Did you know that chart navigation accuracy depends on the chart scale you use? Or that some nautical charts combine feet and fathoms or meters and feet for the depth of the water. If you want to become a more skilled sailing captain, you’ll need to crack the mystery of the treasure chest in your card’s vital title block!

Scan your navigation chart somewhere on the unused land area to find the name of the chart. This title block contains the most important information describing the geographic area, projection type, scale, datum, and chart notes.

All title blocks contain seven key elements that are essential to know before purchasing or using the graphic. Follow this simple overview to see how title blocks appear on any nautical chart in the world.

1. Know your navigation coverage area

Look at the location on the earth’s surface to determine the exact boundaries of your navigation chart. Here’s an example:

EAST COAST OF THE UNITED STATES

MAINE

PENOBSCOT BAY

From top to bottom, this description tells you that the chart area shows the US, along the east coast of Maine, with detailed navigation information for Penobscot Bay.

2. Make Mercator your projection of choice

Most nautical charts are based on a Mercator projection, which shows lines of latitude and longitude as straight lines. This projection allows mariners to draw navigation courses as straight lines from one point to another. This makes browsing easier and less stressful.

3. Use the largest scale available

Choose a chart with a larger scale for the best navigation accuracy. Chart scales are expressed as a ratio of one inch compared to the same number of inches on the earth’s surface.

For example, a graphic with a scale of 1:10,000 means that one inch of the graphic would have to be enlarged 10,000 times to show its actual size. A chart showing a scale of 1:80,000 means that one inch on that chart would have to be magnified 80,000 times to get the actual size. The 1:10,000 scale chart shows more detail, but the 1:80,000 chart covers more area. So the 1:10,000 scale chart will be the larger of the two scales.

4. Check date for GPS compatibility

Make sure your chart shows the 1983 North American Datum or 1984 World Geodetic System. That means when you plot latitude and longitude from a nautical gps on your chart, it will be accurate. If your chart shows different reference information, you will need to set your gps to the proper reference (see your owner’s manual). This resets the GPS so that your latitude and longitude match the latitude and longitude on the chart for accurate and error-free navigation.

5. Know your depth soundings

Depths can be expressed in feet, fathoms (1 fathom = 6 feet), or a combination of fathoms and feet or meters (1 meter = 3.3 feet) and feet. All depths are measured from a low tide level. There is a big difference between the meaning of a 3 on a card showing feet and a 3 on a card showing fathoms.

The chart with fathoms would mean there was 18 feet of water above that spot, more than enough to clear the keels of most cruising or racing sailboats. Understand the sounding size before using any chart to keep your navigation simple and stress-free.

6. Determining how heights are displayed

Next, find a statement that tells you how heights are measured. Find a statement that looks like this: Heights in feet above mean high tide. Mean (average) high water does not take into account periods when the tides have exceptional heights, such as spring tides, after prolonged heavy rainfall or storm surge. For safety reasons, always allow several feet of clearance above the listed height to pass bottom steel under bridges.

For example, if the vertical clearance of a bridge shows 45 feet, that is the average height of the lowest steel at the center of the bridge. On a full moon, when spring tides occur, you can expect much less clearance at high tide. Play it safe and wait for low tide when in doubt.

7. Read your navigation notes for safety

Look around the title block of the chart for the many notes containing safety boating information. These tell you about ship routes, hazards, warnings, transmission channels for marine weather in the area, and provide important precautionary information. Take a few minutes to get a complete picture of the navigation area.

Use these seven quick and easy chart navigation tips to gain a fuller understanding of the awesome power of a navigation chart. You will become a more skilled and confident skipper and navigator, anywhere in the world you choose to sail!