Along the coast, USGS topo maps add a set of symbols borrowed from nautical charts: rocks that cover and uncover with the tide, reefs, wrecks, and depth curves. Most coastal quads show them, and combined topographic-bathymetric maps add full underwater contours. They're great for planning a beach walk, tide-pooling, or a paddle along the shore.
How are rocks shown along the coast on a topo map?
A small star-shaped symbol marks a rock that's bare or awash, meaning at or just above the surface. A star inside a dotted circle is a rock that's dangerous to navigation, and a dotted outline full of stars is a group of rocks.
IN THIS GUIDE
Shorelines · Rocks, reefs, flats, and wrecks · Piers, seawalls, and platforms · Bathymetric features · Using topo maps on the coast · FAQ
Shorelines
The shoreline is the boundary between land and water, usually mapped at the high-water line.
Shoreline
A solid blue line: a surveyed shoreline.
Apparent (edge of vegetation) Topo-bathymetric maps
A blue line marking an apparent shoreline, mapped at the outer edge of marsh or mangrove where the true waterline can't be seen.
Indefinite or unsurveyed
A dashed blue line: a shoreline whose position is indefinite or hasn't been surveyed.
Rocks, reefs, flats, and wrecks
These are the hazards and features you see as the tide goes out.
Foreshore flat
A dotted area between the shoreline and the water, sometimes labeled Mud or Sand: a flat that is exposed at low tide. Watch the incoming tide when you walk on it.
Coral or rock reef
A scalloped or cauliflower-shaped outline, often labeled Reef: a coral or rock reef near the surface.
Rock, bare or awash; dangerous to navigation
A star: a rock that's bare or awash. A star in a dotted circle is a rock that's dangerous to boats.
Group of rocks, bare or awash
A dotted outline filled with stars: a cluster of rocks at or near the surface.
Exposed wreck
A small hull with a mast: a shipwreck that shows above the water. A popular landmark for beachcombers.
Depth curve; sounding
A blue line with depth numbers. Depth curves connect points of equal depth, and soundings are individual measured depths. Check the margin for feet or fathoms.
Piers, seawalls, and platforms
Man-made coastal structures are drawn in black, like other man-made features.
Breakwater, pier, jetty, or wharf
Black outlines extending into the water: a breakwater, pier, jetty, or wharf, drawn to its real shape.
Seawall
A heavy black line along the shore: a seawall or bulkhead.
Oil or gas well; platform
A small circle is an offshore oil or gas well. A solid square is a platform.
Bathymetric features
These appear only on combined topographic-bathymetric maps, which chart the bottom as carefully as the land.
Area exposed at mean low tide; sounding datum line Topo-bathymetric maps
A tinted area along the shore with a line at its outer edge: the area exposed at mean low tide. The line is the datum that water depths are measured from.
Channel Topo-bathymetric maps
Dashed parallel lines: a dredged or natural channel.
Sunken rock Topo-bathymetric maps
A small cross in the water: a rock below the surface, even at low tide.
LEARN TO READ A TOPO MAP
Topo Map Reading Intro: Learn how to read topographic maps in 10 sessions
A one-minute lesson from our Topo Map Reading series.
Watch the full Topo Map Reading series →Using topo maps on the coast
- Check the tides. Flats, rocks, and reefs look very different at high and low tide. Plan beach walks and tide-pooling around a tide table.
- Topo maps aren't for navigation. If you're boating, use current NOAA nautical charts. Topo maps show the coast as it was on the map's date.
- Coastlines move. Storms and erosion shift beaches, inlets, and sandbars. Compare an older map with recent aerial imagery.
- Wrecks and rocks make good landmarks. Exposed wrecks and offshore rocks are easy to spot from the beach.
PUT IT TO USE
See these symbols on a real map
Frequently asked questions
Can you use a topo map for boating?
Not for navigation. Topo maps show coastal features for reference, but boaters should use current NOAA nautical charts, which are updated for hazards and channels.
What does awash mean on a map?
A rock that's awash is at about the water's surface, so waves wash over it. It may be visible at low tide and hidden at high tide.
How is a shipwreck shown on a topo map?
An exposed wreck, one that shows above the water, is a small hull symbol with a mast.
What's the difference between a depth curve and a sounding?
A depth curve is a line connecting points of equal depth. A sounding is a single measured depth, printed as a number.
MORE SYMBOL GUIDES
Water Symbols on Topo Maps: Streams, Lakes, Springs & Wetlands · Contour Lines on Topo Maps: Index, Intermediate & Depression Contours · Vegetation & Surface Symbols: What Green Means on a Topo Map
← Back to all USGS map symbols
Symbol artwork from the U.S. Geological Survey Topographic Map Symbols brochure (public domain).