A couple of comments on statements earlier in this thread:
The original Line of Position (LOP) seems to have been derived from a dawn sighting. I presume, but do not know as a matter of fact, that the tables used for celestial navigation would have given results in true rather than magnetic terms. Drawing the line that indicates one's location at dawn isn't at all dependent on compass readings but a trigonometric calculation based on visual information (seeing the sun break the horizon), altitude, and time. The line comes as the answer to the question, "Where in the world would you have to be to see the sun come over the horizon at that time of day?"
Marty is correct. Celestial navigation is a matter of mathematics, not earth science, so all directions derived from it are True (meaning referenced to the North Pole). The art of celestial navigation is based on spherical trigonometry and the relative motion of the earth and other heavenly bodies, not on the wandering location of our Magnetic North Pole. Latitude and longitude are measured with reference to the True North and South Poles, the Equator and an arbitrary line through the Royal Observatory in Greenwich.
Good point. Some definition clarifications:
Heading = The direction the plane's nose is pointing. This can obviously be very different from the plane's actual direction of travel, especially with wind correction angles. Pilots call this phenomenon 'crabbing'.
Bearing = The direction the plane is actually traveling across the ground (or in this case water). Sometimes referred to as track or course.
* when speaking of these two, it is extremely important to differentiate between TRUE and MAGNETIC *
Magnetic Variation = This is the angle between TRUE north and MAGNETIC north. Sometimes also called magnetic declination. This number has an infinite number of possibilities depending upon the earth's magnetic field and your geographic location.
Magnetic Deviation = Similar to variation, this is the correction angle to account for localized magnetic disturbances of the metal in the airplane itself. This number is usually a fixed number and does not change as long as you are always in the same airplane.
Personally I like the term track to describe where someone or something has actually been. Bearing can mean a couple of things in navigation; usually the direction of a sighting of something besides whatever you're in or on. You can have bearings on different things at the same time, a lighthouse, a radio beacon and a point of land. Three-arm protractors have three arms for a reason. Plus they're fun to play with. More fun than "Bearing constant, range decreasing!", which is navspeak for a collision course.
For my money, 337-157 is pair of reciprocal directions. Nothing more, until you give me a point that it passes through. Now it is a LOP. If you're on it and travelling SSE, now it is a heading of 157 and the 337 is your six o'clock . Hopefully there is some land ahead.
Magnetic North is somewhere in Canada at the moment and travelling Northwest. Magnetic compasses point to it wherever it happens to be, so you have to account for the difference in polar locations in order to get from a celestial observation or other True direction, to a magnetic one. At any particular location, this difference is an angle and it is called Variation, and it changes depending on where you are in the world. Variation is marked on a marine or aeronautical chart by a compass rose, with True North marked on the outer ring and Magnetic on an inner. The variation is listed inside the rose (at least on a NOAA chart) along with the year it was determined and the rate and direction of annual change. So, to get from True to Magnetic, you need to know roughly where you are (Charleston, SC 32 47' N 079 55' W) and the year (2011). The harbor chart 11524 shows variation of 7 deg 15' W in 2008 with an annual increase of 3'. 3' per year for the three years since 2008 gives me a 2011 variation of 7 deg 15' W plus (3 x 3) equals 7 deg 24' W.
So if I take a magnetic compass and sight across the center and 007 degrees 24 minutes, I will be looking at True North.
Unless there's some iron nearby. Cuz that can mess up a compass. So now we need to account for local conditions. Metallic aircraft or boat structure or wires carrying electric current can deflect a compass by interfering with the earth's magnetic field. Once a compass is installed in a ship or aircraft, you can only account for the local (as-installed) weirdness by preparing a
Deviation Card.
Deviation can and will change depending on your heading, so a card will show deviations at a number of directions all around the compass. Most recreational marine compasses have adjustments that allow you to limit deviation with small magnets; big units for ships have large iron balls that are used to compensate for local effects. Either way, unless deviation is zero all around, the only way to accurately account for it is to develop, maintain and use a deviation card.
This is done by "swinging the compass" - actually turning the boat, ship or plane in known directions and reading the compass indications. Take the readings in each direction, compute the errors and appropriate corrections and retain for future use on a deviation card.
Not that I think it had anything to do with the loss, but it's a common mistake to think that if the compass is off by plus 5 degrees heading east, it will also be off by plus five heading west. If you have a tool or wire to the right of the compass, it might make the compass magnet at its north pole deflect to the right (east) when you're heading north, resulting in a low compass reading , but pull it to the right (west) when you are heading south, leading to a high reading .
Check the DC illumination wires on a compass, they are probably twisted together to try and cancel out magnetic effects. The same for other series circuit wiring in a well done binnacle.
Now for the mnemonic:
True
Virgins
Make
Dull
Company
So,
Add
Whiskey
True plus Variation = Magnetic, Magnetic plus Deviation = Compass. Westerly corrections are added, so that's the last part. Works in reverse, too, for when you/re trying to get from the real world to a nautical almanac.
We tend to ignore stuff like this in the days of satellite and other electronic navigation because our guidance systems either are not subject to them or do the compensation for us, but in 1937, this was find it or don't type stuff. Maybe even life or death type stuff and FN took it seriously. They swung the compasses in the Miami hanger before they left on the last test flight and rechecked them once in the air over ranges familiar to FN. He may have been willing to bet his life that the RDF would work, but he wasn't missing any bets on being able to do his own DR. Remember, he'd made numerous RDF departures and approaches, so he would know just how lost they would be without it.