Coordinate formats -help needed.

flapbreaker · 04-26-2008, 08:15 PM

I tried to convert some coordinates from one format to the other but got so confused by all the different ways to display these numbers that I'm not even sure what format I'm dealing with. Part of the problem seems to be that there are some inconsistencies on various websites in either how they label their format or how and where they use periods ect. Anyway maybe someone with a better grasp on this can set me straight.

My Chart plotter shows coordinates in the following manner.

46� 10.117N 124� 27.126W

So, is this Degrees, Decimal, seconds? And would this be abbreviated like DM.MM? On maptech when I select DM.MM it shows coordinates that look like the above but DM.MM doesn't seem to make any sense to me.

Next. I frequently see coordinates in magazines ect in the following format.

46� 10.06'N 124� 27.03'

I can't quite decide what format this is. Again, on maptech if I select DMS I get something similar but not exactly like that. It would show up as.

46� 10'06"n 124� 27'03"

Notice that there is no period used and instead of a hypostrophe they use a quote sign on the end. Are these both DMS or what?

I was trying to use the following converter to convert between the two numbers but it doesn't work when the last number is greater than 60.

BiteFactory · 04-26-2008, 09:17 PM

I think you are pretty close to figuring it out.

dd mm.mmm format gives you degrees and minutes. No seconds are dealt with here. You have 60 minutes in a degree (each minute of latitude is one naut mile). So, the range for 45 degrees lat is 45 00.000 to 45 59.999

The more digits you have after the period (for example: .mmmm) the more accurate of a position. 1 digit = tenth of a nm, 2 digits = hundredth of a nm, etc...

dd mm ss uses seconds (60 seconds in a minute, so each second is 1/60th of a nm in latitude). The range for 45 degrees lat is 45 00' 00" to 45 59' 59"

Pilar · 04-26-2008, 09:22 PM

There are several formats. But lets start at the beginning.

Take Lattitude for example. Think of it as distance from the equator. the first number is always degrees. Around these parts from 43 to 50+ always North for us cold in the winter folks.

N45 .....

W125 would be Longitude or distance from merry old England.

Now for the confusing parts. Degrees, minutes, seconds.

Lattitude Degrees from 0 near the equator to 90 at the north pole

Longitude degrees from 0 near Greenwich England to 180 out in the middle of the pacific at the date line.

Minutes are always 0 to 60 seconds. 60 Minutes is one degree. It can be shown as

N45 34'

Seconds are always 0 to 60 seconds. 60 seconds is one minute.

N45 33' 55"

Now maybe we can decipher your number.

46� 10.117N 124� 27.126W

DM.MMM drop or round the last digit and you get

DM.MM

That last digit is only worth a few feet anyway.

Ok I see degrees, minutes dot decimal minutes. Sometimes they show the 60 seconds as thousandths or hundreths of a minute. The clue is the decimal. 2 places after the decimal so multiply the number 60 by the numbers to the right of the decimal to get seconds.

46� 10.117N 124� 27.126W becomes N46 10' 7", W124 27' 7" notice that seven seconds is the result both times. Not much difference between .117 and .126 minutes.

There is one other way which I personally do not like very much. Degrees decimal degrees.

D.DDDDD

N46.16861, W124.45211

Divide or mulitply by 60 as needed and you can convert any format.

Don't ask me about loran C it makes my head hurt.

Quote:

Originally Posted by flapbreaker

I tried to convert some coordinates from one format to the other but got so confused by all the different ways to display these numbers that I'm not even sure what format I'm dealing with. Part of the problem seems to be that there are some inconsistencies on various websites in either how they label their format or how and where they use periods ect. Anyway maybe someone with a better grasp on this can set me straight.

My Chart plotter shows coordinates in the following manner.

46� 10.117N 124� 27.126W

So, is this Degrees, Decimal, seconds? And would this be abbreviated like DM.MM? On maptech when I select DM.MM it shows coordinates that look like the above but DM.MM doesn't seem to make any sense to me.

Next. I frequently see coordinates in magazines ect in the following format.

46� 10.06'N 124� 27.03'

I can't quite decide what format this is. Again, on maptech if I select DMS I get something similar but not exactly like that. It would show up as.

46� 10'06"n 124� 27'03"

Notice that there is no period used and instead of a hypostrophe they use a quote sign on the end. Are these both DMS or what?

I was trying to use the following converter to convert between the two numbers but it doesn't work when the last number is greater than 60.

Chug · 04-26-2008, 09:35 PM

WOW.. that is detailed..

Reason's I like UTM's.. Chimp Simple.

flapbreaker · 04-27-2008, 07:03 AM

Thanks for the explanation guys. What got me started was the halibut article in the latest fishing and hunting news. They gave a bunch of coordinates but because they were dropping the last M in DM.MMM I thought they were showing dd mm ss. I guess if the last number is larger than 60 it can't be dd mm ss.

This is their format. I say it's DM.MM
46� 10.06' 124� 38.88' (I know, there are NO halibut here)

BiteFactory · 04-27-2008, 07:59 AM

Yep, that is dd mm.mmm

If you see a number like 45 12.02N I think you can assume it is dd mm.mm. I don't think I've seen someone use a period to separate minutes and seconds. Putting the minute tick' in there makes it even more clear.

This format is more common now adays due to gps/chartplotters being commonplace. When you start dealing with paper charts that is where seconds start being used mostly and (hardly) never dd mm.mmm

So - if you know you are dealing with people using paper charts or you have a gps and want to plot your points on a chart - change your gps/chartplotter to use dd mm ss to make life easier. Otherwise set it to dd mm.mmm and forget the seconds.

ron m · 04-27-2008, 03:31 PM

Related, but a bit off the subject is Pilar's comment about dropping the third number to the right of the decimal in the dd mm.mmm format. He's right on that it's only a few feet, like about 6 feet times the digit.

On the water, I hear guys giving their location to the third decimal place and that just makes it more difficult for folks to get the cordinates down. One dicimal place is probably more than enough if you're giving your location for a buddy to come find you for a hot bite.
ron m

HiTekRdNek · 04-28-2008, 10:48 AM

One thing I learned in my navigation class was to match your datum format to that from the chart for the area your in. Makes plotting much easier.

Noyb72 · 04-28-2008, 12:10 PM

I have to do this ALL THE TIME. AIS expresses location degrees, minutes, seconds and digital fire control systems on submarines use degrees, minutes, decimals of a minute.

The thing I haven't seen described here (and that I often come up against,) is having the seconds and needing to find the decimal minutes. The math is just backwards. The way I do it mentally is add a zero and divide by six.

7 seconds = 70/6 = .117 or 117 thousands of a minute.

HiTekRdNek, you have Identified something far more important that an error of a few seconds. Matching datum to format is by far the most important thing you can do. To see how far this can mess you up, build several boxes in a grid shape in Mercator projection on your chart plotter then switch over to stereographic. Your boxes will no longer look like boxes. This error can put you out of position by a considerable margin. People have died from this error.

Ron