How to Read a Nautical Chart
A nautical chart packs an enormous amount of information into a small space. Here's what the numbers, colors and symbols on BayCharts (and any NOAA chart) mean.
Soundings — the scattered numbers
Every small number on the water is a sounding: the depth at that spot. On BayCharts depths are shown in feet. A number with a small subscript — like 2₇ — reads as 2.7 ft. Crucially, soundings are referenced to Mean Lower Low Water (MLLW), roughly the average of the lower of each day's two low tides. Most of the time the real depth is more than charted; at extreme low tides it can be less.
Depth contours and water colors
Lines connecting equal depths divide the water into bands, tinted from darker blue (shallow) to white or pale blue (deep). If your boat draws 4 ft, learn which tint means trouble at a glance rather than reading each sounding.
Buoys and daymarks
The Chesapeake follows the IALA-B rule: red, right, returning — keep red marks to starboard when heading upstream toward a harbor. Green cans are square-ish, red nuns pointed; lighted marks carry a label like Fl(1)R 4s (flashing red every 4 seconds). Tap any mark on BayCharts for its full record.
Hazards
Asterisks are rocks, dotted circles are shoal patches, and a symbol like a sunken hull marks a wreck. Magenta areas call out restricted zones, cable areas and anchorages — tap them for the regulation text.
Practice on the Annapolis chart →