Lessons Index:
6. CHART MAKING FOR NAVIGATORS
Osher Map Library
University of Southern Maine
Charting Neptune's Realm:
From Classical Mythology to Satellite Imagery
An exhibition at the Osher Map Library and Smith Center for Cartographic Education, University of Southern Maine, Portland, 4 April 2000 to 11 January 2001
Donald S. Johnson, guest curator
COMPASS DEVIATION
Osher Map Library Lesson
Charting Neptune's Realm
Peter Rice, Andy Alley
B. Background Information
Sailors headed to New France discovered that as they sailed west from Bordeaux (45° north latitude) toward Cape Breton Island (45° north latitude), they could not 'sail their westing' and reach the intended destination. They ended up somewhere on the coast of New England, well south of Cape Breton. Their compass had given them a false course.
Early mariners had assumed that the compass (magnetic) north and true north were the same. On short voyages in the Mediterranean and around the coast of Africa, this did not make too much of a difference. It was only when they began to cross the uncharted Atlantic Ocean that the differences became acute. Not only were the two directions disharmonious, the difference between true and magnetic north changed all the time.
As an example, a street in London laid out in 1580 on a (magnetic)
north-south line would, measured again in 1812, find that the
northern terminus was seven tenths of a mile off; a difference
of thirty-seven hundred feet in two hundred and thirty-two years.
For the mariner, the problem was acute. In Atlantic storms that
could rage for days or even weeks, the compass was the only navigational
aide available. No sun or star sights could be taken to check
either latitude or longitude. By the time the storm broke, the
ship could be a thousand miles off course. By the time the navigator
had a chance to take a latitude sighting, he would not even know
whether to sail east or west to strike the tiny island that was
his destination. All he could do was to make an educated guess
as to where he was and then plot a course that gave the best
chance of success. After 1761 the captain was able to determine
the longitude (See 'Longitude,
Osher Map Library Lesson 16') using his Harrison chronometer.
Magnetic north is located at approximately 77° north latitude.
It is caused by the molten metal core of the earth as well as
by rotation of the earth around that core. This sets up an electric
charge that establishes the magnetic north pole. The longitude
of the pole varies due to that rotation.
A rudimentary experiment can be conducted in the classroom
that will demonstrate the problem. This is especially true if
the room has a tile floor with the lines perpendicular to the
front of the classroom wall; these are the lines of true north.
Mark a point on the front wall, near the center of the wall,
that will represent magnetic north.
Students can then plot the difference between True North (TN) and Magnetic North (MN) for representative 'latitudes' and 'longitudes' as marked on the floor of the classroom. They will see that the magnetic variation is different for each position.
If the students can plot enough of the points on the floor, they should be able to determine the un-plotted points for compass variation. With these points, they should be able to develop a variation table from which the variation for any point in the room can be developed. This is exactly how chart-makers develop the variation for their charts and maps.
In reality, this does not work, and the early mapmakers found this out. The reason it does not work is two-fold. First, the location of the magnetic pole is not fixed. As was stated in the second and third paragraphs above, the location of the pole changes. Why this is true has not been determined, but we do know that the magnetic north pole is moving west at about three hundred yards each year. The closer to the (true) north pole, the greater the calculated, yearly drift. This drift is called declination, and every modern chart has a declination diagram printed on in. The diagram shows true north, grid (map) north, magnetic north, and the declination (usually yearly) of magnetic north to he east or west.
The second reason why variation is difficult to determine is that the earth is spotted with other magnetic sources (anomalies). Any source of iron ore, such as the Mesabi Range in Minnesota or near Chelyabinsk in Russia, will cause local variation. The presence of copper deposits such as those in Connecticut and Arizona will also affect the compass. Thus the mapmaker is faced with the task of measuring as many points as possible over a significant period of time to produce accurate declination diagrams for maps. Added to the second reason is the fact that iron things surround us. Those little compasses that sit on the dashboards of our cars are next to useless. The car itself has a magnetic field, as does the engine. When we turn the car on, the electric field produced by the ignition system adds another field, and when the transmission begins to turn, another variable is introduced. This variable, produced by the vehicle itself, is called deviation.
The gyrocompass, using the physics of a spinning top, is not affected by magnetism. This compass will give true north at all times. The development of the gyrocompass has allowed us to navigate over long distances with great speed. Without the gyrocompass it would be impossible for an airplane to fly across the Atlantic; the variation would be changing so rapidly that the navigator would never be able to tell the pilot which way to fly.
On ships, the rapidity of the change in variation due to travel is much less rapid, but it does occur. When plotting a course and correcting the compass, the rules of navigation state that the navigator should plot his course on the chart using the nearest compass rose. (See United States Coast and Geodetic Survey Chart of Casco Bay Maine) He uses the nearest compass rose because that is the one that will give the greatest accuracy for local variation. The deviation produced by the ship had been calculated prior to departure from the port by having the ship swung through a full circle with the changes noted on a deviation card that was posted in the chart room.
The computation for determining true north from compass north is memorized by navigators as CDMVT; AE, or 'Can Dead Men Vote Twice; Add East.' (The answer to the question on dead men voting in Chicago under Mayor Daley in the 1960's was 'yes.' His motto was; 'Vote early; vote often.') CDMVT translated was: Compass, Deviation, Magnetic, Variation, True.
As an example, using the chart of Casco Bay, a ship with a compass heading of 90° (due east) could have a deviation of 5° east, and at its current position (latitude/longitude) have a variation of 14° west. The variation is calculated by adding or subtracting as appropriate the variation in declination from the stated and dated compass variation shown on the chart. To establish the true course, the navigator uses CDMVT; AE. He adds easterly difference and subtracts westerly differences:
C D M V T
90 + 5(e) = 95 - 14(w) = 81 true course
Meaning:
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Very simple.
The representation of the change (declination) of the magnetic pole can be shown in the classroom. After the students have plotted one line (latitude) for the variation, move the point on the wall designated as the magnetic pole a meter to the left (west). Have the students recalculate the difference in variation along the base line: this is the declination for a time period (year). The yearly declination is noted on the chart so that if the course were to be plotted six months after the original variation, the declination would be half the shift noted in the declination diagram. on the Casco Chart there is a two minute addition (easterly) for each year. Note that this is an 1896 chart, so by 2000 (104 years later) the shift would have been easterly 208 minutes (3 degrees 28 minutes).
Students can also calculate the predicted variation after a second period of declination. They should predict the variation for a second one-meter westward shift in the magnetic pole. They will find that the new variation is based on where in the room it is measured so that the farther away from the pole the less difference each shift makes. This can, of course, be done mathematically if the distance between the point of observation and the magnetic pole is known.
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