NOTE: This Supplemental Information is a repeat of that which appeared in last week's Supplemental Information…In Greater Depth file.
Ocean scientists use many different graphical representations and some units unique to the field. In this week's Supplemental Information, we describe a few examples of oceanographic charts and define some units commonly used in ocean science. Throughout our study of AMS Ocean Studies, we will encounter these, together with other charts and units. In this supplement, we also briefly explain latitude and longitude as well as time keeping, important information for navigating the high seas.
A chart is a graphical model that assembles and displays data in an organized format that can be readily interpreted. Like all models, a chart is an approximation of actual conditions or the system that is represented. An oceanographic chart is a two-dimensional representation of a portion of a water body that is typically used for navigation purposes. For example, a bathymetric chart is a plot of the depth of the ocean floor (in feet, meters, or fathoms) below the datum, that is, mean sea level. Often contours are drawn connecting points of equal depth. Some U.S. government charts are color-coded with dark blue representing water 0-18 ft deep, light blue 18-36 ft, and very light blue to white more than 36 ft deep. From the contour pattern, mariners can determine the location of sea floor features (e.g., ridges, abyssal plains), the relief of the ocean bottom (the change in elevation between points), and slope or gradient (the inclination of the ocean bottom). The slope is equal to the relief divided by the horizontal distance over which the slope is measured. The closer the spacing of the contours the steeper is the slope. Other oceanographic charts plot the type of sediment or bedrock on the ocean floor. Most navigation charts include both types of data.
In the early days, water depth was determined by lowering a weighted rope or cable to the ocean floor. This line was marked off in 6-foot increments called a fathom, a unit of water depth measurement unique to ocean studies. (One fathom corresponds to 1.83 meters.) According to the Naval Historical Center, the fathom was once defined by an act of British Parliament as "the length of a man's arms around the object of his affections." The word derives from the Old English Faethm, which means "embracing arms." Today, sound waves are used to obtain much more accurate and detailed profiles of the ocean bottom. In the first Essay of Chapter 2 of your textbook, we describe the various techniques used by oceanographers to determine the depth and composition of the seafloor.
Precise location of a ship at sea is extremely important in any oceanographic enterprise, this is especially the case in the open ocean out of sight of any coastal landmarks. As with locations on land, oceanographers use the familiar latitude/longitude grid. The concept of latitude and longitude as imaginary reference lines on the globe by which location could be specified dates to the Egyptian geographer Ptolemy about A.D. 150. As you examine your AMS Ocean Studies globe, note that lines of latitude run east-west forming circles that decrease in circumference from the equator to the poles.
Latitude describes the angular displacement that a point on the Earth's surface is with respect to the equator, while longitude is the angular displacement that the point would be from some reference meridian, usually taken as the Greenwich Prime Meridian. For centuries, angular measurements describing the geographic coordinates of latitude and longitude were expressed in sexagesimal (base-sixty) units. In this numerical system, which originated with the ancient Sumerians (ca. 2000 BC), each degree is divided into 60 minutes of arc (identified by the symbol ') and each minute is divisible by 60 seconds of arc (identified by the symbol "). Therefore, latitude is expressed in degrees where 1 degree = 60 minutes and 1 minute = 60 seconds. The equator is assigned a latitude of 0 degrees. Latitude increases north and south of the equator reaching 90 degrees N at the North Pole and 90 degrees S at the South Pole. Lines of longitude (also called meridians) run north-south and converge toward the North Pole and South Pole. By convention, the 0-degree longitude line (the prime meridian) runs through Greenwich, England. Longitude is measured in degrees west and east of the prime meridian to 180 degrees. (Again, 1 degree of longitude = 60 minutes and 1 minute = 60 seconds.)
With the development of computers and the widespread use of global positioning system (GPS) technology, geographic coordinates are expressed often in terms of degrees and decimal equivalents. Thus, the latitude of the current Tropic of Cancer, which was 23° 26' 22", could be expressed as 23.4394°. Furthermore, in many computer-generated tabulations, the designation of N and S are dropped, with latitudes of locations in the Northern Hemisphere expressed with positive (+) numbers, while Southern Hemisphere locations have negative (–) latitude values. Longitude is sometimes counted in such a way that locations in the Eastern Hemisphere have a positive value, while Western Hemisphere longitudes are negative.
On land (at least in the U.S. and some other English-speaking nations) we commonly express distances in statute miles or simply miles where 1 mile = 5280 ft = 1609.35 m. In ocean navigation, however, distance is given in nautical miles. One nautical mile is equivalent to 1 minute change in latitude at the equator = 1852 m = 6076.103 ft. At 45 degrees N or S, one degree of latitude = 59.96 nautical miles = 69.05 statute miles = 111.1 km. (The slight difference between the distance equivalents of one degree of latitude between the equator and 45 degrees N or S is caused by the oblate spheroidal shape of Earth.) One knot is a speed of one nautical mile per hour and is equivalent to 1.1516 statute miles per hour or 0.515 meters per second. Knots are commonly used as a measure of ocean current velocity, winds at sea, as well as for the speed of ships.
Prior to today's satellite-based navigation (global positioning system), mariners relied on the sun's position in the sky and other astronomical fixes to determine their latitude in the open ocean. As discussed in more detail in the first Essay of Chapter 5 in your textbook, accurate time keeping was key to determining longitude at sea.
NOAA's National Hurricane Center has an interactive "Latitude/Longitude Distance Calculator" at http://www.nhc.noaa.gov/gccalc.shtml that allows you to determine the distance between two points in nautical miles, statute miles or kilometers once you enter the latitude and longitude values of the two locations. Latitudes and longitudes may be entered in any of three different formats, decimal degrees (DD.DD), degrees and decimal minutes (DD:MM.MM) or degrees, minutes, and decimal seconds (DD:MM:SS.SS).
Civil time zones were instituted in the U.S. and Canada in November 1883 to standardize time keeping. Prior to this, time was based on local sun time. The concept of international time zones was officially adopted in November 1884 at the International Meridian Conference in Washington, DC. Because collection and exchange of geophysical (including oceanographic) data are of international concern, use of a single worldwide time system was needed so that all observers around the world could take measurements simultaneously providing, for example, a "snapshot" of weather conditions at sea. By convention, the international system of time keeping is based on the time at the prime meridian and designated Greenwich Mean Time (GMT). This time system is based on the daily rotation of the Earth with respect to a "mean sun." Often, the single letter "Z" (phonetically pronounced "Zulu") is used because this letter identifies the Greenwich time zone. Currently, the approved practice is to use the more precise Coordinated Universal Time or Temps Universel Coordinné system (UTC), which is based on an atomic clock and time reckoned according to the stars (known as mean sidereal time). For practical purposes, GMT and UTC are equivalent.
Earth rotates on its axis with respect to the sun once every 24 hours. Hence, we should have 24 civil time zones of equal width. The 360 degrees of rotation divided by 24 give 15 degrees of width to each time zone. The central meridian of the time zone is then defined as a longitude evenly divisible by 15. If you were located in the U.S. Central Time zone where Central Standard Time (CST) is observed, you would be near 90 degrees W longitude (6 times 15). At any place within this zone, the time would be 6 hours different from the time at Greenwich, England. Earth rotates eastward so that Greenwich is ahead of CST by 6 hours. For example, when it is noon at Greenwich (1200 UTC), it is 6 a.m. in the Central Time Zone. To reduce confusion, all times should be expressed in the 24-hour format, so that 8:45 a.m. corresponds to 0845 and 1:15 p.m. corresponds to 1315. Modifications of the boundaries between time zones were made to accommodate political boundaries of some nations. In fact, some countries adhere to a local civil time that may differ by one half hour from that of the central meridian.
While most of the United States observes Daylight Saving Time during the summer (second Sunday in March to the first Sunday in November), UTC remains fixed and does not adhere to a "summer schedule." Therefore, you will have to adjust the time by one hour for summer. As an example, during the summer, residents of the U.S. Eastern Time Zone lag Greenwich time by only 4 hours, with 0800 EDT=0700 EST=1200 UTC.
Suppose that you would like to know the current time as maintained by the Master Clock at the U.S. Naval Observatory in Washington, DC. Get your clocks or watches ready and then access the current time from the Time Service Department at http://www.usno.navy.mil/USNO/time. The site http://www.time.gov/ also provides an accurate time check.
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Prepared by Edward J. Hopkins, Ph.D., email firstname.lastname@example.org
© Copyright, 2011, The American Meteorological Society.