How to read your nautical charts

Understand how to read your nautical chart, including the title block and other features.

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About nautical charts

The nautical chart is the mariner's road map. Effectively using a chart helps you identify the best route to your destination and prevents accidents. With frequent reference to a chart, you can identify obstacles you wouldn't see with your eyes alone. Obstacles such as rocks and sandbars can stop your boat in its wake and could harm:

  • you
  • your boat
  • the environment

Symbols, abbreviations and terms

Fig. 1

Fig. 1

Page of the symbols, abbreviations and terms for chart 1.

For help using your nautical chart, use the Canadian Hydrographic Service (CHS) Chart 1: Symbols, Abbreviations and Terms. After your nautical chart, this is the most important publication you must have on board.

This chart contains all the symbols, abbreviations and terms used on navigation charts published by the CHS. We use hundreds of these symbols and abbreviations in our charts. They're based on international specifications, which allows nations around the world to use our charts without confusion.

Title block

Fig. 2

Fig. 2

An example of a chart title block.

The first thing you should take notice of when looking at your nautical chart is the title block. It's often overlooked when referring to a chart.

Regional identification

The title blocks' regional identification states the general geographic area of the chart.

Main title

The main title in the title block states the specific geographic area of the chart.

Scale identification

We refer to the relationship between the size of the chart and the earth as 'natural scale.' For example, 1:15 000 means that 1 unit on the chart equals 15 000 units on the earth. The following are examples of different types and scales of charts and their uses.

  • Harbour charts are:
    • large scale, 1:2 001 to 1:20 000
    • used for navigation in harbours or intricate, hazardous, shoal-infested waters
  • Approach charts are:
    • 1:20 001 to 1:50 000
    • used for approaching coasts where a lot of detail is required
  • Coastal charts are:
    • 1:50 001 to 1:150 000
    • used in fisheries charts
    • used to show continuous extensive coverage with sufficient inshore detail to make landfall sightings easy
  • General charts:
    • are 1:150 001 to 1:500 000
    • used in fisheries charts
    • give extensive offshore coverage with sufficient inshore detail to make landfall sightings easy
  • Sailing charts are:
    • 1:500 001 and smaller
    • used for offshore navigation beyond sight of land

Projection identification

Chart projection is a method by which we represent a curved surface (the earth) 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.

When boating, it's essential to be able to recognize features by their shape, such as points of land or shapes of islands. You can compare these shapes to the charted features when attempting 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 equals 6 feet) or metres and decimetres.

Eventually, all Canadian charts will show depths in metres.

Fig. 4

Fig. 4

Example of depths chart.

Elevations note

The elevations note describes the datum used to show the elevation of structures and the clearance of bridges and overhead cables.

Use this note to determine if your boat has enough clearance to travel under the overhead cables and bridges.

Horizontal datum

Fig. 3

Fig. 3

Example of horizontal datum chart.

The horizontal datum describes the starting point used for positioning objects on the surface of the earth. Explanation of horizontal datum charts is available in Chart 1: Symbols, Abbreviations and Terms.

Source classification diagram

The source classification diagram 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. It refers you to Chart 1: Symbols, Abbreviations and Terms.

Bar scales

A bar scale is a graphic scale represented by a line or a bar that is subdivided into nautical:

  • feet
  • miles
  • metres

This bar is used for measuring distances on the chart.

Elevation contours

Elevation contours are lines that connect points of equal elevation.

They're how we show the shape and slope of hills and mountains in graphic form to help you identify them on the chart. Once you identify them on the chart, you can identify your location on the water.

Insets

We use insets when a particular area of the chart can't be represented with enough detail to be safely used by the mariner.

Objects of interest can't always be shown on a chart with a scale of 1:15 000, such as:

  • marinas
  • channels
  • yacht clubs
  • small islands

Hydrographs

We include hydrographs on charts for tidal and non-tidal waters. Hydrographs indicate the:

  • fluctuations in water level over a 1-year period
  • highest and lowest water level ever recorded for each month
  • amount of water to expect above chart datum during any month of the year

Place names

Your chart includes names of places of interest to mariners, such as:

  • cities
  • towns
  • islands
  • points of land
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