Nautical Charts

What a Nautical Chart Shows

A nautical chart is a highly specialized map designed for marine navigation. Unlike land maps, charts show the underwater world just as much as the surface above — every chart is built around the fundamental question: is there enough water here to float my boat?

Charts display water depth (soundings), submerged hazards (rocks, shoals, wrecks), navigation aids (buoys, lighthouses, beacons), coastal features (harbors, anchorages, channels), and tidal information. They also show a limited amount of land detail immediately adjacent to the water — enough to identify landmarks useful for navigation.

Modern charts come in two forms: paper charts (traditional, no batteries required) and electronic charts used with chartplotters and navigation software. Electronic charts — available as raster (scanned images) or vector (data-driven) formats — allow GPS overlay, route planning, and real-time position display. Both forms rely on the same underlying data and symbols.

Sample nautical chart showing depth soundings, contour lines, buoys, a lighthouse, and a harbor entrance — A nautical chart combines underwater depth data with surface navigation aids and coastal features

💡

Paper charts should always be aboard as a backup, even on boats with full electronic chart systems. Electronics fail; paper doesn't.

What is the primary concern a nautical chart is designed to answer?

Where is the nearest port? Is there enough water depth here for safe navigation? What are the tidal patterns for this area? Where are other vessels located?

What is the difference between a raster and a vector electronic chart?

Raster charts work offline; vector charts require internet Raster charts are scanned images; vector charts are data-driven and allow zooming without distortion Raster charts are more accurate; vector charts are for planning only There is no practical difference for cruising sailors

Depth, Datum, and Soundings

Every depth number (sounding) on a chart is measured from a reference level called the chart datum. In the US and many countries, this is Mean Lower Low Water (MLLW) — the average of the lower of the two daily low tides. This means chart depths represent the minimum water you can generally expect. Actual water depth at any moment is the charted depth plus the tide height at that time.

Depth on US charts is given in feet on older charts and meters on newer ones — always check the legend. Soundings shown in italics typically indicate areas that may dry (be above water) at low tide. Dotted contour lines connect points of equal depth — similar to topographic contours on land maps.

The danger sounding — the minimum depth the skipper has set as safe — is the critical number to watch. If charted depth approaches the vessel's draft plus a safety margin, slow down, check the tide, and consider an alternative route.

Diagram showing chart datum (MLLW), mean sea level, high water, and how tidal height adds to charted depth — Actual depth = charted depth + current tide height. Chart datum is set near the lowest predictable water.

Charted depth at the harbor entrance: 8 feet. Current tide height above datum: 3.5 feet. Actual depth now: 8 + 3.5 = 11.5 feet.

Your boat draws 5 feet. With a 2-foot safety margin, you need at least 7 feet. 11.5 feet is safe to proceed — but return at low tide and the charted 8 feet may only give you 8 − 0.5 (a slight negative tide) = 7.5 feet. Much tighter.

A chart shows 6 feet at a channel entrance. The current tide height is 2 feet. What is the actual depth?

4 feet 6 feet 8 feet 12 feet

What does 'chart datum' refer to?

The date the chart was last updated The reference water level from which all depths are measured The depth at which the chart shows shaded blue The average high-water level

Chart Scale and Coverage

Chart scale describes how much the real world is reduced to fit on the chart. A large-scale chart (e.g. 1:10,000) shows a small area in great detail — ideal for harbors, anchorages, and coastal passages. A small-scale chart (e.g. 1:500,000) shows a large area with less detail — useful for passage planning across open water.

A common confusion: 'large scale' sounds like it should mean a big area, but it means a large fraction — 1/10,000 is larger than 1/500,000. Think of it as zooming in (large scale = zoomed in = more detail).

Always use the largest scale chart available for the area you're actually navigating. Small-scale charts omit hazards that are charted in detail on larger-scale versions. In confined or complex waters — channels, harbors, anchorages — the largest scale chart available is essential, not optional.

💡

When approaching an unfamiliar harbor, switch from your ocean passage chart (small scale) to the harbor chart (large scale) well before entering. The larger-scale chart shows rocks, shoals, and channel markers that are invisible on the passage chart.

You are approaching a complex harbor entrance. Which chart scale should you use?

Small scale (1:500,000) for the widest view Large scale (1:10,000 or similar) for maximum detail Scale doesn't matter if you have GPS Medium scale (1:100,000) is always the best compromise

Which of the following is a large-scale chart?

1:500,000 1:250,000 1:50,000 1:12,000

Planning and Monitoring a Passage

Before leaving, passage planning involves marking a route on the chart that avoids all hazards with appropriate margins. Plot waypoints at safe turning points, note the minimum depths along the route, check for hazards at low tide, and identify alternates (bail-out harbors) in case conditions change.

Monitoring position during the voyage involves regularly checking your actual position against the chart — whether through GPS, visual bearings, or dead reckoning. Mark your position at regular intervals. This continuous loop of planning, monitoring, and updating separates systematic navigators from those who are lost and don't know it yet.

Charts require updating as buoys move, new hazards are discovered, and channels change. NOAA (in the US) provides chart corrections through Notice to Mariners. Always confirm that your chart covers the current edition, especially in areas with frequent changes like dredged channels and port approaches.

When passage planning on a chart, what is the most critical check for every waypoint on your route?

That there is a marina nearby That minimum depths along the route are safe at the lowest anticipated tide That the bearing from waypoint to waypoint is a round number That the route stays within 5 miles of shore

Why should charts be checked for currency (latest edition) before use?

Older charts use different color schemes Hazards, buoy positions, and channel depths change over time and may not appear on outdated charts GPS coordinates shift over time and old charts will show errors Chart datum changes yearly

Summary

Nautical charts show depth, hazards, navigation aids, and coastal features — all organized around the question of whether there's enough water.

Actual depth = charted depth + current tide height. Chart datum (MLLW in the US) is near the lowest expected water level.

Large-scale charts show small areas in great detail; always use the largest scale available for your actual navigation area.

Passage planning means plotting a safe route, checking minimum depths at lowest tide, and marking alternates.

Check chart currency — buoy positions, channel depths, and hazards change. Use updated charts and current Notice to Mariners corrections.

Key Terms

Nautical chart: A specialized map of marine areas showing depth, hazards, navigation aids, and coastal features for safe navigation
Sounding: A depth measurement charted on a nautical chart, measured from chart datum
Chart datum: The reference water level from which all depths are measured — typically MLLW (Mean Lower Low Water) in the US
MLLW: Mean Lower Low Water — the average of the lower of the two daily low tides; the standard US chart datum
Large-scale chart: A chart with a large fraction scale (e.g. 1:10,000) showing a small area in great detail
Small-scale chart: A chart with a small fraction scale (e.g. 1:500,000) showing a large area with less detail
Raster chart: An electronic chart created by scanning a paper chart; accurate but pixelates at high zoom
Vector chart: A data-driven electronic chart allowing smooth zoom, layer toggling, and GPS integration
Danger sounding: The minimum depth the skipper has defined as safe for the vessel's draft plus safety margin

What a Nautical Chart Shows

Depth, Datum, and Soundings

Chart Scale and Coverage

Planning and Monitoring a Passage

Summary

Key Terms

Nautical Charts — Quiz

Your chart shows 4 feet at a bar. Your draft is 5 feet. The tide height is currently 3 feet above datum. Is it safe to cross?

What does 'chart datum' represent?

You are entering a marina in an unfamiliar harbor. You have a 1:100,000 coastal chart and a 1:10,000 harbor chart. Which do you use?

Soundings on a chart shown in italics typically indicate:

Which of the following best describes a passage planning best practice?

References & Resources

Related Links