How to Read a Nautical Chart: Depths, Symbols, and Marks (2026)

A nautical chart answers the only question that matters afloat: is there enough water, and what is in the way. Everything printed on it - the numbers, the tints, the little diamonds and stars - exists to answer that faster. This guide walks through the parts that do the real work, in the order you actually use them, and shows how to read official NOAA electronic charts (ENC) in a browser while you plan.

#Depths: soundings, datums, and contours

The scattered numbers are soundings: depth at that point, in the unit stated on the chart (US ENCs commonly show feet or meters - always check). They are referenced to a low water datum, typically Mean Lower Low Water in the US, which means the chart shows roughly the least water you can expect; the tide usually adds to it. Contour lines connect equal depths and the tinted bands between them let you read the bottom at a glance: darker blue is shallower, white is deep. Pick the contour that matches your draft plus safety margin and treat it as a fence.

#Buoys and marks in one minute

• Lateral marks: in IALA-B waters (the Americas), keep red marks to starboard when returning from sea - "red right returning". IALA-A (Europe, most of the rest of the world) reverses it.
• Red marks are typically nuns (cone-topped) with even numbers; green are cans with odd numbers, numbers increasing from seaward.
• Cardinal marks (yellow-black, mostly outside the US) tell you which side is safe by compass direction.
• Isolated danger marks (black-red-black) sit on the hazard itself: give them room.
• Safe water marks (red-white stripes) mean navigable water all around - usually a fairway or landfall mark.

#The symbols that save boats

Three families are worth memorizing. Rocks: an asterisk is a rock that covers and uncovers; a plus sign with dots is awash at low water; danger circles (dotted rings) fence anything the cartographer wants you around, not over. Wrecks: a hull symbol or the dashed "sunken wreck" mark, sometimes with a depth if surveyed. Obstructions: "Obstn" with a depth or a danger circle - pilings, fish havens, cable areas. When a route crosses anything in this family, zoom in and read the fine print; the chart is telling you where boats have already lost.

#Paper, raster, and ENC: what you are actually looking at

NOAA stopped producing traditional paper/raster charts in 2025; the official product now is the electronic navigational chart (ENC), a vector database drawn live with standard S-52 symbology - the same depth tints and symbols described above, but zoomable, queryable, and updated weekly. ENCs drive commercial ship ECDIS and every serious chartplotter. The data is public: you can view it free rather than only through paid chart apps.

That includes while route planning: the Marine OS route planner overlays live NOAA ENC charts on the planning map for US waters - flip on "NOAA ENC charts" in the layer control and the depth tints, contours, marks, and dangers render under your route line as you drag waypoints. It is a planning view, not a type-approved navigation display, but it means the route you sketch respects the water it crosses from the first click.

#Frequently asked questions

Depth fence first, marks second, dangers always. For the route itself, the passage planning guide covers method, and the boat trip planner covers the day-trip version.