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Old 01-30-2008, 02:01 PM
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Default Difference in lat/lon degrees/Min/Seconds

Someone please explain the difference in the different formats of Lat/Lon. Some are in Degrees and fractions and some are in degrees. min. seconds and there is a third of which I am not sure. Which is more accurate? The Hun
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Old 01-30-2008, 02:49 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

They're just different ways of representing the same thing. No difference in accuracy.
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Old 01-30-2008, 06:24 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Going back to old navigation, they divided up a degree into 60 minutes where each minute is divided up into 60 seconds. I think it had something to do with how they used a sextant and chronometer to determine position. (DD MM SS)


This confuses the crap out of most people which is why we prefer to use "decimal degrees" with modern navigation. Here, degrees are again the same but the decimal part ranges from 0 to .9999 (or however many decimals of precision you like).


Here's how to convert back and forth. Say 28 deg 30' 45" (28 degrees, 30 seconds, 45 minutes). Divide minutes by 60, 30/60 = 0.5. Divide seconds by 3600, 45/3600 = 0.0125. Add it all together 28+ 0.50 + 0.0125 = 28.5125. That's decimal degrees for 28 deg 30' 45".

Going backward is kind of the reverse. 28.5125, take the decimal portion. 0.5125 * 60 = 30.75 which is 30.75' which could be fine all by itself. (i.e. 28 deg 30.75'). To get seconds, take the remaining decimal 0.75 * 60 = 45". There you are, 28 deg 30' 45".
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Old 01-31-2008, 02:33 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

The most common expressions are degrees-minutes-seconds or degrees-minutes(with seconds expressed as decimals) Some manufacturers allow you to choose which way you want it pictured, but they all think in binary inside!
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Old 01-31-2008, 03:03 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

On a chart, a nautical mile is equal to one minute of latitude. 60 miles per degree of lattitude. A second of lattitude is equal to 100.771 feet. Prior to GPS, when we used celestial navigation being within 100 feet was almost unheard of (and that is one of the reasons that many charts are way off). So it was not really necessary to know the accuracy of position to more than 100 feet.
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Old 01-31-2008, 03:28 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Think of it this way ...

N 36 54.879 W76 21.215

Take the last 3 digits of LAT/LONG number and multipy them by 60 and that will give you the exact seconds figure.

.879 x 60 = 52.740 roundto the nearesttenth of a second = N 36 56' 52.7"

.215 x 60 = 12.900 round to the nearesttenth of a second = W 76 21' 12."

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Old 01-31-2008, 08:31 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Quote:
thataway - 1/31/2008 2:03 PM

On a chart, a nautical mile is equal to one minute of latitude. 60 miles per degree of lattitude. A second of lattitude is equal to 100.771 feet. Prior to GPS, when we used celestial navigation being within 100 feet was almost unheard of (and that is one of the reasons that many charts are way off). So it was not really necessary to know the accuracy of position to more than 100 feet.
This is not only true, but also where the phrase "a mile a minute" came from. The biggest downside I see with today's electronics is that many newcomers have no idea how to even use a chart & compass. I think these skills are still worth knowing.
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Old 02-02-2008, 04:36 AM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Quote:
bluewaterpirate - 2/1/2008 6:28 AM



Think of it this way ...

N 36 54.879** W76 21.215

Take the last 3 digits of LAT/LONG number and multipy them by 60 and that will give you the exact seconds figure.

.879 x 60 = 52.740******* round*to the nearest*tenth of a second* =** N 36 56' 52.7"

.215 x 60 = 12.900******* round to the nearest*tenth of a second** =** W 76 21' 12."

.
Couple of typo's there Tom
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Old 02-03-2008, 10:55 AM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Just a little tidbit I just found out about. 1 minute of latitude = 1 nautical mile. Always wondered where a nautical mile came from. So 1 degree of latitude is supposed to be 60 nm.

Note that for longitude, I think 1 minute of longitude = 1nm only at the equator. Away from the equator, you have to do some math to factor how far north you are to determine how many nm a degree or minute of longitude is.
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Old 02-03-2008, 02:28 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

I think the phrase "One mile per minute" was coined in the middle of the nineteenth century when steam locomotives capable of that speed were first being discussed. It was predicted that such a speed exceeded the natural limits intended by God, and that bad things would happen. Taking the long view of automobile traffic, there was some truth to that !
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Old 02-05-2008, 06:49 AM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Perhaps a United States Power Squadron or Coast Guard Aux. basic navigation course would be helpful to you. Not only would it answer your Lat/Lon questions but it will also help you understand the difference between true, magnetic, and compass notations. That's a topic for another thread!

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Old 02-05-2008, 01:23 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Check for dyslexia in your parentheses.
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Old 02-05-2008, 02:12 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Thataway and mrchummer you should check your info. I just completedmy 50ton mastersclassesanditis quitefresh in my mind plus I double checkedin case I was mistaken so here goes....
Latitude= horizonal lines
Longitude= vertical lines
Latitude lines(rings around the earth) get shorter as you move away from the equator.
Longitudinal line are all the same. Ie. each longitudinal line is the same length, they vary in distance from one another (horizonally) at different latitudes. The latitudinal lines or rings around the earth are equally spaced from pole to pole so the distance or dimension anywhere on earth from one latitude line to another is the same giving you a consistent reference.
So to measure distance on a chart you would go to the closest longitudinal line and measure the vertical distance on the scale for the minutes (1min= 1nm) as does 60 mins = 60 nm or one degree.
If you were to look at a globe and measured with a set of dividers 1 degree (60nm) at the equator on the latitudinal scale and then tried to use that same 1 degree in Alaska you'd find your dividers would be about 1 1/2 times to large to work up there because the length of the latitudinal distance between each Longitudinal line decreases the further you get away from the equator.
In summary, always measure for distance on the LONGITUDINAL scale and transfer it to what your trying to measure.
Hope this was helpful.
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Old 02-05-2008, 02:40 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

No Latitude/Longitude format is more accurate than another. Charts made through the mid twentieth century most commonly used degrees, minutes, seconds. This format is still commonly found in older editions of British Admiralty and NGA (the old DMA) charts. This format is still used in celestial navigation. The notation would look like this; 3015'40"

As electronic positioning systems started to come into use, hydrographic offices started converting chart scales to degrees, and decimal minutes in the late 1940's. All NOAA charts use this format. The notation would look like this; 3015.3301.

The third format is decimal degrees. Notation would look like this; 30.151535.

It's already been stated that one degree of latitude equals 60 minutes or approximately sixty nautial miles. Since a nautical mile is 6076 feet, one minute is equal to approximately 6000 feet, one second 100 feet and one tenth of a second 10 feet. In decimal minutes one tenth of a minute is 600 feet and one hundredth of a minute is 60 feet and one thousanth of a minute is 6 feet.

The choice of latitude/longitude format is based entirely on the paper chart you use. Every GPS to my knowledge outputs decimal degrees, and the software in the chartplotter or navigation program performs the conversion.
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Old 02-05-2008, 02:50 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

I realized that after I read my post I was trying to explain that you have to use the vertical scale which is used to measure degrees of latitude on a chart. You can't use the horizonal scale which is used to measure degrees of longitude on a chart to measure vertically (unless on the equator).
So my point in the previous post was if you are measuring horizonally on a latitude line you can't accurately use it for a distance going vertically.
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Old 02-05-2008, 05:21 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Quote:
Halacious - 2/5/2008 1:12 PMThataway and mrchummer you should check your info. I just completedmy 50ton mastersclassesanditis quitefresh in my mind plus I double checkedin case I was mistaken so here goes....Latitude= horizonal linesLongitude= vertical lines.
I went back, re-read my post and you explained it better. Even though latitude circles around the globe get shorter, the distance between them is generally constant . So no matter where you're at, the vertical distance between lat lines (along a longitude line for example) should generally be 60nm. That's what I was trying to say, but this is one of those things that could use an illustration.
I think where people can get confused is that horizontal distances between longitude lines goes from 60nm at the equator down to 0nm at the poles. But then again, anyone navigating near the poles probably knows what they're doing anyway. I know I'd hate to navigate up there without GPS.
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Old 02-06-2008, 01:44 PM
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Default Re: Difference in lat/lon degrees/Min/Seconds

I concur. I guess the point I wanted to make to those not so well versed (as I was before taking a class on it) was to Do Not use the Horizonal scale to take your scale readings from. I use the "Horizonal" term for clarity.
The Horizonal scale actually measures Longitude. This is confusing by the written word but visually explained is easier to understand.
And so the Vertical scale measures Latitude.

If I said to measure the latitude scale only in order to get accurate distance I think most people would use the Horizonal (Longitudinal scale).
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Old 02-06-2008, 02:00 PM
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Default RE: Difference in lat/lon degrees/Min/Seconds

It seems to me that the most common format I see used is Degrees/minutes/decimal minutes (read as XX degrees XX point XXX minutes), in cruising guides and such.

The older, more traditional format is Degrees/minutes/seconds

GIS applications generally use Degrees/decimal degrees (read as XX.XXXXXX degrees)

Like the above post say, they are all equally accurate, and you can switch your GPS display to present positions in any format you like, usually in the setup menu.
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Old 02-07-2008, 01:52 AM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Format is totally irrelvant to accuracy in any way.

What some appear to overlook is that there is a difference between the underlying electronic chart format and what the user actually uses is totally different, similar with the coordinate system GPS works in, which means very little to anybody unless you might be a GPS.

What systems "use" and what "users" see is totally different, however the end result to both is the same.
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Old 02-07-2008, 06:37 AM
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Default Re: Difference in lat/lon degrees/Min/Seconds

Anybody know what format the USCG uses? Although there are different formats, I think it makes sense to communicate with the coasties in the same format. Too much opportunity for error if I'm sinking and giving out my lat/lat in minutes and seconds and the helo is trying to convert to decimal (or vice versa).
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