius readings), north, the gas plan, and the single change: legs are r kicks, not 2r. Brief the comparison and the shared-fish caveat, so the student knows what the dive is testing and what it can and cannot conclude. The heading sequence, roles, and counted set are unchanged (Dive 2).
The dive.
-
Descend to the hub; establish neutral buoyancy and trim; set and confirm north.
-
Confirm today's r (carried from the Day-2 calibration); the swim-out is R = r.
-
Run the ring: out r on 000° to center 1, panorama; add 60° (120°), swim r, panorama; through 180°, 240°, 300°, 000° — six panoramas; the closing 060° leg returns to center 1.
-
Ascend with a safety stop.
Post-dive. Compute the six half-radius densities and the site index; compare with the hub's full-radius readings and noise floor. State the result honestly — consistent (within the noise floor) or not — with the shared-fish caveat attached. Debrief what the comparison shows about whether six full-radius stations are a sufficient sample at this site.
Assessment. The dive is satisfactory when the student has run the half-radius ring, produced reconcilable counts and a half-radius site index, made the comparison with the full-radius readings, and can explain both what the comparison suggests and why the overlapping captures make it suggestive rather than decisive.
Depth range. 6–13.5 m / 20–45 ft (Day 2), consistent with the gas plan.