A nautical chart is a map of the water: it shows the depths, the shoreline, hazards such as rocks and wrecks, and aids to navigation such as buoys and lights. To read one, start with the title block and notes, check the units of the depths and the level they are measured from, then use the symbols in U.S. Chart No. 1, the compass rose for directions and the latitude scale for distances in nautical miles.
Read a Chart Step by Step
Read the title block and notes. They give the area, the scale, the datums, the units of the depths and any cautions to read before using the chart.
Check the scale. It tells you how much area the chart covers and how much detail it can show.
Check the depth units and the chart datum. Depths may be in feet, fathoms or meters, measured from the chart datum, on most NOAA charts mean lower low water.
Find the shallow water. Depth contours and blue shading show where the bottom comes up.
Spot the hazards. Look for rocks, wrecks, obstructions and the dotted danger lines around them.
Identify the aids to navigation. Buoys, beacons and lights mark the channels and the dangers.
Measure direction and distance. Use the compass rose for courses and the latitude scale for nautical miles.
What a Nautical Chart Shows
A nautical chart represents hydrographic data: water depths, the shoreline, tide predictions, obstructions to navigation such as rocks and shipwrecks, and navigational aids, NOAA explains. A map, by contrast, shows landforms and routes such as roads. The Coast Guard’s rules for aids to navigation go further: the information on a chart “cannot be obtained from other sources, even ones such as topographic maps, aeronautical charts, or atlases” (33 CFR 62.21).
The symbols, abbreviations and terms on U.S. charts are explained in U.S. Chart No. 1, published by NOAA and the National Geospatial-Intelligence Agency (13th edition, April 15, 2019). It covers the symbols of traditional paper charts, including charts printed with NOAA Custom Chart, and the symbols used to show NOAA electronic navigational charts (ENCs). NOAA’s charts are now ENCs: Coast Survey ended traditional paper chart production and archived the last paper chart on December 5, 2024, and large paper charts can still be printed from current ENC data with NOAA Custom Chart.
Chart Scale
The scale compares a distance on the chart with the distance it stands for. At 1:80,000, one inch on the chart is 80,000 inches on the water, about 1.1 nautical miles. A chart that covers a small area, such as a harbor, is a large-scale chart; one that covers a large area is a small-scale chart. The terms are relative, and the smaller the scale, the less detail a chart can show (Bowditch). Close to shore, use the largest-scale chart you have.
NOAA groups its ENCs into six usage bands, from overview charts for basic voyage planning to berthing charts for maneuvering to a pier or wharf:
| Usage band | Used for | Scales of new NOAA ENCs |
|---|---|---|
| 1 Overview | Basic voyage planning | 1:10,000,000 and 1:3,500,000 |
| 2 General | Coastwise navigation offshore | 1:1,500,000 and 1:700,000 |
| 3 Coastal | Inshore coastwise navigation and wide bays | 1:350,000 and 1:180,000 |
| 4 Approach | Approaching harbors and confined waters | 1:90,000 and 1:45,000 |
| 5 Harbor | Navigating in a harbor | 1:22,000 and 1:12,000 |
| 6 Berthing | Maneuvering to a pier or wharf | 1:4,000 and 1:2,000 |
The scales are those of NOAA’s reschemed ENCs; older ENCs that have not been reschemed yet keep their own scales.
Depths: Soundings, Units and Chart Datum
The numbers on the water are soundings, the measured depths. They may be in feet, fathoms or meters, and the chart says which: traditional charts print the unit in large letters at the top and bottom, such as “SOUNDINGS IN FATHOMS.” ENC data is stored in meters, and NOAA Custom Chart can show depths in meters, feet or fathoms. A fathom is 6 feet; the Fathoms to Feet Converter does the conversion.
Depths are measured from a chosen water level, the chart datum. For most NOAA charts of the United States it is mean lower low water (MLLW), the average height of the lower low tide of each tidal day (NOAA). Because it is an average, the water is sometimes shallower than charted: at lower low water the depth is less than the charted depth about as often as it is greater, and tide tables mark such tides with a minus sign (Bowditch). Heights work the other way: the shoreline and the clearance under bridges and power cables are usually measured from mean high water, which gives you the minimum clearance to expect.
A chart can’t show every sounding, so the shallowest are picked first; even so, shoal water can be missed in a survey, so stay clear of areas where shoals are known to exist (Bowditch).
Depth Contours and Shallow Water
Depth contours are lines connecting points of equal depth, like the contour lines on a land map, and are labeled in the same units as the soundings. A broken contour means the depth line is less certain. Shoal areas are often tinted blue. On a NOAA Custom Chart you can choose the depth at which the shallow-water blue tint starts and show a safety contour for your boat’s draft.
The lettering helps too: names in upright type mark features that stay dry at high water, and names in slanting type mark underwater and floating features (Bowditch).
Rocks, Wrecks and Other Hazards
Section K of Chart No. 1 shows the hazard symbols. What to look for:
- Rocks: a rock that never covers is drawn as a small islet with its height; a rock that covers and uncovers with the tide has its height above the chart datum, such as “Uncov 2 ft”; and underwater rocks are marked whether their depth is known or not.
- Wrecks: separate symbols show a wreck whose hull is always dry, one that covers and uncovers, and sunken wrecks. A sunken wreck with less than 30 meters (about 98 feet) of water over it is considered dangerous and is circled by a dotted line (Bowditch).
- Danger lines: a dotted line drawn around a danger that would not stand out by its symbol alone, or around an area with many dangers that is unsafe to cross.
- Abbreviations: Obstn (obstruction), Subm (submerged), Rep (reported but not confirmed), PA (position approximate), PD (position doubtful) and ED (existence doubtful).
Section J lists the abbreviations for the bottom, which matter most when you anchor: S (sand), M (mud), Cl (clay), G (gravel), St (stones), Rk (rocks), Co (coral) and Sh (shells).
Buoys, Beacons and Lights
U.S. waters use the IALA Region B buoyage system, except U.S. possessions west of the International Date Line and south of 10 degrees north latitude, which use Region A (33 CFR 62.21). The main rule is “red, right, returning”: when returning from seaward, keep red markers on your right-hand, starboard side (U.S. Coast Guard).
| Mark | What it looks like | What it means |
|---|---|---|
| Port-hand mark | Green can or pillar buoy, or a beacon with a green square daymark; odd numbers; green light | Left side of the channel when returning from sea |
| Starboard-hand mark | Red nun or pillar buoy, or a beacon with a red triangular daymark; even numbers; red light | Right side of the channel when returning from sea |
| Preferred channel mark | Red and green horizontal bands | A channel junction; the top band’s color shows the preferred channel |
| Safe water mark | Red and white vertical stripes | Navigable water all around, often the start of a channel or midchannel |
| Isolated danger mark | Black with red horizontal bands and two black spheres on top | A danger you may pass on any side, but not closely |
| Special mark | Solid yellow | A special area, such as an anchorage or a cable or pipeline area |
| Information and regulatory mark | White with orange shapes | A diamond warns of danger, a diamond with a cross keeps boats out, a circle sets operating restrictions and a square gives information |
“Returning from sea” means the conventional direction of buoyage: from seaward into a channel toward the head of navigation, and along the coast clockwise around the land, that is, southward along the Atlantic coast, from Florida to Texas along the Gulf and northward along the Pacific coast. Numbers increase in that direction. On the Intracoastal Waterway, a yellow triangle means keep the mark to starboard and a yellow square to port when heading south along the Atlantic coast or west on the Okeechobee Waterway or along the Gulf Coast, whatever the mark’s number or color (33 CFR 62).
On the chart, a small circle at the base of a buoy symbol marks its approximate position, and the letters C and N show a can or a nun. Lighthouses and other lights are black dots with purple disks or flares, and a purple flare on a buoy means it is lighted. A light’s legend reads like “6” Fl(2) R 10s 80m 19M: light number 6, group flashing with 2 flashes, red, repeating every 10 seconds, 80 meters high, with a range of 19 nautical miles. The charted range is the nominal range, how far the light can be seen when the visibility is 10 nautical miles (Bowditch, Chart No. 1). Whether a light is above your horizon also depends on its height and yours: the Horizon Distance Calculator shows how far away a lighthouse can be seen.
Charted buoy positions are approximate: buoys swing around their moorings and can be dragged off station or sunk, so don’t rely on buoys alone to fix your position (33 CFR 62.23).
The Compass Rose: True and Magnetic
Compass roses are printed at convenient spots on a chart for plotting courses and bearings. The outer ring is graduated in degrees from true north; the inner ring shows magnetic directions. The variation, the angle between true and magnetic north at that place, is printed in the center of the rose with its annual change, so you can update it for the current year (Bowditch). Use true directions for most chart work, and convert with the variation, plus your own compass’s deviation, when you steer by a magnetic compass.
Latitude, Longitude and Distance in Nautical Miles
Latitude measures the distance north or south of the equator, from 0 to 90 degrees, and longitude the distance east or west of the prime meridian through Greenwich, England. Both are given in degrees, minutes and seconds, with 60 minutes in a degree (NOAA). On a chart, latitude is marked along the left and right edges and longitude along the top and bottom.
One minute of latitude is one nautical mile, which is why mariners measure distance on the latitude scale (NOAA). To measure, open a pair of dividers between two points, then set them against the latitude scale on the side of the chart, level with the points: the number of minutes is the distance in nautical miles. Two points 12 minutes of latitude apart are 12 nautical miles, about 13.8 statute miles or 22.2 km; the Miles to Nautical Miles Converter converts miles in either direction, and the Nautical Miles to Kilometers Converter converts kilometers. Don’t use the longitude scale: a degree of longitude shrinks from about 69 miles at the equator to 35 miles at 60 degrees north or south (NOAA). On a small-scale chart, measure at the latitude of the points, because the latitude scale stretches toward the poles (Bowditch).
For a first estimate before you open a chart, the Sea Distance Calculator measures a computed route over water between two points in nautical miles. It is a planning estimate, not a charted navigation track.
Chart Notes, Cautions and Updates
Read the notes before you use a chart. Notes near the title give the scale, the sources of the data, tidal information, the soundings and cautions; other notes cover local magnetic disturbance, the controlling depths of channels, hazards and anchorages (Bowditch). The source diagram, where a chart has one, shows the surveys behind it; be cautious where they are inadequate (Chart No. 1).
Charts change. The Coast Guard’s Local Notices to Mariners report changes to aids to navigation, reported dangers and chart corrections (33 CFR 62.21), and NOAA releases ENC updates every weekday evening. A NOAA Custom Chart has no chart number, so no Local Notice to Mariners is issued for it; NOAA’s Navigation Updates app shows whether its ENC data has changed since you made it. As Bowditch puts it, “a mariner navigating on an uncorrected chart is courting disaster.”
Where to Get Free NOAA Charts
- The NOAA ENC Viewer shows NOAA ENCs in a web browser with the symbols used on electronic chart systems.
- The NOAA Chart Locator downloads NOAA ENCs free for chart plotters and other electronic chart systems.
- NOAA Custom Chart makes a printable PDF chart from the latest ENC data, with depths in meters, feet or fathoms.
- U.S. Chart No. 1 explains every symbol, abbreviation and term.
Frequently Asked Questions
How do you read depth on a nautical chart?
The numbers on the water are soundings, the depths at chart datum. The chart says whether they are in feet, fathoms or meters, and on most NOAA charts the datum is mean lower low water, so at times the water is shallower than charted.
What do the red and green buoys mean?
When you return from sea in U.S. waters, keep red markers on your starboard (right) side and green markers on your port (left) side: red marks the right side of the channel and green the left. Red marks carry even numbers and green marks odd numbers.
What is chart datum?
The water level the charted depths are measured from. For most NOAA charts of the United States it is mean lower low water (MLLW), while heights such as bridge clearances are usually measured from mean high water.
How do you measure distance on a nautical chart?
Open a pair of dividers between the two points, then set them against the latitude scale on the side of the chart, level with the points. Each minute of latitude is one nautical mile; don’t use the longitude scale along the top or bottom.
What is the difference between true and magnetic on a compass rose?
The outer ring shows true directions, measured from true north, and the inner ring shows magnetic directions. The difference, called variation, is printed in the center of the rose with its annual change.
What does a chart scale of 1:80,000 mean?
One unit on the chart stands for 80,000 of the same units on the water, so one inch is about 1.1 nautical miles. A large-scale chart covers a small area in more detail, and a small-scale chart covers a large area with less.
Sources
- NOAA National Ocean Service, What’s the difference between a nautical chart and a map?, What do the numbers mean on a nautical chart?, nautical mile and knot, latitude and longitude (updated Sep 23, 2026; read Oct 19, 2026).
- NOAA and NGA, U.S. Chart No. 1, 13th edition (April 15, 2019): symbol sections, abbreviations and charted light ranges.
- National Geospatial-Intelligence Agency, The American Practical Navigator (Bowditch), Volume I, chapter 4 (nautical charts) and articles 608, 829 and 901.
- NOAA Office of Coast Survey, NOAA ENC, ENC rescheming and usage bands and NOAA Custom Charts (read Oct 19, 2026).
- NOAA Center for Operational Oceanographic Products and Services, tidal datums (read Oct 19, 2026): definition of mean lower low water.
- 33 CFR Part 62, the U.S. Aids to Navigation System (read Oct 19, 2026), and the U.S. Coast Guard’s Boater’s Guide to the Federal Requirements for Recreational Boats (revised Nov 2023).