What is the difference between a large-scale and a small-scale chart?

A large-scale chart covers a small area in great detail — used for harbours, pilotage and close-quarters work. A small-scale chart covers a large area with less detail — used for passage planning and offshore navigation. The memory aid is "large scale, large detail." Always navigate on the largest-scale (most detailed) chart available for where you actually are.

What is chart datum on a nautical chart?

Chart datum is the reference level from which charted depths are measured, usually Lowest Astronomical Tide (LAT) on modern charts. Charted depths are the depth below datum, so the actual depth at any moment equals the charted depth plus the height of tide. Because datum is set at the lowest astronomical tide, the real depth is almost always at least the charted figure.

How to Read Nautical Charts: A Beginner's Guide

Q: How do you measure distance on a nautical chart?

Use the latitude scale on the left or right edge of the chart, never the longitude scale along the top or bottom. One minute of latitude equals one nautical mile. Take the distance between two points with dividers, then lay the dividers against the latitude scale at roughly the same level on the chart to read off the miles. Because lines of longitude converge toward the poles, the longitude scale does not represent a constant distance and must not be used for measuring.

Q: What do the numbers on a nautical chart mean?

The numbers scattered across the water are soundings — the depth of water below chart datum, in metres on modern charts. An underlined number is a drying height, the height a feature uncovers above chart datum at low water. Numbers on land or beside structures are usually heights (of land, lights or bridges) above a height datum. Depth contours join points of equal depth to show the shape of the seabed.

Q: Where can I find what chart symbols mean?

The standard reference for Admiralty charts is publication NP5011, "Symbols and Abbreviations Used on Admiralty Charts" (often just called Chart 5011). It explains every symbol — seabed quality, rocks, wrecks, lights, buoys, beacons and more. Equivalent references exist for other hydrographic offices. It is worth keeping a copy aboard until the common symbols become second nature.

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.

Last updated: 19 June 2026 · By Askolds Hermanis, Founder & Sailing Instructor (SkipperCheck / Nautica, since 2008)

Quick answer: Read a chart in this order — check the scale and units, find your area on the latitude/longitude grid, and measure distance using the latitude scale (1 minute = 1 nautical mile, never the longitude scale). Depths are metres below chart datum; underlined numbers are drying heights. Learn the common symbols (from Chart 5011), and always work on the largest-scale chart for where you are.

Watch: an introduction to reading nautical charts. More clips in our video lessons.

On this page

What a nautical chart is
Chart scale
Latitude, longitude and position
Measuring distance
Depths, drying heights and datum
Symbols and abbreviations
The compass rose
Frequently asked questions

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.

Remember: "large scale, large detail." Always navigate on the largest-scale (most detailed) chart available for where your boat actually is — the small-scale passage chart will not show the rock in the harbour entrance.

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.

Measuring distance

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

Measure distance on the latitude scale — never the longitude scale. One minute of latitude = one nautical mile. Take the distance with dividers, then read it against the latitude scale (the sides) at about the same level on the chart. Lines of longitude converge toward the poles, so the longitude scale does not represent a constant distance.

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:

On the chart	Meaning
S · M · R · Wd · Co	Seabed: sand · mud · rock · weed · coral (useful for anchoring holding)
Asterisk / star symbol	A light or lighthouse (with its characteristics alongside)
+ or rock symbol with depth	An underwater rock or obstruction; a number shows least depth over it
Wk, with hull/mast symbol	A wreck — dangerous or non-dangerous depending on the symbol
Fl, Oc, Iso, Q, LFl	Light characteristics: flashing, occulting, isophase, quick, long-flash

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 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.

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

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