Charts_ Navigation_ and Determining Position Marine Charts A map by lonyoo

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									Charts, Navigation, and Determining Position

Marine Charts
         A map is a two-dimensional representation of the surface of the earth. It may show
relative locations or major landmarks and physical features and usually has some type of grid
system for determining the location of various features. A chart is a type of map that shows
areas of the sea and is usually designed for use in navigation. Because an oceanographer relies
on surface vessels for transportation and as research platforms, marine charts are used as base
maps on which to locate position and plot his data. For this reason a student of oceanography
should learn the use of charts as well as some of the basic principles of navigation and
seamanship. The most critical requirement of all oceanographic work may well be precise
positioning, because data reported from generalized positions are virtually useless to others
wishing to follow up on previous work.
         To precisely describe locations on the surface of the earth, we use a numerical grid
system defined by coordinates of latitude and longitude. These coordinates represent angular
distances relative to reference planes in the earth’s interior. The lines of latitude are called
parallels of latitude (or just parallels), because they are parallel to the equator and to each other.
Measured in degrees of arc along a circle, they specify the angular distance north or south of the
equator, from 0 at the equator to 90 at either pole. One degree of latitude always
corresponds to the same distance over the surface of the earth anywhere on the globe. Each
degree is divided into 60 minutes of arc (1 = 60') and each minute into 60 seconds (1' = 60"),
just as an hour is divided into 60 minutes and each minute into 60 seconds. It is also common to
describe portions of a minute in tenths or hundredths of a minute, for example 23'30" = 23.5'.
Latitude is recorded with its hemisphere notation, north or south: for example the latitude of
Hilo, Hawaii is 1944’N. Lines of longitude, or meridians, are also expressed in degrees and
refers to the angular distance on the earth measured from the prime meridian, 0 (at Greenwich,
England) east or west to 180 (the international dateline). One degree of longitude represents
different distances at different places on the globe, because lines of longitude converge at the
poles. Longitude should always be reported with its hemisphere notations, east or west: for
example the longitude of Hilo is 15505’W. Any specific location is stated by first giving the
line of latitude of its position (in degrees, minutes and seconds) and then giving the line of
longitude (in degrees, minutes and seconds). Remember “Latitude” first, “Longitude” second.
         In addition to latitude and longitude most marine charts give water depth, the
configuration of the shoreline in coastal waters, and other navigation aids such as lights and
important landmarks. On most charts, north is the top of the sheet, latitude scales are given on
the sides, and longitude scales are along the top and bottom. Meridians and parallels are drawn
at given intervals in fine black lines across the chart. The nature of the bottom is specified as
hard (hrd), rocky (rky), gravel (g), shells (sh), sand (s), or coral (co). Various other
abbreviations may be used for the bottom material and color. These are often defined in a key
somewhere on the margins of the chart.

        To locate a position on a chart, first find the desired latitude at points on the scales at
either side of the chart and then connect the two points. Now locate the desired longitude at the
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top and bottom of the chart and connect them. Where the lines cross will be the designated
position. Note that many charts cover an area of less than one degree of latitude or longitude, so
that the scales on the sides of the charts will be in minutes or seconds rather than degrees. This
is an important fact to remember when plotting positions.
         All maps, including charts, are projections depicting the spherical earth’s surface on a
plane. Most marine charts are Mercator projections. On a Mercator projection meridians are
parallel lines, therefore the poles, where the meridians intersect, cannot be shown. There is no
distortion at the equator but great distortion toward the poles. An advantage of Mercator
projections is that true directions are shown as straight lines.
         The ratio of distance on a chart to actual distance on the earth’s surface is the scale. Two
kinds of scales may be provided on a chart: a numeric scale and a graphic scale. A numeric
scale is usually given as a ratio such as 1:25,000. The ratio 1:25,000 means that 1 cm on the
map represents 25,000 cm on the earth’s surface. Though scale can be used to measure
approximate distances, the best method to measure distance on a Mercator chart is to measure 1
minute of latitude at the latitude at which the distance measurement is to be made. One minute
of latitude equals one nautical mile (1' = 1 n.m. = 6,076 ft. = 1.15 mi = 1.852 km). Though
distances are expressed in statute miles (mi) and kilometers (km) on land, at sea distances are
always given in nautical miles (n.m.). To determine the distance between two points from the
chart, measure the distance between them on the chart with dividers. The spread of the dividers
is used to carry the measurement of the latitude scale, where it can be read off in minutes.
Because of variations due to the charts distortion, distance readings should be taken from the
LATITUDE scale at a point approximately level with the boat’s position.
         The unit of speed used at sea is the knot and is defined as one nautical mile per hour. It
is incorrect to speak of knots per hour because this means nautical miles per hour per hour, a unit
of acceleration rather than speed. To convert knots to kilometers per hour multiply knots by
1.85.


Navigation and Piloting
        On the high seas, where there is no immediate danger of grounding, navigation is a
comparatively leisurely process. In the vicinity of shoal waters the situation is different.
Frequent or continuous position information is usually essential to the safety of the vessel. An
error, which on the high seas may be considered small, may in what are called pilot waters be
intolerably large. Frequent changes of course and speed are common. According to Bowditch
(the mariner’s bible), the term piloting is used to mean “the art of safely conducting a vessel on
waters the hazards of which make necessary frequent or continuous positioning with respect to
charted features and close attention to the vessel’s draft with respect to the depth of water.” No
other form of navigation requires the continuous alertness needed in piloting and at no other time
is navigational experience and judgement so valuable. The ability to work rapidly and to
correctly interpret all available information, always keeping “ahead of the vessel,” may mean the
difference between safety and disaster.
        Whether in navigation at sea or piloting close to shore, determining the vessel’s direction
of travel is one of the most fundamental navigational tasks. The “intended direction of travel”


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is the ship’s course, and is expressed in degrees. For example, a course of 180 is due south,
and one of 135 is southeast. In plotting a course at sea one must distinguish between the
magnetic north (to which the north-seeking pole of a magnetic compass points) and true, or
geographic, north. For this reason, one finds a compass rose on all navigational marine charts.
This rose clearly shows the equivalent magnetic and true north directions. Almost all modern
marine compasses are graduated from 0 (north) clockwise through 360. To travel between
any two points, the ship’s navigator plots the desired course on the marine chart and determines
the course to be steered by the vessel. However, wind, ocean currents, and pilot error may
prevent the ship from adhering to a particular course. A ship’s heading is the direction in which
the ship is actually traveling regardless of its prescribed course.


Determining Position
        Piloting makes extensive use of lines of position. A line of position is a line passing
through the vessel’s position, which is determined by observation or measurement. Lines of
position are of great value, but one should always keep in mind that they can be in error because
of imperfections in instruments used for obtaining them and human limitations in operating the
instruments and in interpreting the results. Once a line of position is obtained, an appropriate
label should be placed on the plot of the line of position to avoid possible error or confusion. A
label should include all information essential for identification, but no extraneous information.
        A bearing is the horizontal direction of one point from another. A bearing line is the
direction of an observed object on the chart extending from the observer. It is one of the most
widely used lines of position. It is usually expressed as the angular difference between a
reference direction and the given direction. In navigation, north is generally used as the
reference direction, and angles are measured clockwise through 360. It is customary to
express all bearings in three digits, using preliminary zeros where needed. Thus north is 000
or 360, a direction 7 to the right of north is 007, east is 090, southwest is 225, etc. A
bearing measured using a magnetic compass is referred to as a compass bearing in degrees
magnetic.
The basic principle of determining the vessel position using a bearing is that once a landmark has
been identified and the bearing determined, the vessel can only be on the unique line extending
through both the vessel and the landmark. The line extends out from the landmark along the
reciprocal of the observed bearing. Thus, if a lighthouse is cast of a ship, that ship is west of the
lighthouse. If a beacon bears 156, the observer must be on a line extending 156 + 180 =
336 from the beacon.
        Bearings are obtained by sighting compass, gyro repeater, radar, etc. One type of
bearing can be obtained by eye without measurement. When two objects appear directly in line,
one behind the other, they are said to be “in range,” and together they constitute a range. For
accurately charted objects, a range may provide the most accurate line of position obtainable, and
one of the easiest to observe. So useful is the range in marking a course that artificial ranges,
usually in the form of two lighted beacons, have been installed in line with channels in many
ports. A vessel proceeding along the channel has only to keep the beacons in range to remain in
the center of the channel. If the farther beacon (customarily the higher one) appears to “open


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out” (move) to the right of the forward (lower) beacon, one knows that he is to the right of his
desired track. Similarly, if it opens out to the left, the vessel is off track to the left.
        A line of position, represents a series of possible positions along a line, but not a single
position. However, if two simultaneous, nonparallel line of position are made, the only position
that can be on both lines at the same time is the intersection of these lines. This point is called a
position fix. When drawing the bearing lines to determine the fix it is good practice to plot only
a short part of a line of position in the vicinity of the vessel, to avoid unnecessary confusion and
to reduce the chart wear by erasure. Particularly, one should avoid the drawing of lines through
the chart symbol indicating the landmark used. In the case of a range, a straightedge is placed
along the two objects, and the desired portion of the line if plotted.
        Some consideration should be given to the selection of objects to provide a fix. It is
essential, for instance, that the objects be identified. The angle between lines of position is
important. The ideal is 90. If the angle is small, a slight error in measuring or plotting either
line results in a relatively large error in the indicated position. In the case of a bearing line,
nearby objects are preferable to those at a considerable distance, because the linear (distance)
error resulting from an angular error increases with distance.
        Another consideration is the type of object. Lighthouses, spires, flagpoles, etc., are good
objects because the point of observation is well defined. A large building, most nearby
mountains, a point of land, etc. may leave some reasonable doubt as to the exact point used for
observation. A buoy or a lightship may drag anchor and be out of position. Most buoys are
secured by a single anchor and so have a certain radius of swing as the tide, current and wind
change.
        Although two accurate nonparallel lines of position completely define a position, if they
are taken at the same time, an element of doubt always exists as to the accuracy of the lines.
Additional lines of position can serve as a check on those already obtained, and, usually, to
reduce any existing error. If three lines of position cross at a common point, or form a small
triangle, it is usually a reasonable assumption that the position is reliable, and defined by the
center of the figure. The small triangle is known as a “cocked hat” in nautical jargon. The size
of the cocked hat is a good indicator of the precision of the position fix.




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