Entry & progression
Exam module: Q171. See also: 46 CFR 26.03-4, which requires a UPV to carry charts and nautical publications.
Reading a chart
- Scale. A large-scale chart covers a small area in great detail — the harbour chart. A small-scale chart covers a large area with less detail — the passage chart. Large scale, small area. Candidates reverse this constantly.
- Soundings. Depths below chart datum. Check the units in the title block: metres, feet or fathoms. The same number means three different depths.
- Chart datum. Soundings are measured from it and heights of tide are added to it. On US charts it is normally mean lower low water — a level the tide will sometimes fall below.
- Heights. Charted heights of lights and land are measured from a different datum — normally mean high water. Bridge clearances too. Never mix the two.
- The title block. Datum, units, scale, projection and the survey date. Read it before you read anything else on the sheet.
WARNING — Chart datum is a floor you can go through Mean lower low water is an average, not a minimum. A strong high-pressure system and the right wind will put the water below datum and turn your two-metre sounding into something less. Charted depth is where you start the calculation, not where you finish it.
The publications, and what each is for
| Publication | The question it answers |
|---|---|
| Light List | What does this light do — its characteristic, height, range, and the structure it sits on. Published by the Coast Guard. |
| Coast Pilot | What is it actually like there — the narrative detail no chart can carry: local knowledge, port facilities, regulations, approach descriptions. |
| Tide Tables | Height of tide at a place and time. |
| Tidal Current Tables | Set and drift of the tidal stream — a different book, and a different question. |
| Notices to Mariners | What has changed since the chart was printed. The correction that keeps the chart true. |
| Navigation Rules | Required aboard self-propelled vessels of 12 m or more (33 CFR 83.01(g)). |
The Light List and the chart should agree. When they do not, the Light List is more current — it is corrected far more often than a chart is reprinted. And both defer to the latest Notice to Mariners.
Light characteristics
- F — fixed. Continuous, steady.
- Fl — flashing. Light shorter than dark.
- Oc — occulting. Light longer than dark. The opposite of flashing.
- Iso — isophase. Light and dark equal.
- Q — quick flashing, typically 50–60 per minute.
- Mo(A) — Morse 'A' (· –). The safe water mark.
- Fl(2+1) — composite group flashing. The preferred channel mark.
The period is the time for one complete cycle. Nominal range is the distance a light can be seen in standard visibility; geographic range is limited by the curvature of the earth and depends on the height of the light and the height of your eye. A powerful light on a low structure is limited by geography, not by its bulb.