How to Read Nautical Charts: A Guide for US Boaters

To read a nautical chart, check the title block for the scale, depth units and datum, then read the small numbers (soundings) as water depths below that datum: Mean Lower Low Water (MLLW) on US tidal charts and Low Water Datum (LWD) on the Great Lakes. Darker blue means shallower water, symbols mark hazards and aids to navigation, and you pass channel markers using Red Right Returning.

After that, adjust the charted depth for today's tide or lake level and compare what the chart shows with what you see on the water.

The symbol key for US nautical (marine) charts is U.S. Chart No. 1, produced by NOAA and the National Geospatial-Intelligence Agency. It covers paper charts, including NOAA Custom Chart prints, and the symbols used to show NOAA ENC data electronically. Keep it handy for anything below that you don't recognize.

Read the title block and notes

On traditional NOAA charts, the edition number and date were printed in the lower-left margin. The title block, which holds the scale, depth units and datum, sits in an open area of the chart where it covers the least detail, often over land. NOAA Custom Chart layouts can differ. Before anything else, find:

• Scale. Large scale (for example 1:10,000) shows a small area in great detail. Small scale (for example 1:200,000) shows a large area with little detail.
• Depth units. Feet, fathoms or meters. On NOAA charts the unit is printed in large magenta letters.
• Datum. On US tidal charts, soundings are measured from MLLW. On Great Lakes charts the datum is Low Water Datum (LWD), a fixed level for each lake referenced to IGLD 1985. Inland river charts use a local low-water reference given in the chart notes.
• Notes. Labels such as see Note B point to a note elsewhere on the chart. Read them, since they often hold warnings.

US charts fall into rough scale groups:

• Sailing charts: smaller than about 1:600,000
• General charts: about 1:150,000 to 1:600,000
• Coast charts: about 1:50,000 to 1:150,000
• Harbor charts: larger than 1:50,000

What the small numbers mean

The numbers spread across the water are soundings: water depths measured from the chart datum. On tidal charts that is MLLW, which NOAA defines as the average of the lower low water height of each tidal day, observed over the National Tidal Datum Epoch. So a sounding gives the depth at a low-water level.

Units to know:

• 1 fathom = 6 feet
• 1 meter = 3.28 feet

A small subscript digit splits a value. A 5 with a subscript 2 means 5 fathoms 2 feet on a fathom chart, or 5.2 meters on a meter chart.

Adjust for the tide or water level

On tidal water, actual depth is roughly the charted sounding plus the predicted tide height above MLLW. Get the prediction from NOAA Tides & Currents.

Example: the chart shows 4 ft and the predicted tide is +2.5 ft, so you have about 6.5 ft. On a minus 1.0 ft tide you have about 3 ft. With a 3 ft draft, that leaves no margin.

On the Great Lakes, tide predictions don't apply. Check the current lake level from NOAA GLERL or USACE and add the amount the lake sits above LWD to the charted depth. On rivers, check the chart notes and the current gage reading.

Colors and depth contours

On traditional charts:

• Tan is land.
• Green is intertidal: underwater at high tide and dry at low tide. Underlined numbers there are drying heights above the datum, so they show how far the bottom sticks up at low water.
• Darker blue is the shallowest water.
• Lighter blue is in between.
• White is deeper water, beyond the chart's blue-tint depths.

Depth contours are lines, solid or dashed where approximate, that join points of equal depth and are labeled with that depth.

On ENCs and NOAA Custom Charts, the shallow tint and safety contour depend on the depth you set. Set it to your draft plus a margin so the shallow color matches your boat.

Hazards

• Rocks, wrecks and obstructions in less than 66 feet of water are circled with black danger dots and have a blue interior.
• A dotted line (danger line) around a rock, wreck or obstruction marks it as a hazard to navigation. Treat the whole circled area as dangerous.
• * marks a rock awash and + marks a submerged rock.
• Wrecks have their own symbols. A wreck symbol with no sounding beside it means the depth over it is unknown.
• Common abbreviations include Obstn (obstruction) and rky (rocky).
• Look up any unfamiliar symbol or abbreviation in U.S. Chart No. 1.

Buoys, daymarks and lights

On paper and NOAA charts:

• Diamonds are buoys.
• Squares and triangles are fixed markers (daymarks).
• Magenta teardrops are lights, with the light characteristics printed nearby.

Reading a light label

• Fl G 4s 5M means a green flash, one cycle every 4 seconds, visible 5 nautical miles.
• Q R means a quick-flashing red light.
• Iso means isophase: equal periods of light and dark.

Which side to pass: Red Right Returning

• Keep red aids to starboard (your right) when entering from seaward. Use the same rule heading upstream on a river.
• Red marks are even-numbered nuns with triangular daymarks.
• Green marks are odd-numbered cans with square daymarks. Keep them to port.
• Aid numbers increase as you enter from seaward.
• Preferred-channel marks have horizontal red and green bands. Treat the mark as the color of its top band. If green is on top, keep it to port when returning, and the preferred channel is to your right. If red is on top, keep it to starboard, and the preferred channel is to the mark's left.
• On the Intracoastal Waterway, red is kept to starboard heading south along the Atlantic coast and west along the Gulf. Yellow triangles mean keep the aid to starboard and yellow squares mean keep it to port, even where they sit on aids of the opposite color.

Overhead clearances

Bridge and cable clearances use a different reference from soundings. Always check the title block or chart note for the clearance datum.

• Tidal charts. Clearances are generally referenced to Mean High Water (MHW). When the water is below that level you have more room, and when it is above it you have less.
• Great Lakes. Clearances are referenced to LWD, a low-water level. When the lake is above LWD, subtract that amount from the charted clearance.
• Rivers. Clearance depends on river stage. On the lower Mississippi, NOAA's guidance is to subtract the latest USACE gage river stage from the ENC clearance. NOAA's example at the Crescent City Connection is 166 ft minus 4 ft, for 162 ft. See NOAA's update.

Your boat's height plus a margin must fit under the clearance you work out.

Position, distance and direction

• Latitude and longitude. Paper charts show them along the edges. An app shows your GPS position.
• Distance. One nautical mile equals one minute of latitude, so you can measure on the latitude scale or the bar scale. A nautical mile is 1.15 statute miles, and a knot is one nautical mile per hour.
• Which mile. Most charts use nautical miles. Charts of the Great Lakes, inland lakes and the Intracoastal Waterway use statute miles.
• Compass rose. The magenta compass rose has an outer circle for true north and an inner circle for magnetic north.
• Variation. Magnetic north drifts over time. The center of the rose prints the local variation, the year it applied and the yearly change. Bring it up to the current year using the yearly change. To get a magnetic course from a true course, add westerly variation and subtract easterly variation.

Where to get a current chart

NOAA has cancelled all traditional paper nautical charts to focus on electronic charts, and the USCG no longer issues Notice to Mariners corrections for them. The phase-out was scheduled to finish by January 2025 and also ended NOAA raster charts (RNC) and BookletCharts. Your options now:

• NOAA ENC. NOAA electronic navigational charts are NOAA's primary chart product, free when you download them directly from Coast Survey websites.
• NOAA Custom Chart. This builds a paper chart from the latest ENC data. You can show depths in feet, fathoms or meters, choose the depth where the shallow blue tint starts, and add a safety contour based on your draft.
• An ENC-based app on your phone, tablet or plotter.

NOAA's Office of Coast Survey charts coastal waters and the Great Lakes. The US Army Corps of Engineers makes Inland Electronic Navigational Charts (IENCs) for inland rivers such as the Ohio, Missouri and Lower Mississippi. IENCs show depth contours, buoys, lights, known hazards and reference landmarks.

Use the newest, largest-scale chart you can get for your area.

Check the chart against the water

Charts are a snapshot of conditions when they were made. Buoys can drift and sandbars shift, so treat the chart as a guide, keep a lookout, and watch your depth sounder.

Reading chart symbols in an app

If you use Helm, its Map key explains a selected set of common chart symbols in plain words. Open it from Settings or from the offline charts screen. A few things specific to the app:

• The app draws its own symbols, redrawn from US Chart No. 1 (Edition 13). A green square is a green can and a red triangle is a red nun (the key calls it a red cone).
• Depth shading follows your boat's draft and safety margin from Boat Profile.
• A * after DEPTH in the top strip means the tide or today's water level isn't counted, and the number is the charted depth at low water.
• Striped water means there is no depth data there. No depth number shows (the app shows a dash where depth is unknown), Smart routing won't go through it, and the app tells you to go slowly and watch your depth sounder.

Coastal data comes from NOAA ENCs reprocessed by the app, and it is not an official NOAA product. Inland river data comes from the Army Corps Inland ENC. The app describes itself as a secondary aid that does not replace official up-to-date charts, notices to mariners, a proper lookout or your own judgment. Read more about how depth is shown and where the chart data comes from.

Common questions

What does MLLW mean on a nautical chart?

MLLW is Mean Lower Low Water, the average of the lower low water height of each tidal day over the National Tidal Datum Epoch. Soundings on US tidal charts are measured from it, so add the predicted tide height to estimate how much water you have. Great Lakes charts use Low Water Datum instead.

What do the colors on a nautical chart mean?

Tan is land and green is intertidal ground that covers and uncovers with the tide. Darker blue is the shallowest water, lighter blue is in between, and white is deeper. On ENCs and NOAA Custom Charts you can set the depth where the shallow tint starts.

Are NOAA paper charts still available?

NOAA has cancelled all traditional paper charts. You can download NOAA ENCs free from Coast Survey websites, or make a paper chart from current ENC data with NOAA Custom Chart.

Do lake and river charts use nautical miles?

Charts of the Great Lakes, inland lakes and the Intracoastal Waterway use statute miles. Most other charts use nautical miles. One nautical mile is 1.15 statute miles.

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