Can someone please explain the difference between the following phrases when expressing direction?
a) Northwest
b) North by Northwest
c) North of Northwest
d) North-Northwest
At first, one might think these are simply different ways of expressing the same direction. But, they appeared in a list describing the location of the “population center” of the US since 1790.
{I WAG that these might have specific meanings if degrees were specified, such as 3 degrees north of northwest. Using a pilot’s convention that 0 degrees is North, and West is typically accepted as 270 degrees; my example would translate to 273 degrees on a compass?}
“They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV
But Northwest would be halfway between North and West, so 3 degrees north of northwest is 318 degrees.
Without referring to any “official” list, here’s how I interpret those directions:
a) Northwest = 315 degrees (halfway between N and W)
b) North by Northwest = I always thought this was the same as d), but now I’m not sure.
c) North of Northwest = 315-337.5, exclusive
d) North-Northwest = 337.5 (halfway between N and NW)
It’s really more of a nautical experience than a boy scout adventure.
Back before everyone went to 360° numeric notation, ships compasses were laid out over a card with 32 points on it.
N E W S = 4 points
then the points halfway between N E S W, NE SE SW NW, give four more points
halfway between each of those eight points are NNE ENE ESE SSE SSW WSW WNW NNW giving us a total of 16 points
Those points were, in turn, divided to give up 32 points on the compass. The last 16 points are:
(between N and NNE) North by East
(between NNE and NE) Northeast by North
(between NE and ENE) Northeast by East
(between ENE and E) East by North
(and I’m not about to type out all of them)
So, for one quarter of the compass, the directions are:
North,
North by East,
North Northeast,
Northeast by North,
Northeast,
Northeast by North,
East Northeast,
East by North,
East
Of course, 32 points don’t give you really close sailing instructions, so each point is further divided into quarters, giving 128 points or quarter points on a compass, a quarter point being a little less than 3° on a 360° compass.
Just to chime in with the Boy Scout viewpoint – we never got past the 16 points, and then only for general information. In general, degrees ruled. The military also uses “mils,” of which there are 6400 in a complete circle.
Sure, I’m all for moderation – as long as it’s not excessive.
In the old days of sail, one of the first things sailors had to learn was to recite in order all the points of the compass and that was called “boxing the compass”
Also remember the compass does not point to the true geographic North, or even necessarily to magnetic north.
The difference between true and magnetic north is called variation and changes with place and time.
The difference between compass and magnetic is called deviation and is specific to ship and compass. most of it can be corrected with small magnets (flinders bars) but the remainder must be accounted for and is variable with heading. these corrections are labeled E or W depending on direction.
When I learnt how to go from one to the other I learnt a mnemotechnic aide:
There was a similar phrase to go in the other direction but I cannot recall it now
For a good discussion of these things check out Bowditch, American Practical Navigator volume I, published by the Defense Mapping Agency.
Another last point: the earth’s magnetic field is more horizontal at the equatorial regions but gets progressively more vertical nearer to the poles and a compass will become almost useless near the magnetic poles. Boat’s compasses are made for different regions as a compass that is made for tropical regions will not work so well in very northern latitudes and vice versa.
To fill out the already complete information tomndebb and everyone else has provided, the following is how quarter points go:
north
north 1/4 east
north 1/2 east
north 3/4 east
north by east
north northeast 3/4 north
north northeast 1/2 north
north northeast 1/4 north
north northeast
north northeast 1/4 east
north northeast 1/2 east
north northeast 3/4 east
northeast by north
northeast 3/4 north
northeast 1/2 north
northeast 1/4 north
northeast
As an example, here in Dallas, Texas, the compass points six degrees to the East of true North, but the north magnetic pole is actually two degrees West of true North.
That two degrees is neither variation nor deviation, right? It’s just ignored, as the compass magnetic north is a local magnetic north, rather than the real magnetic “north” pole.
There is such a thing as local magnetic fields. I don’t know what causes them (iron deposits?) but you are warned about them in any navigation class you might take.
Compasses are very imprecise instuments for navigation. When using a compass alone to plot a course, it is known as “dead reckoning” which is about half a step above “guessing”. I use dead reckoning for short trips to Catalina or the Channel Islands (Hell, half the time you can see them from the coast). No matter how well you keep the needle on your desired course, you never end up exactly where you predict. Most of this is due to things like current, but the overall inaccuracy of the compass, also plays a role.
The difference between G.N. (geological North) and M.N. (magnetic North) is called declination. At the bottom of USGS maps there’s a little declination diagram valid for the center of the map. I’m pretty sure that “magnetic North” refers to where your compass points when you’re standing there, taking into account local weirdness as well as the Earth’s magnetic poles.
It’s like InutilisVisEst says, there is local weirdness. It’s not just magnetic mountains, sometimes the differences in the magnetic field are embedded deep in the earth. The Earth is approximately like a big bar magnet, but not quite. Notice what CurtC says, the compass in Dallas points east of north, when the actual ‘north’ magnetic pole (actually a south pole, heh) is west of north.
FWIW, I just pulled out the L.A. sectional chart. The isogonic line nearest to where I live shows 14 degrees of deviation between magnetic north and true north. The Seattle sectional shows local magnetic distrbances in certain areas. Engines, radios, etc. also affect the compass.
“I must leave this planet, if only for an hour.” – Antoine de St. Exupéry
