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Educational
Tips on Reading Your Nautical Chart...
Looking at a nautical chart for the first time can be very confusing.
There is so much information; it can become very complicated if you don't
know what you are looking at and what it means.
The nautical chart is the mariner's road map. Effective use of a chart
helps you identify the best route to your destination and prevents accidents.
Water travel allows freedom of movement. You can travel in whichever
direction you choose. With frequent reference to a chart, you can identify
obstacles not apparent to the naked eye. Rocks and sandbars can stop your
boat in its wake, not to mention the harm it can cause you, your boat,
or the environment.
- The most important publication you must have on-board (aside from
your nautical chart) is Chart No. 1; Symbols, Abbreviations and
Terms (Fig. 1)This book contains all the symbols, abbreviations,
and terms used on navigation charts published by the Canadian Hydrographic
Service. There are hundreds of these symbols and abbreviations used
on charts and they are based on international specifications so that nations
around the world can use our charts without confusion.
Fig. 1
- The first thing you should take notice of when looking at your
nautical chart is
the title block (Fig. 2). It's often overlooked when referring to a chart.
Fig. 2
Several things you should take note of in the title block include:
- The Crest (Fig. 3): the CHScrest is located at the top of the
chart title. CHS is responsible for producing the marine navigation charts
of Canada. Noting the crest will ensure that your chart is official!
Fig.3
- Regional Identification:
CANADA / LAKE ONTARIO states the
general geographic area of the chart.
- Main Title:
TORONTO HARBOUR AND APPROACHES states the specific
geographic area of the chart.
- Scale Identification: "Natural Scale" means the relationship between the size of
the chart and the earth. For example, 1:15 000 means that one unit on
the chart equals 15 000 units on the earth. The different types and
scales of charts and their uses are:
Harbour Charts: are large scale, 1:5 000 - 1:15 000 and are used
for navigation in harbours or intricate, hazardous, shoal-infested
waters.
Approach Charts: 1:15 001 - 1:50 000, are used for approaching
coasts where a lot of detail is required.
Coastal Charts: 1:50 001 - 1:150 000, are used to show continuous
extensive coverage with sufficient inshore detail to make landfall
sightings easy. Fisheries charts use these scales.
General Charts: 1:150 001 - 1:500 000, give extensive offshore
coverage with sufficient inshore detail to make landfall sightings
easy. Fisheries charts use these scales.
Sailing Charts: 1:500 001 and smaller, are used for offshore
navigation beyond sight of land.
- Projection Identification: MERCATOR, chart projection is a method
by which a curved surface, the earth, can be represented on a flat
piece of paper, the chart. The Mercator projection is the most commonly
used for nautical charts. It virtually reduces the shape and direction
distortions that occur during the flattening process. It's important
when boating to be able to recognize features by their shape such
as points of land or shapes of islands. These shapes can be compared
to the charted features in an attempt to determine your position.
- Depths Note: the depths note indicates what units are used for
the depths on the chart. Charts have depths shown in feet and
fathoms
(1 fathom=6 feet) or metres and decimetres. Eventually all Canadian charts
will show depths in metres. It is important to know what units the
depths on the chart are in. You don't want to confuse 3 metres of
water with 3 feet of water.
- Elevations Note: describes the datum that is used to show the
elevation of structures and the clearance of bridges and overhead
cables. You can use this note to determine if the boat has enough
clearance to travel under the overhead cables and bridges.
Fig. 4
- Horizontal Datum: describes the stating point that is used for positioning
objects on the surface of the earth. (Fig. 4) A detailed description
of horizontal datums is a complex topic. To learn more, please see Symbols,
Abbreviations and Terms/Chart No. 1(Fig. 1)
- Source Classification Diagram: this note identifies where data on the chart has come
from and how old it is.
- Symbol Reference Note: the symbol reference note is the last note
in the Title Block and states: For Symbols, Abbreviations and Terms,
see Chart No. 1.(Fig. 1)
- Bar Scales: a bar scale is a graphic scale represented by a line
or a bar that is subdivided into nautical miles, feet, or metres. This
bar is used for measuring distances on the chart.
- Depths: the depth of water is indicated in feet and fathoms
(1 fathom=6
feet) or metres and decimetres. The unit used on a chart will be indicated in the
title block. (Fig. 5)
Fig. 5
- Elevation Contours: lines connecting points of equal elevation. It's
a graphic way of showing the shape and slope of hills and mountains
that might be helpful in identifying them on the chart. Once identified,
it can help identify your location on the water.
- Insets: used when a particular area of the chart cannot be represented
with enough detail to be safely used by the mariner. Such things as
small islands and channels or marinas and yacht clubs may be of interest
but cannot always be shown on a chart with a scale of 1:15 000.
- Hydrographs: a graph that is used on charts in tidal and non-tidal
waters and indicates the fluctuations in water level over a one-year
period. This graph gives the boater an indication of the amount of water
to expect above chart datum during any month of the year. It also indicates
the highest and lowest water level ever recorded for each month.
- Place Names: names of places shown on the chart such as points of
land, islands, cities, and towns, etc., of interest to the mariner.
Bookmark this page and watch for more tips on reading your nautical
chart.
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