Technical Aspects of Charts - SailNet Community
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Technical Aspects of Charts

Let's get into some of the more technical aspects of charts so that you can increase your understanding of this wonderful invention.

A nautical chart represents part of the spherical earth on a plane surface. It shows water depth, the shoreline of adjacent land, topographic features, aids to navigation, and other information useful to mariners. It is the work area on which the navigator plots courses, ascertains positions, and views the relationship of his ship to the surrounding area chart assists the navigator in avoiding dangers and arriving safely at his destination.

An electronic chart is not simply a digital version of a paper chart; it introduces a new navigation methodology with capabilities and limitations very different from paper charts. The electronic chart will eventually become the legal equivalent of the paper chart when approved by the International Maritime Organization and the various governmental agencies that regulate navigation. Currently, however, professional mariners must maintain paper charts on the bridge, or at least have them immediately available when using an electronic chart for navigation. Should a marine accident occur, the nautical chart in use at the time takes on legal significance. In cases of grounding, collision, and other accidents, charts and logbooks become critical records for reconstructing the event and assigning liability. We have discussed electronic charts in previous articles

Because a cartographer cannot transfer a sphere to a flat surface without distortion, he must project the surface of a sphere onto a developable surface such as a cone or a cylinder. A developable surface is one that can be flattened to form a plane through a process known as chart projection. In fact, a sphere cannot be cut and laid out flat without distortion, while a cone or cylinder can be. If you are not convinced, try peeling an orange and laying the peel out flat.

Chart pic 2As the use of electronic charts becomes increasingly widespread, it is important to remember that the same cartographic principles that are used on paper charts also apply to their depiction on computer screens. Each projection has certain preferable features. However, as the area covered by the chart becomes smaller, the differences between various projections become less noticeable. On the largest scale chart, such as a harbor chart, all projections are practically identical. In all cases the chart will show the true shape and angular relationship of physical features in their correct relative proportions.

The type of developable surface to which the spherical surface is transferred determines the projection's classification. The name of a projection indicates its type and its principal features. Mariners most frequently use a Mercator projection, classified as a cylindrical projection upon a plane with the cylinder tangent along the equator. To better understand this, visualize a cylinder placed over a transparent globe of the earth. If a light bulb is placed inside the center of the globe, the outline of the continents, as well as latitude and longitude lines, will be projected onto the cylinder. When you see a rectangular map of the world, a Mercator projection is the most likely one that was used.

Mercator ProjectionThe Mercator is the most common projection used in maritime navigation, primarily because rhumb lines plot as straight lines. A rhumb line is a line that crosses all meridians at the same angle.

The nature of a sphere is such that any point on it is exactly like any other point. In order for points to be precisely located on the sphere we call the earth, some system of reference points must be used. Over the last 400 years a coordinate system has been developed using imaginary lines known as latitude and longitude.

Latitude was easy to determine and understand since the North Star had been used for centuries to sail a constant East-West line. The reference points were the North and South poles and the distance halfway between them became known as the equator or zero degrees. Parallels of latitude were constructed North and South of the equator. Latitude is measured in degrees up to 90 both North and South of the equator.

Unlike latitude there was no natural starting points for the second component of the coordinate system which became known as longitude. For many years each country used their own reference point as the zero line or starting point and proceeded East and West from there around the word to finally reach the 180 degree point on the opposite side of the earth. Eventually the observatory at Greenwich England was accepted by international agreement as the zero degree or starting point for longitude to be used by all nations. Longitude is measured in degrees up to 180 in both an East and West direction from Greenwich.

Once this coordinate system was added to the nautical chart it became relatively easy to measure both true direction and distance. Distance, as previously defined, is measured by the length of a line joining two points. The most common unit is the nautical mile defined as one minute of arc on a meridian (one minute of latitude), or 6,076 feet.

Closely related to the concept of distance is speed, which determines the rate of change of position. Speed is usually expressed in nautical miles per hour and is know as knots. Thus a speed of 6 nautical miles per hour and 6 knots are the same thing. It is incorrect to say 6 knots per hour unless referring to acceleration.

Direction is the position of one point relative to another without reference to the distance between them. The time honored point system for specifying a direction as north, north-northeast, northeast, east-northeast, east, etc. is not adequate for modern navigation. It has been replaced for most purposes by a numerical system. This system divides the horizon into 360 degrees starting with north at 000 degrees and continuing clockwise through east at 090 degrees, south at 180 degrees, west at 270 degrees and back to north at 360 degrees or 000 again. Many modern compasses are laid out in both degrees and the cardinal headings. Generally speaking, all courses should use three numbers, i.e. 010 degrees and unless otherwise stated should be referenced to true north.

Since determination of direction is one of the most important aspects of navigation, the various terms involved should be clearly understood and will be the subject of a future article

Jim Sexton is offline  
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