A nautical chart looks like a map, but it is built for a different job. A road map shows where you can go; a chart mainly shows where you cannot go: the rocks, shoals, wrecks and shallow water hidden under the surface. Learning to read one is the foundation of safe navigation, whether you use paper, a chartplotter or a phone app.

Quick answer: start with the title block to find the chart’s scale, depth units and datum. Depths (soundings) show the water at the lowest normal tide, so actual depth is usually more. Measure distance on the latitude scale at the side of the chart, where one minute of latitude equals one nautical mile. Symbols and abbreviations are standardized and explained in a free key: U.S. Chart No. 1 from NOAA, or the UK equivalent, NP5011. Buoys follow the IALA system, which uses red to port entering harbor in Region A and red to starboard in Region B.

What a Nautical Chart Shows

A chart combines several layers of information on one sheet or screen:

• Water: depths, depth contours, seabed type, hazards such as rocks, wrecks and obstructions, and areas that dry at low tide.
• Aids to navigation: buoys, beacons, lighthouses and their light characteristics, fog signals and radar reflectors.
• Land: the coastline, heights of hills and conspicuous objects such as towers, chimneys and churches that you can use to take bearings.
• Regulation: channels, traffic separation schemes, anchorages, restricted and prohibited areas, cables and pipelines.
• Navigation tools: the latitude and longitude grid, compass roses and tidal information.

Chart Datum, Scale and Projection

Read the title block first

Every chart has a title block, usually on the land area. It tells you the chart number and name, the scale, the units used for depths and heights (meters on most charts worldwide; feet or fathoms on some US charts), the horizontal datum (usually WGS 84, which matches GPS), the vertical datum, the edition date and the projection. Misreading feet as meters has put plenty of boats aground.

Chart datum

Depths are measured from chart datum, a deliberately low water level. Most hydrographic offices, including the UKHO, use Lowest Astronomical Tide (LAT); NOAA uses Mean Lower Low Water (MLLW). Because chart datum is low, the water is usually deeper than charted. To find the actual depth, add the height of tide from the tide tables, as explained in our guide to tides for sailors.

Heights of lights, land and bridge clearances are measured from a high water level instead, typically mean high water springs on Admiralty charts and mean high water on most NOAA charts. Using a high reference means the real clearance is usually greater than charted, but on a big spring tide it may be less. The title block or chart notes state which reference is used.

Scale

Scale is expressed as a ratio. A large-scale chart such as 1:12,000 shows a small area (a harbor) in great detail; a small-scale chart such as 1:1,000,000 covers a large area (an ocean passage) with little detail. Always use the largest-scale chart available for coastal pilotage and harbor entry. On a chartplotter, zooming in on a small-scale chart does not add detail; it just enlarges the coarse data, which can hide a rock.

Projection

Most navigation charts use the Mercator projection, which stretches high latitudes but has a useful property: a straight line drawn on the chart is a line of constant compass bearing (a rhumb line). That is why you can lay off a course with a ruler.

Latitude, Longitude and Measuring Distance

Latitude is printed on the left and right borders of the chart, longitude on the top and bottom. Positions are given as degrees and minutes, for example 50°48.5’N 001°17.2’W.

The key rule: one minute of latitude equals one nautical mile (1,852 meters). To measure distance, open a pair of dividers between two points, then hold them against the latitude scale at the side of the chart, roughly level with the points you measured. Never use the longitude scale along the top or bottom: lines of longitude converge toward the poles, so a minute of longitude is shorter than a nautical mile everywhere except the equator.

Speed in knots is nautical miles per hour, so distance divided by speed gives passage time. For example, 12 miles at 5 knots takes 2 hours 24 minutes.

Depths, Contours and Seabed Types

The small numbers scattered across the water are soundings, depths below chart datum. On metric charts a sounding of 4₅ means 4.5 meters. An underlined number such as 1₂ is a drying height: the seabed there is 1.2 meters above chart datum and uncovers at low tide.

Depth contours join points of equal depth, much like contour lines on a hiking map. Areas of shallow water are tinted blue (often darker blue for the shallowest band), deeper water is white, and drying areas are usually green. Pick a safety contour that suits your boat’s draft plus a margin and treat everything inside it as off limits unless you have checked the tide.

Seabed types are shown with abbreviations, which matter when choosing an anchorage:

Abbreviation	Meaning	Abbreviation	Meaning
S	Sand	R	Rock
M	Mud	Sh	Shells
Cy	Clay	G	Gravel
Wd	Weed	Co	Coral

Combinations such as “fS” (fine sand) or “S.M” (sand and mud) are common. Our guide to anchoring in different seabed types explains which bottoms hold well.

Chart Symbols and Abbreviations

You do not need to memorize every symbol. NOAA publishes U.S. Chart No. 1 as a free download, and the UKHO publishes NP5011 for Admiralty charts. Both follow the International Hydrographic Organization standard, so most symbols are the same worldwide. Some symbols to recognize early:

• Rocks: an asterisk-like symbol is a rock that covers and uncovers; a plus sign with dots in the corners is a rock awash at chart datum; a plain plus sign is a dangerous underwater rock.
• Wrecks: a hull-shaped symbol shows a wreck with part of its hull visible; a horizontal line crossed by three short strokes shows a sunken wreck (circled by a dotted line if dangerous).
• Anchorages are marked with an anchor symbol; prohibited anchoring areas with an anchor crossed out.
• Cables and pipelines are wavy or dashed magenta lines; never anchor on them.
• Conspicuous objects on land are labeled “conspic” or “CONSPIC” and make reliable bearing targets.

Buoys, Lights and Marks (IALA A and B)

Lateral marks

Lateral marks show the sides of a channel. The world is split into two IALA buoyage regions:

• Region A (Europe, Africa, most of Asia, Australia and New Zealand): red cans to port and green cones to starboard when entering harbor or following the direction of buoyage.
• Region B (North, Central and South America, Japan, South Korea and the Philippines): reversed, so red to starboard when returning from sea (“red right returning”).

Charts show the direction of buoyage with an arrow where it is not obvious.

Other marks (same in both regions)

• Cardinal marks: black and yellow, with two cone topmarks. They show where the safe water is: pass north of a north cardinal, south of a south cardinal, and so on.
• Isolated danger: black with red bands and two black balls on top; marks a small hazard with safe water all around.
• Safe water: red and white vertical stripes with a single red ball; often marks a channel entrance or mid-channel.
• Special marks: yellow, with a yellow X topmark; used for areas such as outfalls, racing marks or water-ski zones.
• Emergency wreck marking buoys: blue and yellow vertical stripes, used temporarily for new wrecks.

Light characteristics

Next to each light, the chart prints a short code. For example, Fl(2) R 10s 15m 12M means: group flashing two flashes, red, repeating every 10 seconds, the light is 15 meters above the high-water reference, and its nominal range is 12 nautical miles. Other common characteristics are F (fixed), Oc (occulting: more light than dark), Iso (isophase: equal light and dark), Q (quick flashing) and LFl (long flash). At night you identify a lighthouse or buoy by timing its pattern with a watch. Vessels’ own lights work differently and are covered in our guide to sailboat navigation lights.

Compass Rose, Variation and Plotting a Course

Compass roses are printed in several places on each chart. The outer ring shows true directions, aligned with the meridians. The inner ring shows magnetic directions and is labeled with the local magnetic variation and its annual change, for example “Var 1°30’W (2020) 9’E”. Because variation changes each year, update the figure to the current year before using it.

To convert a true course to a magnetic one, add westerly variation and subtract easterly variation. The memory aid “west is best, east is least” works the other way, from compass to true. With a true course of 090° and variation of 3°W, steer 093° magnetic. Your compass may also have deviation caused by the boat’s own metal and electronics, recorded on a deviation card.

To plot a course on paper:

1. Draw a straight line from your starting point to your destination, checking that it clears all hazards with a margin.
2. Use a plotter or parallel rules to transfer the line to the nearest compass rose and read off the true bearing.
3. Measure the distance on the latitude scale.
4. Allow for tidal stream and leeway, then convert the course to magnetic for the helm.
5. Mark waypoints and note clearing bearings or depth contours that warn you if you stray.

Good habits like these are worth including in your sailing pre-departure checklist, and they are what make navigating without a chart or compass possible in an emergency. Once you are following the channels on your chart alongside other traffic, you also need a solid grasp of sailboat right of way rules.

Paper vs Electronic Charts

Charting is moving rapidly to electronic formats. NOAA cancelled its last traditional paper chart at the end of 2024 and now produces Electronic Navigational Charts (ENCs), with the NOAA Custom Chart tool letting users print their own paper charts from ENC data. The UKHO has said it will continue its paper chart service until at least 2030.

Format	Strengths	Weaknesses
Paper charts	No power needed; big overview; easy to plan on; reliable backup	Must be corrected by hand from Notices to Mariners; bulky; no live position
Chartplotter / ENC	Live position, AIS and radar overlay; updated electronically	Depends on power and electronics; easy to miss hazards when zoomed out
Tablet and phone apps	Cheap, portable, good for planning and backup	Battery and screen visibility; not all apps use official data

Most cruising sailors navigate electronically and keep paper charts or an independent backup device aboard. Electronic charts use the same symbols as paper ones. For options, see our guides to the best GPS units and chartplotters and best sailing apps, and if you are planning longer passages, offshore sailing for beginners.

How to Read a Nautical Chart: Summary

• Start with the title block: scale, units, datums and edition.
• Soundings are measured from a low chart datum; add height of tide for actual depth. Underlined figures are drying heights.
• Measure distance on the latitude scale: one minute of latitude is one nautical mile.
• Use Chart No. 1 or NP5011 to decode symbols and abbreviations.
• Know your IALA region: red to port entering in Region A, red to starboard in Region B. Cardinal and other marks are the same everywhere.
• Apply up-to-date variation when converting true courses to magnetic.
• Use the largest-scale chart available and keep charts corrected.

Spend an evening with a chart of your home waters and Chart No. 1 side by side, and when you are ready to understand how the wind on your boat works, read true wind vs apparent wind.