Did you know that chart navigation accuracy depends on the scale of chart you use? Or that some nautical charts combine feet and fathoms or meters and feet for water depth. If you want to become a more skilled sailing skipper, you will need to crack the mystery of your chart’s vital title block treasure chest!
Scan your navigational chart somewhere in the unused land area for the name of the chart. This title block are contains the most important information that describes the geographic area, type of projection, scale, datum, and chart notes.
All title blocks contain seven key elements that are vital for you to know before you purchase or use the chart. Follow this easy summary to learn 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 limits of your navigation chart. Here’s an example:
UNITED STATES EAST COAST
From top to bottom, this description tells you that the chart area shows the US, along the east coast in the state 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 navigators to plot sailing courses as straight lines from one point to another. This makes navigation easier and less stressful.
3. Use the largest scale available
Choose a chart with a larger scale for the best accuracy in navigation. 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 chart with a scale of 1:10,000 means that one inch of the chart would need to be blown up 10,000 times to show the actual size. A chart that shows a scale of 1:80,000 means that one inch on that chart would need to be blown up 80,000 times for the actual size. The 1:10,000 scale chart shows greater 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 datum for GPS compatibility
Make sure your chart states North American Datum of 1983 or World Geodetic System 1984. That means when you plot the latitude and longitude from a nautical gps onto your chart, it will be accurate. If your chart shows different datum information, you will need to set your gps to the appropriate datum (see your owner’s manual). This resets the gps so that its lat and long matches the lat and long of the chart for error-free, accurate navigation.
5. Know your depth soundings
Depths can be expressed as 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 water tidal stage. There’s a huge difference between the meaning of a 3 on a chart that shows feet and a 3 on a chart that shows fathoms.
The chart with fathoms would mean that you had 18 feet of water over that spot–more than enough to clear the keels of the most any cruising or racing sailboat. Understand the sounding measurement before you use any chart to keep your navigation simple and stress-free.
6. Determine how heights are shown
Next, look for a statement that tells you how heights are measured. Find a statement that looks similar to this: Heights in feet above Mean High Water. Mean (average) high water does not take into account periods when tides have exceptional heights, such as those at spring tides, after heavy, extended rainfall, or storm surge. For safety’s sake, always allow several feet of clearance above the stated height to clear the lower steel under bridges.
For example, if a bridge vertical clearance shows 45 feet, that’s the average height of the lowest steel in the center of the bridge. At 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 chart title block at the numerous notes that contain navigation safety information. These tell you about vessel routes, dangers, warnings, marine weather broadcast channels in the area, and give important cautionary information. Spend a few minutes to get a complete picture of the navigation area.
Use these seven fast, easy chart navigation tips to get a more complete understanding of the awesome power of a navigational chart. You will become a more skilled, confident sailing skipper and navigator–wherever in the world you choose to cruise!