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Navigational Charts: Submerged Objects & Obstructions

Do you know these lesser-known submerged object chart symbols? It would be a good idea to get up to speed.

Visual displaying a chart on a computer showing what lies beneath water.

Photo: Getty Images/Davyd Volkov

There’s a lot going on beneath the waves that can have a negative impact on your boating day. Your charts tell a story, and knowing what their symbols mean may save your bacon one day. Here’s a list of some of the more obscure symbols to avoid, along with their meanings.


Learn what to do if your vessel strikes a submerged object.
Infographics displaying visuals for groins and fishing strakes and fish traps.


Engineering structures perpendicular to the shoreline meant to “catch” or inhibit the alongshore movement of sand. They can be individual structures, but are usually placed in multiple structure “groin fields.”

Fishing Strakes and Fish Traps

Permanent or semi-permanent devices used for catching fish, consisting of a net or other structure that diverts the fish into an enclosure arranged so egress is more difficult than ingress. They typically (but not always) feature a row of poles that generally extend out from the shore to the head of the net.

Infographic displaying distributed remains of a wreck

Distributed Remains of Wreck

This is foul ground that (unless indicated) is not dangerous to surface navigation but is to be avoided by vessels anchoring or trolling. It typically indicates the remains of wreck or a cleared platform. As wreckage tends to shift position over time, always anchor well clear of these areas or risk losing your anchor.

Infographic showing unexplored ordnance

Unexploded Ordnance

Different agencies may use different symbols. The symbol below left is a NOAA symbol; the one on the right is the NGA symbol.

Unexploded ordnance (UXO) are explosive weapons (e.g., bombs, bullets, shells, grenades, mines) that were disposed of or did not explode when they were employed and still pose a risk of detonation. Ocean disposal (dumping) of munitions was an accepted international practice until 1970, when it was prohibited by the Department of Defense. In 1972 Congress also passed the Marine Protection, Research, and Sanctuaries Act banning ocean disposal of munitions and other pollutants.

Danger and location chart with abbreviations

Infographic displaying rock beneath the water surface.

Rock Awash

The basic rock symbol looks like a plus sign (not shown here) and indicates rock that’s beneath the water surface all the time. The rock awash symbol (which resembles an asterisk) means the rock will uncover (become visible) at low tide.

Infographic displaying obstructions


An easy-to-overlook symbol, obstructions are where damage to keels, props, and shafts can often occur. Some charts may only display the abbreviation “Obstn,” but most will display dots around a circle or square to warn mariners that some unknown hazard lurks beneath the surface. Small circles of dots could indicate anything from old piling remnants or stumps, to submerged post and poles.

How To Read Vector Electronic Navigational Charts

Legend explaining abbreviations

Infographic displaying ruins and submerged man-made structures


Submerged man-made structures that can include anything from wharf foundations to buildings. Typically they’re located near shore, but that’s not always the case. Another symbol on this chart is the well-known wreck symbol to the right of the broken line structure labeled “ruins” on the chart.

A Word On Electronic Navigational Charts

The symbols, abbreviations, and terms used in Electronic Navigational Charts, or ENCs, on an Electronic Chart Display and Information System (ECDIS) is now shown on NOAA U.S. Chart No. 1. It can be downloaded at no cost directly from NOAA at (click “Legend” at the bottom of the page) or purchased from numerous online retailers.

A major difference between paper charts and ENCs is the ability of ECDIS to display the same feature differently depending on user settings. A good example is setting an electronic chartplotter to display wrecks, rocks, and other obstructions only if they’re a navigational hazard.

One major advantage of ECDIS over paper charts is the ability for the user to obtain more information about a feature simply by clicking or hovering the cursor over the item or symbol in question. — F.L.

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Frank Lanier

Contributing Editor, BoatUS Magazine

Capt. Frank Lanier is a SAMS Accredited Marine Surveyor with more than 40 years of experience in the marine and diving industries. He’s also an author, public speaker, and multiple award-winning journalist whose articles on boat maintenance, repair, and seamanship appear regularly in numerous marine publications worldwide. He can be reached via his YouTube channel “Everything Boats with Capt. Frank Lanier” and website