Navigating poorly charted or uncharted waters safely

Electronic charts look precise. The coastline is sharp, the soundings are neat, and the boat symbol moves smoothly across the screen. That precision is often false. In many remote cruising grounds, the data behind the chart comes from surveys made long ago with lead lines and sextants, and nobody has checked it since.

Skippers who cruise these areas safely do not trust any single source. They plan carefully, read the water with their own eyes, move slowly and keep a way out. This guide sets out how they do it.

Why charts can be wrong

• Old surveys. Parts of many tropical and remote regions were last surveyed in the nineteenth or early twentieth century. Soundings may be sparse, and the space between them was never measured at all.
• Datum offsets. Older charts were drawn on local horizontal datums. When they are converted for use with GPS, which uses WGS 84, the whole chart can be shifted from its true position, sometimes by a significant distance. Your GPS position can be accurate while the reef on the chart is in the wrong place.
• Changing seabeds. Sand banks move, river mouths silt up, and coral grows. Storms can rearrange a channel in one season.
• Missing features. A single isolated rock or coral head between survey lines may simply not be there on the chart.

Check the source data

Before you rely on a chart, find out how good it is.

• Look at the survey information. Paper charts usually include a source diagram showing when and how each area was surveyed. Official electronic charts (ENCs) carry quality information, often shown as zones of confidence, or CATZOC.
• Understand zones of confidence in general terms. CATZOC categories run from the highest confidence (A1), through B and C, to D (poor) and U (unassessed). Many chart plotters can display them. In lower categories, expect larger position errors and uncharted dangers.
• Check the datum in the chart notes and how your plotter or app handles it.
• Read the notices to mariners and any warnings for the area, and use the most recent editions of charts and pilot books.

Cross-check with other sources

No single source is enough, so compare several.

1. Satellite imagery. Many navigation apps can overlay satellite images on the chart. In clear water, reefs, sand and channels are often visible. Remember that images can be old, cloud and glare can hide features, and the imagery itself may not line up perfectly with the chart.
2. Pilot books and cruising guides. These often give approach directions, waypoints and warnings based on recent visits.
3. Recorded tracks. Tracks from other cruisers can show a route that worked for them. They do not show the draft, tide or conditions on the day, so treat them as a hint, not a guarantee.
4. Your own earlier tracks. If you entered safely, your inbound track is your most reliable route out.

When sources disagree, assume the worst one is correct until you can prove otherwise.

Eyeball navigation and light

In clear tropical water, your eyes are the most important instrument. Good conditions make the difference.

• Have the sun high and behind you. Light from above and behind lets you see into the water. Looking towards a low sun produces glare that hides everything. Plan arrivals in the middle of the day, and avoid entering a reef area late in the afternoon when the sun is ahead of you.
• Wear polarised sunglasses. They cut surface glare and show colour changes far more clearly.
• Put a lookout high and forward. Someone at the bow, or better on the spreaders or a raised position, sees much further into the water than the helm. Agree simple hand signals in advance.
• Learn to read the colours. In general, deep water looks dark blue, shallower water over sand turns lighter blue and then pale green, brown or dark patches can be coral or rock just below the surface, and white or very pale areas are often very shallow. Cloud shadows can look like reefs, and real reefs can hide under ripples.

Slow down and sound

Speed is the enemy. At low speed you have time to stop, and if you touch, the damage is usually smaller.

• Watch the depth sounder continuously, and know whether it reads from the transducer, the waterline or the keel.
• Set a shallow-water alarm with a sensible margin for your draft and the sea state.
• Use forward-looking sonar if you have it. It can warn of rising ground ahead, but its range and beam are limited, and it can miss a narrow coral head. It supports your lookout; it does not replace it.
• Consider a tender ahead. In a narrow or doubtful pass, sending the tender in first with a handheld sounder or lead line is a long-established technique.

Lay a safe track

1. Plan the route on the chart and imagery before you arrive, staying well clear of any doubtful areas.
2. Use clearing lines and bearings on visible marks where possible, rather than relying only on GPS.
3. Keep wide margins around charted dangers, larger in lower confidence zones.
4. Record your track as you go, and use it to leave by the same route.
5. Decide your abort point in advance: where you will turn back if the light, weather or sea state is not right.

Anchoring in unknown ground

• Look before you drop. Inspect the bottom from the bow, with a mask and snorkel, or from the tender. Choose sand where possible.
• Protect coral and seagrass. Do not anchor on live coral. Many places have rules about this, and some require mooring buoys.
• Allow for swinging room and for wind shifts that could put you on a reef at night.
• Consider a trip line where the anchor could foul on rock or coral.
• Plan a night exit. If the wind changes after dark, you may need to leave without good light. Make sure your recorded track gives you a safe way out.

Local knowledge

People who live and work on the water often know far more than any chart. Fishermen, harbour staff, local pilots and other cruisers can tell you about new shoals, changed channels and seasonal conditions. Where local pilots are available or required, use them. Always respect local rules and customs, including permits for protected or restricted areas, which vary by country.

Contingency and communications

• Leave a passage plan with someone ashore, with your route and expected arrival time.
• Carry reliable communications beyond VHF range, such as a satellite phone or satellite messenger, and a registered EPIRB.
• Know the nearest help: where the closest port, haul-out facility and medical support are, and how long it would take to reach them.
• Prepare for a grounding. Know how to check for hull damage and water ingress, and how you would try to get off safely.

Choosing a yacht for remote cruising

A shallow draft, strong hull, good visibility from the helm and high positions for a lookout all help in poorly charted waters. Many owners choose catamarans or sailing yachts with these features in mind. Browse the full catalogue on Yachtxchange, and before you buy, use our surveyor directory to find a surveyor who can check the hull, keel and steering, which take the most stress in a grounding. More guides are on our resources page.

This article is general information only. It does not replace official charts, pilot books, notices to mariners, local regulations, proper training or good seamanship. The skipper is always responsible for the safety of the yacht and crew.