Nautical Chart Symbols & Abbreviations: Nature Of Seabed

Nautical charts are filled with so much information that it is inevitable some things would be abbreviated.

One such abbreviation is for the terms describing the nature of the seabed.

For example, rather than writing the word “sand” hundreds of times on each chart, the universal abbreviation “S” is used instead.

All the abbreviations are contained within official publications such as “Chart 1” in the US, and “Chart 5011” in the UK.

To save you time, however, I have compiled a list of the most common ones I could find and put them together into a simple-to-read cheatsheet.

Please note: The cheat sheet should not be used for navigation as you should always consult official publications. It is a good starting point for making your own notes though.

You’ll find that I have broken the sheet down into sections, starting from the most commonly used at the top.

Types Of Seabed

There are a small number of internationally recognised abbreviations to describe the nature of the seabed on nautical charts.

Some charts may use their own versions of these abbreviations, but in general, if you know these then you will be able to work out the meaning on most charts.

As these are the most important description when it comes to the seabed, you’ll notice that all of these symbols are capitalised. The importance of this will become more apparent when we talk about qualifying terms in the next section.

Symbol	Description
S	Sand
M	Mud
Cy	Clay
Si	Silt
St	Stones
G	Gravel
P	Pebbles
Cb	Cobbles
R	Rock; Rocky
Bo	Boulder(s)
Co	CoralCoralline Algae
Sh	Shells (skeletal remains)
Wd	Weed (including kelp)
Sg	Seagrass

Qualifying Terms

With nautical charts, qualifying terms are used to add further detail for navigators to better understand the nature of the seabed.

For example, while the seabed type may be “M”, or mud, there may be a qualifying term attached to it telling you that it is “sy M”, or sticky mud.

Most qualifying terms can be used to describe multiple types of seabed, but there are some that are limited to sand, “f”, “m”, and “c” describing “fine”, “medium” and “course” respectively.

One thing you’ll notice about all the qualifying terms is that they are in lower case letters. This is done on purpose so that it is easier to differentiate them from the more general seabed symbols.

Symbol	Description
f	Fine (sand only)
m	Medium (sand only)
c	Coarse (sand only)
bk	Broken
sy	Sticky
so	Soft
sf	Stiff
v	Volcanic
ca	Calcareous
h	Hard

Layered Seabed (with examples)

Occasionally, you will find a seabed with two different characteristics layered on top of each other. This would be a layered seabed.

Layered seabeds are different to mixed seabeds because they are separated into distinct layers.

For example, you might find a layer of sand has accumulated over an underlying mud sea floor.

It is important to take note of layering on a chart because it can give you critical information for operations such as anchoring.

If you see that there is a layer of weed over an underlying mud seafloor, you will expect your anchor to have far less holding power than if there was no weed.

Layered seabeds are characterised with a “/” symbol, with the top layer listed first.

For example.

Symbol	Description
S/M	Sand over mud
M/S	Mud over sand
Sh/S	Shells over sand
Co/S	Coral over sand
M/R	Mud over rocks

Mixed Seabed Terms (with examples)

A mixed seabed is one in which different constituents are mixed together across the seafloor. It is different to the layered seabed, because the different elements are mixed together rather than being layered on top of each other.

To show a mixed seabed on a chart, the different constituent parts are listed in order, with the main constituent first.

For example, if you had a seafloor that is comprised mainly of sand, with some shells and pebbles mixed in, it would be written as:

S.Sh.P

The use of a full stop between the terms is optional, so the same mixture could also be written as:

S Sh P

Further examples of mixed seabeds are:

Symbol	Description
S.Sh.P	Sand with shells and pebbles
M St	Mud with stones
G Cy	Gravel with clay
Cb.S.Sh	Cobbles with sand and shells
Si M	Silt with mud

Supplementary Abbreviations

Up until this point, the abbreviations that we have discussed have been the international symbols and abbreviations.

While these are found on most charts, there may also be symbols and abbreviations that are specific to the national hydrographic office that has produced the chart.

For example, in the United States, NOAA maintains a list of supplementary abbreviations that they may use on their charts.

NOAA Supplementary Abbreviations

Charts produced by NOAA may use supplementary abbreviations that are specific to US charts.

Supplementary abbreviations may be used in the same way as international abbreviations. They can be layered or used to show different mixtures of seabeds.

The full list of these is published in Chart 1, but I have included a selection of them in the table below.

Symbol	Description
Grd	Ground
Oz	Ooze
Ml	Mark
Sn	Shingle
Ck	Chalk
Qz	Quartz
Sch	Schist
Co Hd	Colar head
Mds	Madrepores
Vol Ash	Volcanic ash
La	Lava
Pm	Pumice
T	Tufa
Sc	Scoriae
Cn	Cinders
Mn	Manganese
Oys	Oysters
Ms	Mussels
Spg	Sponge
K	Kelp
Grs	Grass
Stg	Sea-tangle
Spi	Spicules
Fr	Foraminifera
Gl	Globigerina
Di	Diatoms
Rd	Radiolaria
Pt	Pteropods
Po	Polyzoa
Cir	Cirripedia
Fu	Fucus
Ma	Mattes
sml	Small
lrg	Large
rt	Rotten
str	Streaky
spk	Speckled
gty	Gritty
dec	Decayed
fly	Flinty
glac	Glacial
ten	Tenacious
wh	White
bl; bk	Black
vi	Violet
bu	Blue
gn	Green
yl	Yellow
or	Orange
rd	Red
br	Brown
ch	Chocolate
gy	Gray
lt	Light
dk	Dark
vard	Varied
unev	Uneven