Why Nautical Charts Still Matter
GPS chartplotters are everywhere, and they're genuinely useful — but every experienced mariner will tell you the same thing: understanding the underlying chart is non-negotiable. Electronics fail, batteries die, and screens crack. A sailor who can read a paper chart and work with compass and dividers is never truly lost. More importantly, understanding charts makes your chartplotter use far more intelligent and safe.
What Is a Nautical Chart?
A nautical chart is a specialized map of a body of water, designed specifically for navigation. Unlike a road map, a chart shows you what's under the water as much as what's above it. Charts are published by hydrographic offices — NOAA in the United States, the UKHO for British waters — and they're kept as up to date as possible, though local Notices to Mariners (NtMs) should always be checked for recent changes.
Key Chart Elements
Depth Soundings
Numbers scattered across the water areas represent depth. In most modern charts they're in meters, though older US charts may use feet or fathoms. The datum used (the reference zero point) is typically Mean Lower Low Water (MLLW) in the US or Chart Datum elsewhere — meaning actual depths at high tide will be greater than shown. Always know your datum, and always add tide height to chart depth when calculating clearance.
Contour Lines (Depth Curves)
Just like a topographic map has contour lines for elevation, charts have depth contours — lines connecting equal depths. The 5m, 10m, and 20m contours are commonly shown. These help you visualize the underwater terrain at a glance. Closely spaced contours mean a steep bottom; widely spaced means a gradual shoaling.
Buoys and Aids to Navigation
Charts show the position, color, shape, and light characteristics of every buoy and beacon. In the IALA system used in the US (Region B), red buoys are kept to starboard when returning ("red right returning"). In IALA Region A (most of the world), this is reversed. Light characteristics — shown as abbreviations like Fl R 4s (Flashing Red, 4-second period) — let you identify aids at night.
Shoreline Features
The chart distinguishes between firm shorelines, sandy beaches, mud flats, and rock. Drying areas (which are above water at low tide but underwater at high tide) are typically shown with a specific hatching or color. These are critical — a flat that looks harmless at high water can strand you at low tide.
Hazards
Rocks are marked with specific symbols depending on whether they're always submerged, awash, or only cover at high water. A cross (+) indicates a dangerous wreck. Overfalls and tide rips are also marked.
Scale and the Compass Rose
Every chart has a compass rose — a circular diagram showing both True North (outer ring) and Magnetic North (inner ring). The angular difference between them is magnetic variation, and you must account for it when converting between compass bearings and chart bearings. The scale of the chart matters greatly — a large-scale chart (1:5,000) shows a small area in great detail, while a small-scale chart (1:500,000) covers vast areas with little detail. Use large-scale charts for pilotage in harbors and inlets.
Chart Symbols You Must Know
| Symbol | Meaning |
|---|---|
| PA | Position approximate |
| Wk | Wreck |
| † | Dangerous wreck |
| Rk | Rock (above water) |
| ⊕ | Submerged rock at chart datum |
| Obstn | Obstruction |
| Anch | Anchorage area |
The full symbol reference is found in NOAA Chart No. 1 (US) or Chart 5011 (UK/admiralty) — both available free as PDFs and worth keeping aboard.
Getting Started with Real Charts
The best way to learn is to download a free NOAA raster chart of waters you know, and simply spend an hour identifying every symbol. Match what you see on the chart to what you know about the real location. Recognizing a rocky shoal you've sailed past dozens of times on the chart cements the knowledge in a way that no amount of reading can.
Then take that chart out on the water with you. Reference it constantly, even when your GPS is working perfectly. That habit is what separates competent mariners from people who are just along for the ride.