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Compass and its directions

  1. Jul 5, 2010 #1
    The linked picture shows a compass rose showing the four cardinal directions, the four ordinal directions, plus eight further divisions.


    a compass point that is two points east of due south : S22°30′E
    [M-W Col. Dic.]

    I couldn't understand the 'two points' part. Please help me with it.

    I also didn't understand the direction designated by S22°30′E. A compass is divided into 360 degrees. Does it mean 22 degrees and 30 minutes in the direction of east from due south? It would be kind of you if you can help me with it.

    Thank you, in advance.
  2. jcsd
  3. Jul 5, 2010 #2
    In the old days the compass was divided into 32 points of 11 1/4 degrees.
  4. Jul 5, 2010 #3
    Thanks, AC.

    Here is a 32-points compass:

    Does it mean by two points M-W meant first point 'S' and the second point 'S by E'? This makes SSE two points away of due S.

    Is S22°30′E the same as 22°30′ SE? If it is, would the degrees be calculated from due S?

    Please help me with the above questions.
  5. Jul 6, 2010 #4
    The table you linked to has
    That is, 22.50° (or two points = 2 x 11.25°) east of due south (180°). The first letter in the bearing is either due north or due south.

    The "bearing" you quoted 22°30' SE is ambiguous. Is it 22°30' east of SE or south of SE? That's why the bearing is built the way it is.
  6. Jul 6, 2010 #5
    Where did this quote come from? I've never seen "S22°30′E" before, and I was a (admitedly Air Force) navigator, but it does seem it would follow the pattern "south by southeast" not "22°30′ SE" or even 22°30′ south of east. So the most likely interpretation is 157.5° as in the preceding post by dulrich.
  7. Jul 6, 2010 #6
    Thanks a lot, Dulrich.

    Do you mean to say that the bearing of due west is quoted as N90°W, or S90°W?
  8. Jul 6, 2010 #7
  9. Jul 7, 2010 #8
    So in the early days of navigation we have the 32 points, which probably means the helmsman could only hold a heading plus/minus 5 degrees. At first I thought this would lead to a problem with celestial fixes; but the astrolabe only does altitude of the object anyway, which still provides a position fix.

    You do need something more accurate for surveying (thanks, dulrich). You would have a fixed card with 360 ticks, probably without numbers assigned and a north-south needle. Easiest way to read this is the number of degrees from the nearest head/tail of the needle. First thing you write down is whether it's the north end or the south end, then you read off the number of degrees east or west of that.
  10. Jul 11, 2010 #9
    Hi AC

    The quote come from M-W's Collegiate Dictionary as I mentioned in the start of this thread.

    I don't know the reason why you haven't ever seen "S22°30′E" before. Perhaps, you are making a point that the bearing SSE is not written this way as far as you can tell, or you could be concerned about the format "S...E". If it's the former, the dictionary is simply defining, explaining where that point is located on the compass. If the latter, it's my general view that bearing is mostly written this way - e.g. check this link quoted by Dulrich: http://academic.brooklyn.cuny.edu/geology/leveson/core/linksa/comp.html

    Thank you for all the help. Please correct me if I'm wrong.
  11. Jul 11, 2010 #10
    Other than surveying (I haven't located my book on that, a passing fancy years ago), I do not believe "S22°30′E" is used anywhere in the U.S. That would include land navigation, which I had a course in during survival training. The Army likes to use a grid system for coordinates for coordinates, but definitely uses 360 degree azimuth for bearing.

    I still have an Army "lensamatic" hand compass. It has a "floating" card, which is to say the whole thing turns; it doesn't have a needle. It is marked with N-E-S-W and 360-degree azimuth; "S22°30′E" would be awkward to read requiring subtraction and thereby inducing errors.

    I do not know how surveying transits are marked. Surveyors do tend to hold on to old traditions.
    Last edited: Jul 12, 2010
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