Chartwork foundations
Scope: RYA Day Skipper theory syllabus area: chartwork. Builds on: nothing; start here.
What a chart actually tells you
A nautical chart is a scaled picture of the seabed, the coastline and the aids that help you avoid both. It is also a compromise. Every chart was surveyed at a particular time, with the technology of that time, and has been corrected since only as far as someone has bothered to apply the corrections. Before you plan anything, read the title block:
| Title block item | Why it matters |
|---|---|
| Scale (e.g. 1:25,000) | Large scale shows detail for pilotage; small scale for passages |
| Projection | Almost always Mercator for coastal work, so latitude scale = distance scale |
| Depth units and datum | Depths in metres below chart datum |
| Height datum | Lights and land measured above a high-water level |
| Horizontal datum | Must match your GPS set-up, or positions will be offset |
| Corrections | Small corrections are noted bottom-left; compare with the latest Notices to Mariners |
Depths, drying heights and heights
- Soundings are depths below chart datum, which on UK charts is set close to the lowest predicted tide. On most days you will have more water than charted.
- Drying heights are shown as underlined figures in areas that uncover. A figure of 1.5 underlined means the seabed is 1.5 m above datum, so you need a height of tide of more than 1.5 m before there is any water at all.
- Heights of lights and land are measured from a high-water level. That keeps the figure conservative when you calculate how far off you will see a light or whether the mast will clear a bridge.
Symbols worth knowing by sight
You will not memorise the full symbols publication, and no one expects you to. Know the everyday ones: rock that covers and uncovers, rock awash, wreck (dangerous and non-dangerous), foul ground, overfalls and tide rips, anchorage, prohibited areas, cable and pipeline areas, contour lines, and the abbreviations used for seabed type (S, M, Sh, R and so on) that matter when choosing an anchorage.
TIP — Seabed notes A tender run ashore for a picnic depends on the seabed as much as on the depth. Sand and mud hold an anchor; rock and weed do not. The chart tells you before the guests do.
Publications that go with the chart
The chart does not stand alone. For coastal work you need an almanac or tide tables for heights and times, a tidal stream atlas or the chart's own tidal diamond table, a list of lights for detail beyond the chart, and pilotage notes for harbours. Electronic versions exist for all of these; the principles below apply whichever format you use.
CAUTION — Survey age Many inshore areas, including popular superyacht anchorages in the Mediterranean and Caribbean, were last surveyed long ago. The source diagram on the chart tells you how old the data is. Old lead-line surveys can miss isolated rocks between the lines.
The yacht angle
On a superyacht the bridge team keeps the ship's charts corrected, but deck crew running tenders and water toys are often working off a phone chartplotter or a laminated harbour plan. The same rules apply. Know the datum, know whether a figure is a depth or a drying height, and know how old the survey is. MGN 599 expects the person in charge of any passage to be familiar with the navigational hazards, and a tender run is a passage.