Know Chart, Compass and Things Magnetic, your GPS died don't be Pathetic



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Fewer and fewer people use a compass for navigation as GPS devices have become ever more popular. Relying solely on an electronic device for your safety is a mistake. You should also know how to use a compass, understand its limitations and be prepared to navigate the old fashioned way.




Navigation (Latin Navigare "to sail") is the science of moving from one place to another. It requires locating one's position relative to some other known point. It tells you where you are in relation to that point and how to get to any other point whose position relative to the known point is also known, i.e. how to get to where you want to go.

In recent years some very effective, efficient and complicated systems have been devised to easily and accurately tell you where you are and how to get from one place to another. The Global Positioning System was created by the United States Defense Department for the purposes of waging war. It is currently used much more commonly and benignly for all types of navigation from the largest of sea going tankers, to the smallest of handheld devices on the foredeck of a kayak, to a computer assisted navigation system in the dash of automobiles. The system uses several dozen low orbiting satelites. A receiver processes electronic signals and sophisticated computations triangulate the position of the GPS receiver from a minimum of three satellites. As long as a GPS has power and a couple of satellites to talk to, they are a remarkable and convenient device to show you how to get where you want to go.

A GPS is a great tool. However, like any electronic device in the middle of a salt water environment, particularly a remote and/or turbulent environment, they often fail in the most critical of times. I was on a ten day trip in the Everglades in 2004. My fellow kayaker was carrying two GPS devices which we had little real need for until we got way back in the interior of the mangrove swamp in Hell's Bay. There was a period of confusion when a GPS reading would have been very helpful. Unfortunately one of the GPS units had leaked and fried itself the day before and the last of our batteries had died that morning in the other. It was a chart and a compass that finally got us back on track. The accuracy and convenience of a GPS lull many users into a dangerous reliance soly on these devices. I have seen several examples of how a misiterpreted or imporperly set GPS can lead people off in the wrong direction. Because the GPS said so, they followed it off in a direction 45 degrees off course, in spite of what their eyes, a chart and a compass should have told them.

A modern GPS combines the functions of the two most important pieces of traditional near shore navigational equipment, the chart and the compass. Using these old reliable items you can figure out what a GPS computes for you - where you are and how to get where you want to go. So let's look at how to use them so that we are not floating forlornly when our marvelous electronic device quits on us. But first lets get some background.

At first, getting around was simply a matter a knowing a sequence of locations based on experience. Recognizable features where linked together to provide a path from one place to another - " From the squared topped mountain, go to the river, follow downstream until it forks and then head for the next mountain" and so on. It is like going to the grovery store. You aren't really navigating. You are simply repeating a series of known directions and distances confirmed by recognized landmarks.

That is basically the way early travelers proceded. But not all object were within eyesight. For objects not immediately visible, general directions were described relative to the daily rising and setting of the sun, moon and stars. This was the first navigation.

Knowledge of how to get from place to place was passed from person to person verbally or experientially. Eventually maps where made that indicated the spatial relationship of one place to another. Now you didn't have to have been there yourself, or even talk to anyone who had. It was on the map and you could figure it out yourself.


Ancient Maps


World Map, Babylon 600 B.C.


Orbis Terrarum 20 A.D. Roman Map


Any map has to be oriented (from Latin oriens "the East" ) and centered, that is something has to be at the top and something in the middle. In the early Roman and European Middle Ages many maps were drawn with the east at the top representing the direction of Jurusalem and centered on Rome. This eastern orientation tradition extended well into the 17th century (See John Smith Map 1612 Chesapeake Bay). On Chinese maps the top was to the south. Many early nautical maps where drawn facing the coast that they depicted, referencing a universal direction only in a legend. Today most maps are oriented with geographic norh at the top, but some maps like polar maps are not.

In order to figure out a direction of travel, a map must be aligned to the earth's surface, also called oriented. This can be done is the most basic of ways by observing the sun's rise and set, up in the east and down in the west, and travel across the sky during the day from east to west. ( This process can be made more accurate by compensating for the seasons.) At night ancient peoples have long used the nearly motionless star, the one we call Polaris, to navigate Since it is currently located "over" the axis of rotation of the planet, it moves very little during the night as the earth spins. It is mentioned in Assyrian stone tablets, Mayan temples and was used by Bedouins for navigation across the trackless deserts. Using this constant star visible throughout the Northern hemispehere as a reference, maps could be "oriented" so that a direction of travel to a point on the map could be determined.

If you can orient a map and know where you are, you can figure out which direction to go in order to get to a desired location on the map. If you know the directions to two points on an oriented map (i.e. directions to three different points) you can figure out where you are.



So if you know you are in Rome and know the direction to Athens (A) you can orient your map and determine the direction to sail to Cyrene (B). If you were at sea and you could see (or determine) the direction to Cyrene and Athens, then you would know that you are at the junction of the two lines on your (east) oriented map and therefore would know where you are. That is really all there is to navigating.

Frequently a chart ( nautical map) is all we really need to navigate in a kayak in benign conditions with easily identifiable landmarks. All the other tools just make it easier or more accurate to determine what direction we should follow. The directions to two identifiable landmarks that can be located on the map determines where you are. Direction to one identifiable landmark and knowing where you are will let you determine a location to any other point on the map. The accuracy of any determination depends on the accuracy of your observations.

Knowing the direction to some identified point is therefore an absolute necessity in order to be able to navigate using a chart or map. For moving around on the surface of the earth, any point on the earth would do just fine, such as the Great Pyramid of Cheops, the City of Rome, the Imperial Palance in China, or the British Royal Observatory in Greenwich England. Unless you can see that point, you need way to tell direction in order to orient your map. The indications of directions can be simple, some of the most obvious items in our world, the sun and stars. The sun rose in one place, crossed the sky, and set in another. The next day it did it again, but in a slightly different position that changed during the seasons. But the pattern repeated. It was predictable. As long as you knew the time of year, you know the direction the sun rose, how high in the sky it got at midday and where it set. At night the stars did the same thing. They rose in one direction, whirled around the sky and disappear as the sun reappeared. They all moved in predictable yearly patterns. All except one. That one stayed in the same place all night, every night, all year long. While the moon, sun and other stars all moved that one stayed in the same place all the time (at least on the time frame we are interested in). People figured out that you could use the sun, moon and stars, particularly the unmoving one, to guide your travels. With the development of geometry and astronomy in Greek and Roman times, the importance of the constant star, pole star, lodestar, polaris and all the other names given to it by many civilizations became more dominant.

But the sun and stars are only available when the weather is good, and the skies are clear. When storms came up and clouds rolled in, the direction to orient your map could not be determined. At sea out of sight of land, where everything looked like everything else, there was no way to determine if you were still headed in the same direction. Directions relative to the wind, waves or clouds all changed on the otherwise featureless sea. Without a constantly available indicator of direction, orienting yourself was possible only in good weather and at widely separated times of the day even on good days. So sailors never ventured out of sight a land, paying heavily when storms drove their ships up on the coast or "off the edge of the world."

What was needed was some way of constantly indicating a constant direction to a known point on the earth in order to orient a map. The first device was a compass. A compass is a piece of magnetic material suspended in some manner so that it aligns with the magnetic field of the earth. You can hold such a device in any horizontal direction and the indicator adjusts itself to point in the same direction. Where there is no visual evidence of a constant direction, a compass will provide one.

From written history the invention of the compass occured around 1000 A.D. in China. There is some archaeological evidence of a compass device in Olmec civilization in central Mexico over a thousand years earlier. Early chinese compasses (200 BC) where made directly from lodestone - a magnetic iron. Later 600 AD, Chinese had learned to magnetise metals by stroking them with magnetite or lodestone or heated to red hot condition and allowed to cool while aligned in the north direction. These were floated on water, placed on points or suspended by silk threads. These later configurations where much more portable and practical on a moving ship than the balanced cup of the picture below. These Chinese designs had the indicator pointing south, so Chinese maps were drawn oriented in that direction.


Early Chinese Compass


These early crude compasses allowed sailors to go out of view of the land, be at sea under stormy conditions and still get to where they wanted to go as the needle pointed in a "constant" direction. With the discovery of geometry, astrology and the fact that the world was a sphere, not flat, there developed a great interest in using the sun, moon and stars to calculate from their relative positions where on earth the observer was located.

When the true nature of our world became better known, a system of describing the location of any point on its assumed sperical surface was required. The earth was known to rotate around an axis which pointed at the unmoving star, Polaris to us. This point where the axis line intersects the earth surface is called the North Pole. On the opposite side of the big blue ball is the South Pole. The equator is the circle at the maximum radius of the sphere bisecting the axis on a plane at right angles between the two poles, the waistband of the earth so to speak.

To uniquely describe any position on the surface of a sphere of known radius, requires the intersection of two arcs. Circles are the most easily described. Lines intersecting at right angles are mostly easily used in calculations. Thus two sets of circles intersecting at right angles are used to describe a position on the earth's surface. Lines of latitide are circles on parallel planes with the equator of constant angle with the equatorial plane from the center of the earth. The range of angles is designated from 90 degress south to 90 degrees north (the angle Theta in the figure below). Their partner is a line of longitude a series of circles that intersect at the north and south poles and identified as an angle (Phi in the figure below) between the circle and an arbitrarily chosen special circle of longitude called the Prime Meridian. Many cities around the world have been used as the starting point for the calculation, a confusing and complicating system. Not until 1884 was the universal meridian chosen to run through the British Royal Observatory in Greenwich England marked by a large brass marker. As Britain ruled all of the seas and most of the world I guess there wasn't a whole lot of argument at the time. Lines of longitude run from 180 degrees East to 180 degress West of the Prime Meridian. Specification of the two angles Theta and Phi define a point on the sperical surface. The angles are reported as degrees, minutes and seconds or degrees, such as 39 degree, 14 minutes and 16 seconds North or degrees and fractions thereof 39.2433 degrees North.



This system was used to create charts of increasing accuracy and it is the basis of the system used today. Charts and maps are laid out on a two dimensional grid representation of the three dimensional grid of geodetic latitude and longitude. The rules used to project the sperical grid onto a two dimensional flat map is called a projection. The Mercator projection is the most common one in use today. It is siply the projection of the sprical grid onto a cylinder of the same diamter as the sphere attached at the equator. Think of a light at the center of the spere shining trough a translucent ball with latitude and longitude lines drawn on it. Where the shadows of the lines fall on the cylinder is where the lines of latitude and longitude a drawn on the flat cylindrical map. The difference between the Mercator grid and the sperical grid are small for small areas, but the Mercator projection seriously distorts the scale, angle and area of the flat map as it nears the poles. A one foot circumference circle center on the exact pole would be a line as long as the 24000 mile circumpherence of the equator. That is why one never estimates distances from degress of longitude on a Mercator Projection chart. Only degrees of latitude give constant distance at any location. Because of this distortion mercator projection maps make Greenland look bigger than Africa although it is one fiftenth its size. Because of this distortion this Equatorial Mercator projection map is useless for polar maps.




So now we have a chart system based on the rotational axis of the earth and we can use the sun and stars to tell us the line of latitude for our position. We have this device called a compass that aligns with the magnetic field of the earth. So it is telling us which way to the North Pole right? Well, sort of.

Well why not exactly? What are the magnetic lines of the earth anyway? What creates them?

The rotation of the magnetic molten metals in the core of the earth and their flow toward the surface because of heat generated at the core of the inner planet cause the magnetic field. We should be thankful as that magnetic field keeps us safe from the radiation showered upon us by our sun. Without our magnetic field the radiation might fry us. The interplay of the magnetic field and the ions sent out by the sun, the solar wind, interplay to give us the spectacular auroras, the northern and southern polar lights.

The magnetic field lines of the earth are nearly the same as that from a simple magnet. Have you ever seen a magnet placed in a bunch of loose iron filings?. The iron filings line up in a shape nearly the same as the earth's magnetic field as shown here



Iron filings around a magnet


Magnetic dipole schematic + north end of magnet -1 south end of magnet


Simplified Earth Magnetic field and response of a compass.
Note that the south magnetic pole is near the north geographic pole. The north end of a magnetic compass needle is attracted to the south end of the earth's south magnetic pole just as the south end of a magnet is attracted to the north end of another magnet.



Unfortunately for simplicitiy's sake, the axis of the magnetic dipole does not allign with the rotational axis of the earth, the North and South poles. The magnetic pole is not the same point as the geographic poles. Currently the magnetic axis differs from the rotational axis by about a dozen degrees. The difference in location between these two points, the North Pole and the South magnetic pole, is a different direction along the surface of the earth depending upon where you are on the earth. Given the location of the magnetic pole, it is just a geometric problem to figure out the difference between the magnetic pole and the North Pole. A simple calculation and a correction for any particular place on the globe would be all that was needed to compensate for the difference.

Not only is the magnetic pole not the same as the geographic pole, but the magnetic pole wanders around as the currents in the core of the Earth change over time. This is a slow drift and makes a difference of a few to tens of seconds each year in middle latitudes and more up closer to the poles. Over long periods of time, the magnetic poles wander great distances from the rotational pole. At even longer time periods, the magnetic field weakens and even reverses, switching the south magnetic pole for the north magnetic pole. This happens on average every 250,000 years so I wouldn't be losing a lot of sleep over this one even though we are overdue for another reversal. It has been about 750,000 years since the last one.

In addition to the problem with the difference in the two poles, the magnetic lines of force can be affected by local deflections caused by other magnets and magnetic materials. Types of magnets are other compases, electronic equipment that create magnetic fields, metals exposed to a magnet and any iron object. Place two compases next to each other. Their needles will both move from the direction they were originally pointing. Place your VHF radio next to your compass and turn it on. The needle is likely to move. Magnetic materials are iron deposits in rocks on shore and under the water, varying thicknesses of the basalt sea floor, and any significant piece of iron placed near your compass. Put a iron knife next to your compass and the reading will change.

So on every chart there is an indicator of the difference between magnetic north, the direction indicated by the compass aligning parrallel to the magnetic line of force and true north. It is called the compass rose. It shows the variation between magnetic north and true north for that location and for a given date. In addition it will list the drift, the change in that variation over a year from the given date of the variation. To get the current variation, you must multiply the number of years since the variation was published by the yearly drift and adjust the variation by adding or subtracting the drift from the stated variation to get the current variation. As the speed of variation does change with time, this is an approximation that is less and less valid as the chart gets older. That variation is then used to translate from the magnetic north as indicated by the compass and true north as indicated by the grid line on the chart.




The figure above is a piece of a compass rose. The true north rose is always the outer ring and the magnetic ring is one the inside. The variation between magnetic and true is written on the magnetic north indicator. it is 6 degrees 40 minutes to the west as of 1992. The drift is 8 minutes east each year. Since this 2009 it has been seventeen years since the variation was measured. It is probably time for a new chart, but without a new one we must multiply 17 times 8 minutes for a drift of 136 minutes, or two degrees 16 minutes east. Adjusting the variation of 6 degrees 40 minutes west by two degress 16 minutes east, the calculated variation in 4 degrees 24 minutes West. Any bearing taken from the latitude longitude grid of this chart would have to have 4 deg 24' ADDED to get the compass bearing. An easy way to remember this is the anagram POW MFer, which stands for Plus On West Map to Field.

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