Degrees, Minutes, Seconds vs Decimal Degrees

[StuM]

Originally Posted by chrisr

i have just - belatedly - worked out what rob_p was talking about (converting true to compass ?)

we, being young gentlemen officers, did not know about such things as whisky and virgins and were simply taught "error east, compass least. error west, compass best"

after a few floggings it stuck - but no doubt whiskey & virgins works too

cheers,

After 50 odd years of assorted military, flying and boating experience using chart and compass, ( see I can do it too ) I still don't get the point of all this whisky/virgin cr*p.

It's far better to actually understand the concept of what you are doing rather than rely on some stupid rhymes or mnemonics. Just visualise the compass rose you see on every chart and think about the numeric values of the two North pointers..

If the variation is East (positive), the magnetic needle is rotated to the East (right/clockwise) of vertical so magnetic bearings are smaller ( e.g. the 0°M lines up with say 10°T). Conversely, if it's West (negative), magnetic bearings are larger. (Mod 360 )

[Jim Cate]

True Virgins Make Dull Companions

Ping, you forgot the add-on "Add Whisky"!

This is old school stuff, for all the young punks now use Rohypnol for that purpose, but it doesn't scan very well in the mnemonic.

Jim

[Cyrus Safdari]

Originally Posted by CaptTom

Base 10 really doesn't have a lot going for it, except for the number of fingers we humans have.

Actually base 12 (and 60) was used by Sumerians and Babylonians etc presumably because we have 12 finger knuckles on each hand (using the thumb only to count them) this is still common around Asia
12 is also useful because it is divisible by odd and even numbers. 60 has even more factors

[SeanPatrick]

Originally Posted by Martkimwat

Then as I said, his terminology is wrong, ie ambiguous, he should have stated
"to change 37.3 decimal minutes to decimal degrees". I thought he was trying to explain how to convert decimal minutes into minutes and seconds, is why I could not make sense of it. Also might have helped in the example, to use another unit of numbers that did not coincidently end up so close to the same numbers in the subsequent calculated answer.

Apologies. I was pretty drunk when I wrote that. The fact that it makes any sense at all is pretty amazing.


And "Thank you!" to all of you who helped explain what I meant.

Originally Posted by Martkimwat

(as an aside, have never seen a full stop used to denote the multiplication sign, normally use x or *, where does that come from? again ambiguous because I thought he meant 6.6 as in 6 decimal point 6)

I [obviously] don't know how your computer (or Mac, tablet or whatever) displays special characters, but the symbol I used is not a full stop. It is a "multiplication dot". (See here, here and here.) The difference being that a full stop is at the base of a line of text and the multiplication dot is slightly elevated to the middle of the line. This was used almost exclusively in textbooks when I was in school and, AFAIK, is a well-known symbol. Honestly, I have trouble believing that you have never encountered it. But then, I also don't know anything about you.



But, thank you for reading my posts, considering them and offering some thoughtful, sincere feedback. Cheers!


PS


I am no scholar. I barely passed my high school geometry class and I never went to college. So, I am sorry if my terminology is wrong and my posts are confusing. I'm doing the best I can. But I believe that, having made my own perpetual almanac, accurate from the years -500 to 2030 and which calculates accurate fixes from two or more celestial sights and also plans great circle routes between any two places on Earth ... I'm doing okay. But that's just my opinion. And you know what they say about opinions ...

[RaymondR]

Originally Posted by ranger42c

???

-Chris

I'm confused now myself.

I think what I meant to say was "all marine areas are required to use graticular coordinates" which I think is required by the Law of the Sea Convention.

[Benz]

I'm surprised no one has mentioned the H.0. 229 and 249 navigation tables, which are why I use Degrees/Minutes/tenths of minutes in plotting. The currently available nautical almanac uses that notation as well, so it's easy to stay with it.
I would gladly use Deg/Min/Sec if the ephemeris and sight reduction tables used them, but you have to change with the times.

[olewsaa]

Units on a sphere or even circle is not a trivial task, well it is for the mathematicians whoi uses radians, where a full circle is 2 times Pi. Not so easy for very day life. Since acient times the circle was divided up into 360 degrees, then using minutes and seconds to divide each degree, that came from the clock, also old with 12 times 2 hours.

Countless attempts to go all metric have been proposed, like 400 degree cirle, metric clock and metric calendar. We still use 360 degrees, but subdegree units are harder. For computing using floating point numbers make perfect sense, 8 or 16 desimal digits (32 or 64 bit floats).
For paper chart work one NM is 1/60th of a degree or one minute. One minute is then divided onto cables (1/10 of a mile. The actual lenght of a cable is debated, bit close, but pay attention). This make mins and decimal minutes very practical for paper chart work. For programming charts and computer work like google floating point number make more sense.

Nowhere is there any reference to any lenght in length units, this does not really apply to lenght when we are living on a sphere. A flat surface is a special case, we live on a sphere and it's not even perfect.

I suspect that in the future with even more apps and mobiles it will all be floating point numbers with decimal degrees.

[dbraymer]

So I have a very good chart plotting system, by nobeltec, visual mariner descendent, and I am sailing along the coast of Roatan, and somehow one freaking tile of the chart is missing. so I have sailed into a "white hole" on the chart plotter. I know that I am pretty close to a harbor, at a yacht club, but it is not anywhere upon the display data.

So I do have a cell phome, and we are within a mile or two of the shore, so I call the wife in Michigan and ask her to go on the internet and look up this coastal resort. She reads me some numbers and I plot it out on an ancient paper chart, and the course is "take a right out into the ocean, and sail out of the sight of land"

Took me quite a bit of time to realize that the internet advertisement was in modern decimal, whereas I am trained and I prefer the older methods, because of the relationship between nautical miles and minutes, at least latitude wise...

So in the case that someone is referring you to a location, it can and did become confusing, and it could have cost time and money, so be sure about the reference that you are receiving.

[The Sun King]

I was taught Degrees, Minutes, Seconds for pointing airplanes, but largely followed railroad tracks and eyeballed water tower names! Now my phone GPS reads decimal degrees. My archaeology buddies like to confuse me more by insisting that Universal Transverse Mercator (UTM) is the only accurate means of locating things.



I learned to translate Metric and American units of measure in my head in elementary school and have been learning to translate various spatial measuring schemes as well. Sometimes, though, if in sight of land, it helps to just look out the window for landmarks.

[sailorladd]

Report it in degrees as you would work it on the chart...

[KJThomas]

Originally Posted by chrisr

FIFY

believe the length of a minute of latitude varies slightly...more towards the poles...less towards the equator, because the earth is not a perfect sphere.

1852m / 6080ft is a handy average

cheers,

1852m is the length of a minute of Latitude at the equator. With a few decimals added if you want to be pedantic.

That is, the definition!

On Mercator charts, which we all use unless we are near the poles, where the latitude scale would have to be near infinity, the longitude scale is adjusted, so the latitude scale divisions still equal 1852m. The datum used, such as WGS 84, is to adjust for the fact the earth is not a perfect sphere. There may be minor differences between the charts scale and actual distance on the earths surface due to imperfect datums, but the Cartographers intention is that the latitude scale on a Mercator chart corresponds exactly to the real distances on the chart, at that latitude.

[StuM]

Originally Posted by KJThomas

1852m is the length of a minute of Latitude at the equator. With a few decimals added if you want to be pedantic.

That is, the definition!

No, 1852m is the SI definition of a Nautical Mile. It's an "average" value of a minute of latitude. (at approximately +/-45° ).There is no "definition of the length of a minute of latitude". A minute of latitude is what it is and its length varies with distance from the equator. A minute of latitude at the equator is actually 1843m (to the nearest whole number).

[chrisr]

to make it even more complicated, a UK nautical mile is 6080 feet (about 1853.182m)

fortunately for all practical purposes it doesn't matter - but interesting none the less

meanwhile : this variability of the length of a minute of latitude, depending upon what latitude you are in, is the reason why you should always use the latitude scale nearest your position when measuring distances

cheers,

[StuM]

Originally Posted by chrisr

meanwhile : this variability of the length of a minute of latitude, depending upon what latitude you are in, is the reason why you should always use the latitude scale nearest your position when measuring distances

cheers,

With a variation of around 20 centimetres (8 inches) over 60NM (1° of latltude) , I sincerely doubt that you can measure accurately enough for the difference between the spacing of the most northerly and most southerly lines of latitude on your chart to be detectable

[NPCampbell]

Originally Posted by StuM

No, 1852m is the SI definition of a Nautical Mile. It's an "average" value of a minute of latitude. (at approximately +/-45° ).There is no "definition of the length of a minute of latitude". A minute of latitude is what it is and its length varies with distance from the equator. A minute of latitude at the equator is actually 1843m (to the nearest whole number).

What is the definition of the power of one horse? One horsepower? Or 745.7 watts?

The definition of "the (arc) length of a minute of latitude" is a nautical mile. Just because we may be referring to the SI standard of nautical miles (sometimes) doesn't change the original definition that stood for hundreds of years. Every 3rd blue moon when you pull out your paper chart and plot a course you measure distance using dividers and the latitude scales ... because a minute of latitude is a nautical mile.

I find it amazing that the term was contrived 500 years ago and that the accuracy of a minute of latitude equals 1 SI nm to an accuracy of +/- .005 SI nm at any latitude.

I guess you could generalize this and say that the arc length of an arcminute anywhere on Earth measured in any direction is basically a nautical mile.