Reading Nautical Charts II

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Larry De RidderIn our February 2016 issue we began a discussion on how to get the most out of your charts and chart-plotters. If you missed that article it can be accessed here.

To continue the discussion, it’s important to realize that some of the chart data you are looking at is quite old. Recently added or updated data was likely gathered with a multi-beam full-coverage sonar unit. However, just a few years ago the state of the art was a single beam echo sounder, and prior to 1940 readings were taken with a chunk of lead tied to the end of a line, and lowered to the bottom periodically. In fact, given how rarely NOAA actually updates charts in most areas, more than 50 percent of the information found on NOAA charts is based on surveys done before 1940 using a sextant to locate a position and a lead line to measure depth. Some of that data is pre-1900! Older measurements, depending on the depths, may have been taken anywhere from 50 to 400 yards apart, with the assumption there was not much change between the two marks. Thus there are many bottom irregularities, such as smaller reefs or submerged hills, which are still being located and marked on chart updates. Other items, such as wrecks or other manmade objects may have appeared since the last chart update, or be the result of natural changes.

Given the space constraints on even the largest chart plotter displays, it really is a good idea to spend time studying the actual paper NOAA chart for the area you will be navigating. It’s the only way to really appreciate the “big picture” without losing the details as you “zoom out” on a plotter. Each NOAA chart, except international charts, has a five digit identification number. For example the chart for Point Arena to Trinidad Head is number 18620. The number can be found in the lower left corner of the chart. Immediately above that is the Edition of that particular chart, and the month and year it was produced. Charts produced since 2003 include a specific date showing the most recent update from the Local Notice to Mariners that was incorporated into the data. The Feds regularly produce and distribute updates, so that paper chart owners can manually mark their charts with the newest data. When you update your chart plotter chip you are getting new data incorporating these Notices, which may be important for safety or perhaps simply show the most recently charted wreck you may want to fish.

As mentioned in the prior article, depths can be displayed in several ways. Individual numbers represent individual soundings, regardless of whether it was a sophisticated sonar array or a 1920’s lead line reading. Large block letters at the top of the chart will define the measurement scale in use, such as “SOUNDINGS IN FATHOMS”. This would indicate that a number such as 3.5 would indicate a depth of 3.5 fathoms, or 21 feet. Depth contours which are solid indicate that every point on the line is at the same depth, while a broken or indefinite bottom contour line would indicate the data is incomplete. For example, the side limits of dredged channels are marked with broken lines, because these tend to change frequently and are not considered reliable due to the fact they can change rapidly. For smaller channels, the charted depth only applies to the central 80% of the channel width, and is not considered reliable for the 10% along each edge. For larger channels charts will show a separate table of depths for cross-section portions of a channel, labeled as “left outside quarter”, “left inside quarter”, “right inside quarter” and “right outside quarter”. This data is serious reading for bar pilots! If a chart area is too small to indicate all the depths present, the shallower numbers are given preference, as a safety measure.

Charts start with what is called a “horizontal datum”, which forms the basis for computations of horizontal positions. In your chart plotter’s menu options you likely can find a list of options available to you for use as a basis for position calculations. It’s important to have the right starting point. NOAA always uses the North American Datum of 1983 (NAD 83), which is the equivalent of the World Geodetic System of 1984 (WGS 84). If you specify the wrong coordinate starting point for your GPS, such as NAD 27, you might find yourself hundreds of yards off from where you think you are. If you take your boat and GPS international, you need to know what the basis is for the local charts at your destination, and may need to alter your chart plotter settings.

Another feature shown on paper Mercator charts is the compass rose and magnetic variation. It’s pictured as a circle graduated in degrees, clockwise from 0 degrees at the top all the way around till 360 degrees equals the starting point. There may be more than one diagram on a chart, and if there are multiples they may vary slightly. These are used by navigators to assist with laying out a course. They will show both a True North starting point and a Magnetic North starting point. Because magnetic north does not equal true north, and moves a bit every year, magnetic north on a General Chart may not be the same everywhere. Use the compass rose nearest your location. If you are on the water and using your chart to plot a course, you need to know which of these compass rose marks you will be using!

When laying out a course, it’s important to take into account both Magnetic Variation and Magnetic Deviation. Magnetic Variation is the angular difference between the two sets of marks. Magnetic Variation is given as degrees and minutes, either West or East of the designated heading. A west variation is subtracted from the magnetic to obtain a true course, while an easterly variation would be added. To throw one last complication into the mix, each boat has a unique magnetic field, which causes a compass deviation. This is a measure of how much your own boat’s magnetic compass is affected by the boat itself, and its contents. Serious long-distance navigators will “swing the boat” in increments and calculate how their own vessel’s compass is affected by the boat’s hull and contents on each course, to make the final adjustment between true and magnetic headings. This process will be repeated each time there is a boat modification, particularly if it involves something with a magnetic field, such as installing new speakers or a new compass. You can see why Navigator is an important position on any large vessel!

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