NAVIGATION

Most of us are familiar with some form of navigation. If we drive, in fact we are navigating over a system of roads; essentially a grid pattern superimposed on a flat surface. Navigation is of the utmost importance in oceanography, for everything from getting the ship to the study area, to accurate (and precise) positioning of a transect in the study area. In the absence of roads ocean navigation necessarily makes use of latitude and longitude; essentially a grid pattern arranged to fit over the sphere of the earth. Lines of latitude run east-west, while lines of longitude run north-south, but that is not their only difference:

Latitude

• starts at zero degrees at the equator
• goes to 90 degrees at each pole
• is determined by measuring angle between lines with points at the surface of the earth at the equator, at the center of the earth, and at the surface of the earth at the given line of latitude.
• lines produce circles about the earth at evenly spaced intervals, and the circles get smaller toward the poles

Longitude

• lines pass through the poles and converge at the poles
• therefore they are all great circles
• What is a great circle?
  • It is a circle about the circumference of a sphere which:
  • if prescribing a plane, cuts the sphere exactly in half.
  • prescribes the a line which, when traveled, is the shortest distance between two points over the surface of a sphere.
• How is longitude determined?
  • It is counted degrees east or west of the prime meridian (That line of longitude which passes through Greenwich, England)
  • therefore, maximum longitude is180 degrees angular distance from prime meridian.
  • Is determined in fashion similar to latitude.

For convenient measurement of distance, mariners developed the nautical mile (nm), to fit measures of latitude and longitude:

• one degree of latitude = 60 nm, therefore one minute of latitude = 1nm everywhere on earth
• one degree of longitude = 60 nm only at the equator. Why?
  • because lines of longitude converge
• how do we adjust for that?
  • trigonometry: 60nm*cos(latitude) gives the distance for one degree of longitude at any latitude. Examples: 60nm*cos (0) = 60, the equator. 60nm*cos(90) = 0, the poles.
  • mercator charts: navigation charts which represent small surfaces of the earth as flat areas.

Successful navigation involves careful attention to vectors, quantities which have both magnitude and direction. The value "a 10kt current" is not a vector, however "a 10kt current from the northeast" certainly is a vector. Vectors are used to contend with the multiple of forces acting on a vessel at sea. Vectors for wind, current, and ship speed can be expressed on a nautical chart as arrows with length (magnitude) and direction and can be added end to end to resolve the course of a ship.

To determine direction, navigators use the magnetic compass, the arrow of which always points north, magnetic north that is. In doing so however, one must realize that the magnetic north pole and the geographic north pole are not in the same place. In fact, magnetic north lies right now at about 70 degrees north latitude and 100 degrees west longitude. This brings up the concept of magnetic declination, the angle between compass north, and true north.

The relationship between the geographic north pole, the magnetic north pole, and declination at any specific point on the earth's surface is very complicated. One reason is that the magnetic north pole is not stationary. This indicated on charts on the compass rose which shows the annual change in declination. However, this annual change is projected from change that occurred in previous years and, thus, is only a prediction of what is likely to happen. Such predictions are useful for the near future, say to five years, but if the rate at which the magnetic pole moves changes, that is if it speeds up, or slows down, or changes direction, then the predictions made from the record of past motion becomes progressively more inaccurate.

The second source of error, and an even more difficult problem, is that the earth's magnetic field is not a perfect dipole. It has irregularities that show up as bends in the lines of equal declination. One manifestation of this is that the line of zero declination should be on the precise longitude of the magnetic north pole, i.e., 100 degrees West longitude, or just a short distance west of San Antonio, Texas. In fact, the line of zero declination is in the eastern Gulf of Mexico, closer to Panama City, Florida.

Because the position of the magnetic poles is constantly changing and because the rate of change and its direction are not entirely predictible, the compass roses on navigation charts can become obsolete in a very short time.

Last Updated: 2 February, 1998