All About Charts, Part Two - SailNet Community

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Old 11-29-2004
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All About Charts, Part Two

In Part I, we read all the information in the borders and Title Block of the chart. Here, in Part II, we will note that there are other features of the chart you must know and understand. Fortunately, when you look at the chart itself, you will find important standard features that make it easy for you to understand the information being portrayed.

They are:

Colors. On NOAA charts, land is usually colored yellow, tidal flats greenish yellow and shoal areas are tinted blue. A light green or purple tint indicates areas that have been swept clean with a wire drag. These areas are outlined with a broken limiting line and the clear depth is indicated with a characteristic symbol under the numbers. White areas on the chart are considered the high seas.

On electronic charts, a variety of other color schemes may be used. Raster scanned NOAA charts use the standard NOAA color scheme, but raster scanned charts for other countries use the standard color scheme of the country of origin. Vector charts use a color scheme determined by the manufacturer and are usually close to the NOAA chart colors for US areas. The colors you see displayed on your screen will vary depending on the manufacturer, your color palette settings and the intensity and contrast settings.

Contour lines and Soundings. Lines connecting points of equal depth or elevation are called contour lines. The solid black line next to coastlines indicates the mean high water mark. The first contour line seaward from the shoreline is the 6-foot (2 meters or 1 fathom) line. The next contour line seaward from the 6-foot line is the 18-foot (5 meters or 3 fathoms) line. From the shore line to this contour line is considered shoal area and is often tinted in blue. Additional contour lines are spaced at appropriate intervals out to the 100-fathom line. A broken contour line is used whenever the reliability of the depth contour is questionable.

Numbers denote individual soundings: they may be in either standard upright font or italics. Both types of lettering may be used on the same chart to distinguish between sounding data obtained from different surveys, different datums or from smaller scale charts. Standard type is used for features which are dry at high water while italics is reserved for underwater and floating features. This general rule is true for all lettering on a chart except for numbers used to show heights, elevations and soundings. By evaluating the type of lettering you can easily determine if a feature is visible at high tide. For example, if the name of a rock is given in standard font letters, it constitutes a small islet; if its name is in italic letters, the rock is a reef that is covered at high tide.

Soundings shown are the least depths to be expected under average conditions. Most NOS charts of US waters are based upon mean lower low water. You will also see soundings based on mean low water or mean low water springs. When we get to the section on tides and currents, I will cover these sounding datums in more detail. For now, just be aware that there will be times when the actual depth of water is less than what is shown on the chart.

Chart Abbreviations and symbols. Chart No. 1 is not a chart at all, but a book containing a key to all of the chart symbols. Subjects covered include general features, topography, hydrography and aids to navigation. There is also a complete index of abbreviations and an explanation of the IALA buoyage system used in US waters. You need to be familiar with most of these symbols as it is not always apparent what these symbols represent. Many of them you need to know by heart to avoid dangers, especially the ones for rocks, wrecks, tide rips, eddies, piles, dolphins, snags, fish stakes and fish traps. For example you need to know the difference between a * symbol and a + symbol, especially when an * also has an underlined number enclosed in parentheses next to it: i.e., *(2). This is a rock that is covered at high tide and protrudes above the sounding datum by the indicated number of feet.

Aids to Navigation: This subject requires a separate article to cover in detail and will be featured in our next discussion . I highly recommend the various seamanship courses taught by the US Coast Guard and US Power Squadrons for you to become totally familiar with the US Buoyage System and use of these navaids. I also suggest that you read and study the chapter on Aids to Navigation in Bowditch, using Chart No. 1 and your local area chart familiarize yourself with the symbols. Aids to Navigation are shown by symbols listed in Sections P through S in Chart No. 1.

On the chart, aids to navigation symbols are exaggerated in size relative to the chart's scale. "Position Approximate" circles are used on floating aids to indicate that their location varies around the mooring. The type and number of aides to navigation shown on a chart and the amount of information given varies with the scale of the chart.

Light houses and other navigation lights are shown as black dots with purple disks or as purple flare symbols. Small triangles mark red daybeacons and small squares mark all others. A variety of buoy symbols are used to show the different types of navaids on the chart and a legend element in italic type near the buoy symbol provides details about the buoy. There is no significance to the angle at which a buoy symbol appears on the chart: it is arranged to avoid interference with other chart details or features.

Miscellaneous Chart Features:. Compass roses are placed at convenient locations on Mercator charts to make it easier to plot bearings and courses with the parallel ruler. The outer circle is graduated in degrees with zero (360) at true north. The inner circle is also graduated in degrees but it is measured from magnetic north. In the center of the compass rose you will see the magnetic variation along with the annual change. When a chart is reprinted, the magnetic information is updated to the latest epoch.

Usual or average currents are sometimes shown on charts with the arrow's pointer giving the direction of flow and numbers showing the current's speed. Depending on tides and weather conditions, direction and speed of any current may differ considerably from what is shown on the chart.

Anchorage areas are labeled with a variety of magenta, black or green lines and anchorage berths are shown as purple circles. Spoil areas are shown with short broken lines, and on NOS charts they are also tinted blue and labeled. These areas contain no soundings and should be avoided.

Land areas, with spot elevations, cities, roads, commercial radio stations and prominent landmarks are also shown on the chart if they can be of value to the mariner for bearings. Later in this series covering coastal navigation, I will go into detail as to how to use the symbols ashore to help determine your position.

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