How to Read Nautical Charts for Beginners: Depths, Symbols, Scale and Distance

A nautical chart packs an enormous amount into a single sheet — depths, dangers, lights, the shape of the coast. This guide shows you how to read it: scale, the lat/long grid, measuring distance the right way, depths and datum, and the symbols that matter most.

What a nautical chart is

A nautical chart is a scaled map of a sea area produced by a national hydrographic office (such as the UK Hydrographic Office's Admiralty charts, or NOAA in the United States). Unlike a road map, it concentrates on what is under and around the water: depths, the nature of the seabed, rocks and wrecks, lights and buoys, tidal information and the coastline. Even in an age of chart plotters, knowing how to read the chart — paper or electronic — is the foundation of safe navigation.

Chart scale

Scale tells you how much area a chart covers and in how much detail:

• Small-scale charts (e.g. 1:150,000) cover a large area with less detail — for passage planning and offshore work.
• Large-scale charts (e.g. 1:7,500) cover a small area in fine detail — for harbours, approaches and pilotage.

Latitude, longitude and position

Position on a chart is given as latitude (north/south of the equator, 0°–90°) and longitude (east/west of the Greenwich meridian, 0°–180°). Each degree divides into 60 minutes, and minutes into decimals or seconds.

• Latitude is read up the left and right edges of the chart.
• Longitude is read along the top and bottom.
• To plot a position, run across from the latitude and down from the longitude until they meet.

Reading a position off the chart quickly matters on the radio too: a Mayday call must include your position — latitude and longitude, or a bearing and distance from a named navigational mark or harbour. Charter skippers are often asked for a VHF radio certificate alongside their skipper licence; if you still need one, our VHF SRC certificate course runs fully online and covers distress, urgency and routine calls, including passing a position unambiguously.

Measuring distance

This is the single most important chartwork habit, and the one beginners most often get wrong:

Because the latitude scale itself stretches slightly toward the poles on a Mercator chart, always read distance against the part of the latitude scale nearest to where you are measuring.

Depths, drying heights and datum

• Soundings — the numbers in the water are depths below chart datum, in metres on modern charts.
• Chart datum — usually Lowest Astronomical Tide (LAT). Actual depth = charted depth + height of tide.
• Drying heights — an underlined number is the height a feature dries (uncovers) above datum at low water.
• Depth contours — lines joining equal depths, shaded to show shallow water, give you the shape of the seabed at a glance.
• Heights — numbers on land, lights or bridges are heights above a height datum (often Mean High Water Springs).

Symbols and abbreviations

Charts use a standard symbol language. A few you will use constantly:

The full reference for Admiralty charts is NP5011 (Chart 5011), "Symbols and Abbreviations". Keep a copy aboard until the common symbols are automatic.

The compass rose

The compass rose printed on the chart shows true north (outer ring) and magnetic north (inner ring), with the local magnetic variation and the year it was measured. Use it to convert between true and magnetic bearings when plotting courses or taking bearings to fix your position.

Frequently asked questions

How do I read a nautical chart as a complete beginner?

Start with four things: the scale (how much sea the sheet covers), the depth numbers in the water (metres below chart datum — the lowest tide), the coloured depth areas and contour lines, and the symbols for dangers, buoys and lights. Then find your position with the latitude and longitude scales on the edges, and measure distance on the latitude scale only — one minute is one nautical mile. Everything else on the chart builds on those four.

How do I read the depths on a nautical chart?

Each number in the water is a sounding: the depth in metres (older charts: feet or fathoms — the chart's title says which) below chart datum, which is roughly the lowest tide, so the real depth is usually a little more. A small subscript digit is a decimal (7₂ = 7.2 m). An underlined number is a drying height: that spot is above the datum at low water. The shaded areas and contour lines group the depths — blue tints for shallow water, white for deep.

Is a nautical map the same as a nautical chart?

At sea the word is chart. A map shows the land; a nautical chart shows the water — depths, dangers, buoys, lights and the coast as seen from the sea — and is made for plotting positions and courses. If you searched for "nautical map", this guide is what you want.

How do you measure distance on a nautical chart?

Use the latitude scale on the sides — one minute of latitude equals one nautical mile. Never use the longitude scale, because lines of longitude converge toward the poles and don't represent a constant distance.

What do the numbers on a nautical chart mean?

Numbers in the water are soundings — depth below chart datum in metres. Underlined numbers are drying heights (above datum). Numbers on land or by structures are usually heights above a height datum.

What is the difference between large-scale and small-scale charts?

Large-scale charts show a small area in great detail (harbours, pilotage); small-scale charts show a large area with less detail (passage planning). "Large scale, large detail" — navigate on the largest-scale chart available for where you are.

Where can I find what chart symbols mean?

For Admiralty charts, publication NP5011 ("Chart 5011") lists every symbol and abbreviation. Other hydrographic offices publish equivalents. Keep one aboard until the common symbols become second nature.

Related reading

• Position Fixing & the Running Fix — turning bearings into a position on the chart
• Understanding Tides — chart datum and height of tide
• Free sailing video lessons — chartwork, tides, buoyage and more
• Maritime glossary — navigation and COLREG terms