Reading Nautical Charts

Larry De RidderEducation

Larry De RidderBy Larry De Ridder

We’ve received a couple of requests recently for information to help boaters interpret what their charts display. So, here goes. First off, there are four different types of charts, classed primarily by the area size they cover and the level of detail displayed. They start with Sailing Charts, which generally are drawn to a scale of 1:600,000 or smaller. The shoreline and topography are generalized and really they’re only useful if you are navigating offshore for extreme distances (e.g. leaving Eureka and headed for Honolulu). As a practical matter most of us will never need a Sailing Chart. The next level is General Charts. These are intended for coastwise navigation well offshore of most hazards, and are drawn on scales from 1:150,000 to 1:600,000. This level of detail would be useful if you were leaving Eureka and headed for San Francisco, and staying well offshore. Third is Coastal Charts, designed for inshore coastwise navigation, entering and leaving harbors, and navigating large inland waterways. The scales range from 1:50,000 to 1:150,000. The last type, providing the most detail and the smallest area of coverage are Harbor Charts. These provide a great deal of detail intended for navigation and anchorages in harbors and small waterways. The scale is generally larger than 1:50,000.

To provide some sense of size, 1:600,000 means one inch on paper represents 600,000 inches of real world, which equals 50,000 feet, or about 8.23 nautical miles per inch of chart. Note: a nautical mile is 6,076 feet, but for quick math navigators pretend it is 6,000 feet. Thus a Harbor Chart at the 1:50,000 scale means that one inch on paper = about 0.68 nautical mile, or about 0.79 statute mile.

On our GPS/chart plotters, government paper charts have been copied onto a computer chip. The accuracy of your chip display depends on the accuracy of the chart it was derived from, the way it was copied and the number of pixels in your display. Generally, nearshore stuff is very accurate, as it was taken from Harbor and Coastal Charts, but what you see 50 miles out tuna fishing is less accurate, because it was taken from a General Chart. In areas where the coastline changes regularly (say, due to storm-driven sand bars), the government is always playing catch-up with underwater changes, so you can’t take your GPS chart’s accuracy for granted. There are two basic ways in which chart plotter manufacturers display their data on-screen. One technique is to very carefully exactly copy the chart as a whole. The primary advantage is that nothing is missed or misplaced. However, this can result in a slower scroll, and uses a great deal of memory. It also means that nothing on the chart can be modified without a complete memory update, because everything is one large file. The alternate method is to break the chart data down into “layers” of detail. This allows the user to omit displaying features deemed unnecessary, which speeds the display. For example, if you don’t want shoreline navigational features shown, or you are irritated with the horizontal and vertical latitude and longitude lines, you can opt not to display them. However, there is a flip side to this type of display. If there is a “box” displayed somewhere (for example the dump site off Eureka), instead of exactly drawing the box as on the original chart, the computer goes to a particular place with instructions to draw a box of a particular dimension. This is much faster than exactly copying the original paper chart “box”. But a side result of this type of reproduction is that sometimes the display isn’t as accurate on fine details, because of how the data was stored. Remember also that a chart is a 2-dimensional representation of part of a 3-D world. Thus at the fringes of the chart there will be some distortion. The larger an area the chart covers, the greater the peripheral distortion. And finally, remember that your GPS position isn’t always spot-on. The key issue here is never trust your GPS display if it directly contradicts what you can see! Just because a GPS says the reef is 30 feet to the side, if your depth sounder indicates it is below you, exercise extreme caution.

Speaking of depth, it’s important to know how that is measured. With the tide continually cycling, clearly the depth to underwater hazards is continually changing. In most 24-hour days we have two tidal cycles, and thus two high and two low tides. One high tide is higher than the other, and one low is lower than the other. So, in a typical day we have a higher high tide, a lower high tide, a higher low tide, and a lower low tide. The baseline for underwater depths is based on the average of the lower low tides in the region displayed, called “mean lower low water”. This means most of the time the water will be a little deeper than shown, but at the lowest part of the lower low tide, the water could be shallower than shown, and certainly will be on the lower low Spring Tide. Clearly it’s important to be aware of the tidal cycle you are fishing, if you are in and around reefs, shoals or other underwater hazards. By the same token, if you navigate an area with overhead clearance issues (e.g. bridges and power lines), then you must be concerned about the height of the obstruction above the water. In this case the baseline clearance is given from the average of the higher high tides. Again, this gives a little extra room for clearance except at the top of the higher Spring Tides.

Looking at a physical chart you will note many features not shown on a chart plotter copy. These include items such as the chart title and number, various publication and editor’s notes, corner coordinates, linear scale references, etc. Some items, such as cautionary notes, may appear on your GPS if you move your cursor to the object in question. On paper charts you will also likely find a compass rose, showing the magnetic variation for the region in question (remember that true north is not the same as magnetic north).

Perhaps the one graphical item we look at most frequently on our chart plotters is the water depth around us – particularly when we are reef hopping for bottom fish. One advantage of the “layered” style of producing the electronic versions of charts that most of us use is that we can choose whether our machine displays depth in feet, meters or fathoms. Even within the “fathoms” style, you can have “fathoms and fractions” or “fathoms and feet”. If the first, 6.1 means 6.1 fathoms; in the latter it means 6 fathoms plus 1 foot. If you are in a metric display mode, 6.1 would be 6.1 meters, which is about 20 feet. It’s important to know which scale your display is showing! On a paper chart shallow areas are usually shown in various blue colors, and most color chart plotters have retained that pattern. Soundings are supplemented by depth contours, meaning every point along the line is believed to be the same depth. The closer the lines are to one another, the faster the depth is changing. If the line is broken or dotted, it means the reliability of the soundings is uncertain. Certain specialized charts called bathymetric charts emphasize these contour lines. Periodically there will be notes indicating whether the bottom is mud, sand or rock. That’s important information to have prior to anchoring, since it could influence your decision on what anchor style to use, or even whether to anchor at all.

Other features shown include aids to navigation such as buoys and established navigation channels. A lighthouse, for example, will be shown with lines radiating from it to show its side-to-side limits of visibility. It will also show an abbreviation code indicating the height and light characteristics which positively distinguish this light from other lights. Channel markings will indicate where large vessels may be encountered. Here that isn’t much of an issue, but offshore of Los Angeles that’s really important! Operating Areas may show where the Navy has reserved an area for war games, live fire ranges or anchorages.

Well, we’re out of room for this issue. We’ll continue in a future edition.[/cs_text][/cs_column][/cs_row][/cs_section]