Download -1- I. Introduction The ocean, unlike the land, does not have

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NAVIGATION AND MARINE CHARTS
I. Introduction
The ocean, unlike the land, does not have topography, roads or street signs. Hence an issue from earliest
times was how to determine your location when you are at sea. Early sailors stayed within sight of land and
anchored at night. With the rise of empires (such as the Roman) it was possible to maintain bonfires along
the coast line thus allowing ships to sail at night. However in many cases the shortest distance between two
points was across open water and out of sight of land. Thus it was necessary to develop a system that would
permit travel without reference to landmarks.
II. Latitude and longitude
The fundamental coordinate system used to locate
one’s position on the earth is latitude and longitude
(Figure 1). Lines of latitude run parallel to the equator
(east-west) and go from 0o at the equator to 90o at the
poles. Lines of longitude run north-south. The reference
point for longitude is the Prime Meridian of the Earth
which passes through Greenwich, England. Longitude is
measured east or west of Greenwich and goes from 0o at
Greenwich to 180o at the International Date Line. Each
degree contains 60 minutes of arc and each minute
contains 60 seconds of arc. We can specify the location of
any point on the surface of the earth in terms of longitude
and latitude. For example, the Weather Lab at the
University of Massachusetts Lowell, one of the focal
points of the Merrimack Valley, is located at 42o38'47"N
latitude and 71o19'24"W longitude.
Figure 1. Latitude and longitude coordinate
system for the earth. From Pipkin et. al., 1987.
Laboratory Exercises in Oceanography, 2nd Ed.
New York: Freeman, p. 21.
III. Determination of latitude and longitude
Determination of latitude can be done in a number of
ways. For example, if you are in the Northern Hemisphere
there is a star currently aligned with the rotational axis of the earth. To determine latitude one only needs to
know the angle of that star (the North Star) above the northern horizon. The Southern Hemisphere does not
have a pole star, so the problem is a bit more complicated but still relatively straightforward. During the day
the angle of the sun in the sky can be used to determine latitude. Since the maximum altitude of the sun varies
with the day of the year, because of the axis tilt of the earth, one needs to know the date in order to do a
precise latitude determination.
In principle the determination of longitude is very simple. The earth completes one rotation in 24 hours.
Given that there are 360 degrees in a circle, each hour the earth rotates 15 degrees. If one knows the time in
Greenwich, England, than one can compare that time to when the sun, for example, is at its highest point
(high noon) at your location. Let’s suppose that your local high noon occurs at 2 PM GMT (Greenwich Mean
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Time). You are two hours, hence 30 degrees from Greenwich. Since high noon at your location occurs later
than it does at Greenwich you must be west of Greenwich. Thus your longitude is 30oW. Unfortunately it
proved very difficult to maintain accurate time on a ship. In the 1800's the British Admiralty held a
competition to design a clock which could keep accurate time on board a sailing vessel. Such a clock was
ultimately built by Harrison, and the “Harrison Chronometer” is on display at the Greenwich Observatory.
Prior to the development of an accurate time keeping device for shipboard use, position was determined
by a combination of latitude determinations and dead reckoning. Latitude determinations were made
throughout the course of a day. It was assumed that the ship traveled in a straight line between each
determination. The distance traveled was determined from the velocity of the vessel. The velocity was
measured by dropping a log over the side of the ship to which was attached a rope with knots at equally
spaced intervals. An hour glass was used to time the passage of the knots. Knowing the total length of line
payed out, and the time, one can calculate the velocity. This was the origin of the term knot which is used
to measure the velocity of ships at sea. Today we set the knot (nautical mile hour-1) = 1' of latitude hour-1.
Hence 1 nautical mile = 1.15 statute miles and 1o of latitude = 60 nautical miles or 69 statute miles. To
convert knots to kilometers per hour multiply knots by 1.85. Before the development of the Harrison
Chronometer one had to know the diameter of the earth in order to determine longitude. During the voyages
of Columbus the earth was thought to be much smaller than its actually is, thus leading to Columbus’s
confusion about his location.
IV. The marine chart
Most marine charts give water depths, the configuration of the shoreline in coastal waters, and other
navigation aids such as lights and important landmarks. Water depths are also shown on charts. This is
particularly important in shallow waters since you need to have enough water to “float your boat”. It is
important to note the units of depth (feet, fathom = 6 feet, or meters) and the datum plane used for the depth
measurements. Remember that the ocean is influenced by the tides so the height of water changes throughout
a tidal cycle. If there is a 6 foot tidal range than the difference in water depth between low and high tide
would be 6 feet. If the datum plane was high tide, then during low tide the water depth would be 6 feet less
than that shown on the chart. Also included on some charts are the types of materials that make up the
bottom, hard (hrd), rocky (rky), gravel (g), shells (sh), sand (s), or coral (co).
V. Plotting a course
In plotting a course at sea we must distinguish
between magnetic and true north. For this purpose a
compass rose (Figure 2) appears on all navigational
marine charts. Almost all modern marine compasses are
graduated from 0o (north) clockwise through 360o. There
are 32 points on the compass, the four cardinal points,
north (0o), east (90o), south (180o ) and west (270o ), the 16
intercardinal points (illustrated in Figure 2) and eight
additional points that fall between each cardinal and
intercardinal point.
Figure 2. The compass rose. From Pipkin et. al.,
1987. Laboratory Exercises in Oceanography, 2nd
Ed. New York: Freeman, p. 24.
A ship’s course, in degrees, is the intended direction
of travel. A ship’s heading is the direction the ship is
actually going. A bearing is the direction from one point to another and is expressed as an angle from north.
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1. Answer the questions using the following table:
City
Latitude
Longitude
New York, NY
40o38'N
73o50'W
Reno, NV
39o30'N
119o46'W
Los Angeles, CA
33o42'N
118o15'W
Hana Bay, HI
20o45'N
155o59'W
San Francisco, CA
37o19'N
122o25'W
a. Which city is farthest north?
b. Is Reno, Nevada, east or west of Los Angeles?
c. What is the time difference (to the nearest hour) between New York and San Francisco?
d. What is the time difference (to the nearest minute) between Hana Bay and Los Angeles?
e. How far north is San Francisco from Los Angeles?
Nautical miles
Statute miles
2. The distance between two points is 60 statute miles. What is the distance in nautical miles?
3. A sailboat is doing 8 knots. Convert the sailboat’s speed to miles per hour and kilometers per hour.
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Figure 3. Fictitious chart of Port Cochron. From Pipkin et. al., 1987. Laboratory Exercises in
Oceanography, 2nd Ed. New York: Freeman, p. 26.
4. The following questions relate to the fictitious chart of Port Cochron (Figure 3). To answer the questions
you will need parallel rulers or two triangles and an inexpensive drafting compass.
a. Locate a ship station at 43o00'45"N, 154o59'45"E and mark the location with numeral 1. What is the
depth of water (in fathoms) at this location? What is the bottom sediment?
b. From Station 1, the point you just labeled, determine the bearing on Lehman Island. The compass rose
on the chart is an aid for measuring directions. Place a straightedge on the two points and transfer that
direction to the compass rose by parallel motion with parallel rulers or two triangles.
The bearing is
degrees true and
degrees magnetic.
Do the same for the AERO (aeronautical) beacon on land.
The bearing is
degrees true and
degrees magnetic.
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c. From Station 1 you set off on your course for Lehman Island. The vessel is suddenly engulfed in fog.
The navigator fixes your position by determining the radar distance to the south light (0.7 km) and the
north light (0.8 km) of the harbor entrance. With dividers or a compass locate your position west of
the harbor entrance and determine the actual course and the ship’s distance from Lehman island.
Course
Distance is
true and
magnetic.
kilometers.
VI. Bathymetry
Bathymetry is the measuring and charting of the topography of the sea floor. At one time soundings (depth
measurements) were done by dropping a lead weighted line from a vessel and measuring the amount of line
payed out. In more recent times depths are determined by echo sounding. In this method a sound pulse is sent
out by a ship. The length of time it takes the pulse to reach the bottom and return to the ship is used to
determine the depth. The speed of sound in sea water is, on average, 1463 m sec-1. If it takes two seconds for
a sound pulse to make the round trip from ship to ocean bottom the depth of water is 1463 m.
5. A sound pulse is sent out. The pulse returns in 2.553 seconds. Calculate the water depth.
The water depths obtained by soundings can be used to construct a topographic map of the sea floor. This
type of map is referred to as a bathymetric chart. The principles are exactly the same as those used earlier
in the course to construct a topographic map. The only difference is that we are contouring depths. Hence
small numbers indicate high areas on the sea floor while large numbers indicate low areas.
6. Figure 4 is part of the National Oceanic and Atmospheric Administration (NOAA) Chart 18746. Contour
the soundings shown on this chart using a 50-fathom contour interval (isobath). Note that the soundings
are in fathoms. Where appropriate draw in the 10- and 20-fathom isobaths to give more detail. Remember
to label the isobaths.
7. On the graph paper (Figure 5) construct cross sections along the two profile (AB and XYZ) lines shown
on the chart. The upper graph is vertically exaggerated and the lower graph is true scale with no vertical
exaggeration. Draw both profiles at the two different scales.
8. Briefly describe and compare the true and exaggerated shapes of the topography of Newport Beach,
California.
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Figure 4. National Oceanic and Atmospheric Administration Chart 18746, San Pedro Channel, California. From
Pipkin et. al., 1987. Laboratory Exercises in Oceanography, 2nd Ed. New York: Freeman, p. 8.
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Figure 5. Profile grids. From Pipkin et. al., 1987. Laboratory Exercises in Oceanography, 2nd Ed. New York:
Freeman, p. 9.
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