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Discussion Starter · #1 ·
why dont all gps numbers look the same? some look like this wildwood reef lat 38 52.159 lon 74 41.599 while others look like this
latt 38d 58.700 lon 74d 39.817
while others are 38.58700 74.39817 ? none of them are the same as what the sofware is asking for? I dont get it
 

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Different platforms use different formatting methods for displaying lat/lon information. There are 3 general formats that you will see. Before you do anything with the numbers, you need to first decide which format they are in. See the more detailed definitions below.

1) dd mm.mmm - This is the format favored by most GPS units. It represents degrees followed by minutes (with decimal places). 3 decimal places is standard as with DGPS, that represents about 10 foot of accuracy.

2) dd mm ss - Degrees, minutes and seconds. This was a standard for many years and still appears in many publications/notations. (May also appear without seconds, which implies 00 as the seconds.)

3) dd.ddd - Degrees with decimal places. Not widely used except in some conversion processes where the number of decimal points varies.

Ok, so once you decide which is which, how do you convert?

dd mm ss -> dd mm.mmm - Keep the dd mm part. Divide the ss part by 60, and that equals the decimal part of your minutes.
Ex. - 39 30' 45" -> 39 30.750'

dd mm.mmm -> dd mm ss - Keep the dd mm part. Multiple the decimal part of the minutes by 60 and that equals the seconds.
Ex. - 38 15.250' -> 38 15' 15"

dd.ddd -> dd mm.mmm - Multiple the decimal part of the degrees by 60 to give you the minutes including decimal points.
Ex. - 40.100 -> 40 06.000'

dd mm.mmm -> dd.ddd - Divide the minutes (including decimal) by 60 to give you the decimal part of the degrees.
Ex. - 74 15.000' -> 74.250

dd - degrees of latitude or longitude. Should include a N/S (north/south, for latitude) or E/W (east/west, for longitude) designator to tell in which hemisphere or side of the Prime Meridian (0 degrees longitude), respectively, but for most local charts, this is left off because of the "assumption" you know on which side of the equator/Prime Meridian you are (for this area, anyway). May use the "d" as shorthand for degrees, or the degree symbol (a small o as a superscript above the number).

mm - minutes of latitude or longitude. There are always 60 minutes in a degree of latitude or longitude, however the distance that a degree represents differs for lat and lon. 1 minute of latitude is equivalent to 1 nautical mile. The distance represented by a minute of longitude varies based on your latitude...the further north you are from the equator, 1 minute of longitude is equal to a lesser distance than closer to the equator. For the latitudes NJ spans, 1 minute of longitude is less than 1 nautical mile. Minutes are sometimes followed by the single quote (') notation. (39 30')

ss - seconds, a way to divide minutes into 60 increments. Seconds are sometimes followed by the double quote (") notation. (74 07' 30")

[ 05-24-2005, 05:16 PM: Message edited by: captadamnj ]
 

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Discussion Starter · #3 ·
thank you for taking the time to explain that to me. now I just have to digest all that
 

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Discussion Starter · #4 ·
ok which format is 38d 57.000 is that the ddd? I dont understand why the d is there in the first place? it just makes it more confusing. do I then divide 000 by 60 to get the number i need, or am I just stupid and cant figure it out
 

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So, consider that the earth spins 360 degress in 24 hours. So, it spins 15 degrees in one hour. Note the time on your watch when the sun is the highest in the sky. Greenwich Mean Time (the time in Greenwich England at the Prime Meridian) will be five hours earlier. So, 5 x 15 degrees yields our approximate longitude, 75 degrees west.

A carpenter (Harrison) finally constructed a clock that would keep accurate time at sea, telling the crew what time it was in Greenwich England. It was only then that an accurate longitude could be determined. Why? As Capt Adam posted above, the meridians of longitude are not parallel as are the parallels of latitude.

Soooo, even today, our highly reliable GPS system of satellites remain a matter of time.
 

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Originally posted by Verse:
ok which format is 38d 57.000 is that the ddd? I dont understand why the d is there in the first place? it just makes it more confusing. do I then divide 000 by 60 to get the number i need, or am I just stupid and cant figure it out
The "d" is just indicating degrees. 38d 57.000 is merely 38 degrees, 57 minutes, and 000 seconds. You just punch those numbers into a GPS.
 

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Wait. I just reread Adam's post and I'm also confused about those other formats. Just go with Adam's post since I rarely encounter those formats and therefore don't use them. So I can't give an answer.
 

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38d 57.000 is dd mm.mmm format. It is using the optional "d" designator to indicate degrees. So that means 38 degrees 57.000 minutes. While it does represent 0 seconds, you would not read 57.000 as "57 minutes and 0 seconds". You would read it as "57 decimal 0 minutes".

The "d" in your original post just indicates that the 38 and 74 represents degrees. The first 2 examples you gave are in dd mm.mmm format...just one used the optional "d" and the other didn't. The 3rd example you gave "looks" like dd.ddd (all degrees/decimal of degrees), given that your second set of numbers has the same exact digits, just different decimal places, it was probably written incorrectly. But therein is the reason for understanding the different formats. 38d 58.700 is a very different location than 38.58700 (which would be 38d 35.220 using my conversion method). Make sense?

I could have simply skipped the long explanation, I suppose, but maybe someone will find the information useful.
 

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Thanks Capt. I'm gonna' revisit the topic so I understand it in the future.
 

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Discussion Starter · #10 ·
thanks for the help, I am just way confused now. I am using a program called easygps that allows me to punch the numbers in from my desktop and load it to my reciever.
 

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Verse,
On that program you are using there should be a way to find out what format they are using.

You also want to make sure it is in the same format that your gps is using.

Once you find those out, let us know what format and we can narrow some things down for you.

CaptAdams post excellent.
 

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Originally posted by TWIN D'S:
So, consider that the earth spins 360 degress in 24 hours. So, it spins 15 degrees in one hour. Note the time on your watch when the sun is the highest in the sky. Greenwich Mean Time (the time in Greenwich England at the Prime Meridian) will be five hours earlier. So, 5 x 15 degrees yields our approximate longitude, 75 degrees west.

A carpenter (Harrison) finally constructed a clock that would keep accurate time at sea, telling the crew what time it was in Greenwich England. It was only then that an accurate longitude could be determined. Why? As Capt Adam posted above, the meridians of longitude are not parallel as are the parallels of latitude.

Soooo, even today, our highly reliable GPS system of satellites remain a matter of time.
Parallel, huh?? So is that why the Loran and GPS TDs are the same - ie the 43 line in the Pond?

Being the hardheaded ex-engineer that I am keeps me convinced that "ditto" applies to latitude. Just tougher to calculate, a few more variables, maybe iterative procedure etc that can do better than 0.2 miles. Might keep me from anchoring on the cable between the Fort and the Elbow. ;)
 

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Parallel, huh?? So is that why the Loran and GPS TDs are the same - ie the 43 line in the Pond?
Latitude is truly parallel from a mathematical geometric sense...that is, each line of latitude is everywhere equidistant from the next and never intersect.

TD's (from loran), on the other hand, are not parallel...they are curved (hyperbolic, specifically, to be matchematically accurate). The reason why the 2 and the 4 line are widely used here in NJ (for those that still use loran) is because the "curves" (the proper terms is LOP, Line Of Position) from those transmitting stations (Nantucket for the 2 line and NC for the 4 line) become nearly parallel over our waters...and more importantly, they cross at near right angles, making the lines more useful.

Another useful correlation regarding Lat/Lon and LOP's (only off of NJ for LOP's) -

To maintain the same Latitude - steer a course of 90/270 degrees TRUE.
To maintain the same Longitude - steer a course of 0/180 degrees TRUE.
To maintain the same "2" line - steer a course of approx. 30/210 degrees MAGNETIC
To maintain the same "4" line - steer a course of approx. 100/280 degrees MAGNETIC

Moreover, due to the timing of the land based signals, a change in 10 microseconds on the 2/4 lines represents a distance of approximately 1 nm, making it convenient as a distance estimating tool.

Ex. - If I am at 26900 43100 and want to run to position 26900 43030, I can quickly estimate I am about 7 miles away. (43100 microseconds - 43030 microseconds = 70 microseconds, and every 10 microseconds is about 1 nm)

[ 05-25-2005, 10:00 AM: Message edited by: captadamnj ]
 

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Discussion Starter · #18 ·
after all that I look up and on the sofware it converts it for you! I amaze myself with my own stupidity.
Capt Adam after looking at all the readings that I had put into the gps, in what I thought was right before you told me different were the wrong coordinates. I would have ended up in bermuda without your help. Thanks
 
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