Reading a nautical chart means treating it as a navigation tool rather than a road map. It shows depths, hazards, the shape of the shoreline and aids to navigation, at a scale chosen for a particular kind of water. For a beginner, the method is to work in a fixed order: start with the title block, then check the scale and chart type, the depth and height units, the datum and projection, and finally the symbols. According to the Canadian Hydrographic Service (2022), the title block is the place to begin on a paper chart.
Why start with the title block?
The title block is the printed information around the chart area. It tells you what the chart covers, how it was produced and how far you can rely on it. The Canadian Hydrographic Service (CHS), part of Fisheries and Oceans Canada, recommends reading it first. Its contents include the seal or crest of the hydrographic office, the regional identification, the main title, the scale and chart type, the projection, the depths note, the elevations note, the horizontal datum, the sources note and the symbol reference note.
Each item changes how you interpret the rest of the chart. The main title names the specific area, and a title written as from and to indicates the upstream direction, which matters on rivers and channels. The sources note states where the chart’s data came from and how old it is. An older survey may not show a sandbar or rock that has since shifted, so the date is part of your safety check, not a formality.
How does scale tell you how much detail to expect?
CHS uses the natural scale, which is the ratio between the chart and the earth. A scale of 1:15,000 means that one unit on the chart represents 15,000 of the same units on the ground or water. The beginner’s rule is simple: the larger the number after the colon, the more area is compressed onto the paper and the less detail each part of the chart can carry.
CHS groups its charts by scale:
- Harbour charts, 1:2,001 to 1:20,000, for harbours and for hazardous or shoal-filled waters.
- Approach charts, 1:20,001 to 1:50,000, for approaching coasts that need a lot of detail.
- Coastal charts, 1:50,001 to 1:150,000, which give continuous coverage with enough inshore detail to make landfall easy to spot.
- General charts, 1:150,001 to 1:500,000, for extensive offshore coverage with enough inshore detail for landfall.
- Overview or sailing charts, 1:500,001 and smaller, for offshore navigation beyond sight of land.
Use the chart type as a guide to what the chart is designed to show. A general chart may show the coast as a clean line but will not reveal the rocks near a marina entrance. When you move from open water into a harbour or a narrow passage, switch to the largest-scale chart you carry, because the detail you need is on that chart and not on the overview.
How are depths and heights shown?
Depths on Canadian charts are printed either in feet and fathoms, where one fathom equals six feet, or in metres and decimetres. The depths note in the title block tells you which system the chart uses, so read it before you interpret any figure. CHS states that eventually all Canadian charts will show depths in metres, so you will meet both systems for some years. Mixing them up is a common beginner error. A figure of 3 fathoms read as 3 feet makes the water look six times shallower than it is.
Heights work differently. The elevations note gives the datum for structure heights and for bridge and overhead cable clearances, which is the reference you need to decide whether your boat can pass beneath them. Do not treat a published clearance as a fixed value for your trip. In tidal or fluctuating water, the height available changes with the water level at the time of passage. The official sources consulted do not set out a conversion method for this, so check the tide or water-level information for that place and time, and keep a clear safety margin.
Why do projection and datum matter?
A projection describes how the curved surface of the earth is shown on flat paper. According to CHS, the Mercator projection is the most common for nautical charts. It minimizes distortion of shape and direction, which is why the outline of coasts and islands helps you confirm your position. Mercator does not show every area at equal scale, however, so measure distances close to your planned route rather than across a large part of the sheet.
The horizontal datum is the reference starting point for positioning objects on the earth’s surface. If your GPS receiver or plotter uses a different datum from the chart, the position on the screen may be offset from where it falls on the paper. Before plotting a position, check that the device’s datum setting matches the chart’s horizontal datum, and consult the manufacturer’s documentation for how to change that setting.
Where do you find the symbols and abbreviations?
The symbol reference note, the last item in the title block, points to Canadian Chart 1, Symbols, Abbreviations and Terms. This publication from CHS explains the symbols, abbreviations and terms used on its nautical charts. It is based on the chart specifications of the International Hydrographic Organization, and it is available free online, where CHS posts updates. Keep a current copy with your charts, in printed or digital form, so you can check unfamiliar symbols on the water.
Two points help beginners. Abbreviations are listed without full stops, and lettering and capitalization can vary from chart to chart without any change in meaning. A symbol that looks unfamiliar is usually explained in Chart 1, so look there before guessing.
When should you trust a chart, and when should you slow down?
A chart records surveyed conditions at a given time, so it is an aid to judgement, not a substitute for it. CHS advises referring to the chart often during a trip, because hazards such as rocks and sandbars may not be visible from the water. Keep the chart in use from departure to arrival, not only when you plan the route.
Compare what you see with what the chart shows: the shape of headlands, the position of buoys and beacons, the line of the shore and the colour of the water over shallow areas. If the chart and the water disagree, slow down and assess your position before going on. Treat the disagreement as a reason to stop or reduce speed, avoid unknown water, and check your position with every means you have. If you are unsure, seek help rather than press on.
How can a beginner practise chart reading?
Practise on water you know well, in good visibility and calm conditions, with a current chart of that area. Work through the following steps:
- Read the title block and note the scale, chart type, depth units, datum and the date given in the sources note.
- Pick a feature you can see ashore, such as a headland, a light or a prominent building, and find it on the chart by its shape.
- Read several depth figures along your route and identify areas marked as hazardous, including rocks and shoals.
- Find a buoy or beacon you can see and confirm its symbol in Canadian Chart 1.
- Compare the same stretch of water on a harbour or approach chart and on a coastal or general chart, and note what each one leaves out.
Repeat the exercise until you can name the main features without looking at the legend. Only then move on to planning a passage with a route, distances and a margin for error.
Frequently asked questions
Are electronic charts read in the same way as paper charts?
The same logic applies: check the scale, datum, date and units before you trust a position. Menus, symbol displays and default settings differ between devices, so check the manufacturer’s documentation for your display.
Can I use a chart issued by another country in Canadian waters?
Charts are issued by national hydrographic offices, and their conventions can differ. The official sources consulted for this article do not address that question, so use current CHS charts for Canadian waters and check with the issuing authority for any other chart.
Why might a chart depth differ from my sounder reading?
Chart depths are referenced to the chart’s own datum, while a sounder shows the water depth at that moment, which can change with the tide. Check the sounder’s unit and offset settings against the manufacturer’s documentation.
