Tips On Reading Your Nautical Chart
Looking at a nautical chart for the first time can be confusing. There is a great deal of information and it may be difficult to decipher what you're looking at and what it means.
Essentially, 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 not apparent to the naked eye. Rocks and sandbars and other obstacles can stop your boat in its wake, and could harm you, your boat, and the environment.
In addition to your nautical chart, the most important publication you must have on board 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. Hundreds of these symbols and abbreviations are used on our charts. They are based on international specifications allowing nations around the world to use our charts without confusion.
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.
Several things you should take note of in the title block include:
- 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" refers to 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, 1:2 001 - 1:20 000 and are used for navigation in harbours or intricate, hazardous, shoal-infested waters.
- Approach Charts
- 1:20 001 - 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. When boating, it is essential 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 the units of depth on your chart. 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 your boat has enough clearance to travel under the overhead cables and bridges.
- Horizontal Datum
- describes the stating point that is used for positioning objects on the surface of the earth. (Fig. 3) 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.
- 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. 4)
- 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.
- 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.
- 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, towns, and more, of interest to the mariner.
- Date modified: