Sunday, March 22, 2009

Nautical Charts

To travel safely in a boat, you must have knowledge of water depths, shoals, and channels. You should also know the location of aids to navigation and landmarks, and where ports and harbors can be found. At any near shore position, you can measure the depth of water and see some landmarks, but for true safety you should know the depth ahead, the actual location of the aids to navigation you can see, and where more navigational aids will be on the course you follow. To plan the best route you should know the dangers to navigation along the way. This information can be found by updating your charts from notice to mariners (NTM) or by purchasing print on demand nautical charts.

A nautical chart is a representation on a plane surface of a portion of the earth's surface showing the water, natural and man-made features of interest to a navigator. A map is a similar but used on land in which shows roads, and cities. A chart's basic purpose is to give you information that lets you make the right decision in time to avoid danger. Your charts should be accurate. Even a small error in charting the position of a submerged obstruction can be a danger to your vessel.

Geographic Coordinates
Charts show a grid of intersecting lines to aid in describing a specific position on the water. These lines are charted representations of a system of geographic coordinates that exist on the earth's surface.

Meridians and Parallels
Geographic coordinates are defined by two sets of great and small circles. One is a set of great circles each of which passes through the north and south geographic poles, these are the Meridians of Longitude. The other set is a series of circles each established by a plane cutting through the earth perpendicular to the polar axis. The largest of these is midway between the poles and passes through the center of the earth, becoming a great circle, this is the Equator. Other parallel planes form small circles known as the Parallels of Latitude.

Geographic coordinates are measured in terms of Degrees. The meridian that passes through Greenwich, England, is the reference for all measurements of longitude and is designated as the Prime Meridian, or 0 degrees. The longitude of any position on earth is described as East or West of Greenwich, to a maximum in either direction of 180°. Parallels of latitude are measured in degrees north or south from the equator, from 0° at the equator to 90° at each pole. For greater precision in position, degrees are subdivided into Minutes (60 minutes = 1 degree) and Seconds (60 seconds = 1 minute). In some cases, minutes are divided decimally in tenths. One degree of latitude is equal to 60 nautical miles, one minute of latitude is approximately one nautical mile.

Direction is defined as the angle between a line connecting one point with another point and a base or reference line extending from the original point toward the True or Magnetic North Pole. This angle is measured in degrees clockwise. Direction on charts can be described as so many degrees True (T) or so many degrees Magnetic (M). The difference between these directions is Variation and must be allowed for.

Measurement of Direction
To facilitate the measurement of direction, as in plotting bearings and laying out courses, charts have a Compass Rose printed on them. A compass rose has two or three concentric circles, several inches in diameter and accurately subdivided. The outer circle has its zero at true north, this is emphasized with a star. The inner circle or circles are oriented to magnetic north. The middle circle, if there are three, is magnetic direction expressed in degrees, with an arrow printed over the zero point to indicate magnetic north. The inner most circle is also magnetic direction, but in terms of Points, and half and quarter points, (One point = 11 1/4 degrees.)

The difference between the two sets of circles is magnetic variation at that location on the compass rose. The amount of the variation and its direction (Easterly or Westerly) is given in words and figures in the center of the rose, together with a statement of the year that such variation existed and the annual rate of change. Each chart has several compass roses printed on it in locations where they do not conflict with navigational information. For large area charts, the magnetic variation can differ for various portions of the chart. Check each chart when you first start to use it, and make sure, you use the compass rose nearest the area for which you are plotting. Depending on a chart's type and scale, graduations on its compass rose circles may be for intervals of 1 degree, 2 degrees, or 5 degrees.

Distances on charts are measured in statute or in nautical miles. The Statute (Land) Mile is 5,280 feet. The Nautical Mile is 6,076.1 feet (1,852 m) is used on ocean and coastal waters. You might have to convert from one unit to the other. This is not hard to do, 1 nautical mile = 1.15 statute miles, or roughly 7 nautical miles. One kilometer = 0.62 statute or 0.54 nautical miles. In navigation, distances of up to a mile or so usually expressed in Yards, a unit that is the same no matter which mile is used on the chart. Meter can also come in use for short distances, 1 meter = 1.094 yards.

Because a chart is a representation of navigable water area, actual distances must be scaled down to much shorter dimensions on paper. This reduction is the Scale of the chart, and it can be expressed as a ratio, 1:80,000 meaning that 1 unit on the chart represents 80,000 units on the actual land or water surface. The ratio of chart to actual distance can also be expressed as a Numerical or Equivalent Scale, such as "1 inch = 1.1 miles, another way of expressing a 1:80,000 scale. Try and fix in your mind the scale of the chart you are using.