GPS Format differences

[Mark Mocho]

n another thread, I briefly touched on the need to use the proper coordinate format when reporting things like landout position. There are three main formats used by GPS. Many are familiar with the traditional "Degrees, minutes, seconds" (DDD.MM.SS), but the default used by most Search and Rescue personnel is "Degrees, decimal degrees" (DDD.dddd). These are definitely NOT interchangeable and can give wildly different locations. The issue is switching from a system using Base 60 (60 minutes to one degree) to a Base 10 (10 or 100 or 1000 0r 10000 units per degree). As an example, I used Google Earth to drop a pin at a point at Moriarty airport. The point is at 34 degrees 59 minutes 38.65 seconds N, 104 degrees 59 minutes 36.68 seconds W. Using the same numbers, I entered them in the "Degrees, decimal degrees" format (34.5938 N, 104.5938 W) and dropped another pin. The points are 35.68 miles apart. See illustraion below. Make sure everybody is on the same page, or you might turn a retrieve into a major problem!

[Mark Mocho]

I forgot to list the third format used by GPS. It is "Degrees, decimal minutes" (DD.MM.mmmm). This is close to the "Degrees, Minutes, Seconds" format (DD.MM.SS.ss) but can still give position differences of a mile or so. The issue revolves around points that are close to the limit of 60 minutes per degree and 60 seconds per minute. If the position is right at an even degree like 35.00.00 ,105.00.00, there is no position error. But when you get close to the next degree and second (34.59.59, 104.59.59) the difference is significant.

[Stefan Will]

But the decimal representation of 34:59:38.65N 104:59:36.68W is 34.9941N, 104.9935W (the fractional degrees for 59 minutes is 0.983, not 59…

I think we would all be in real trouble if the decimal and DMS coordinates were that far off.

On Jun 11, 2026, at 9:23 AM, Mark Mocho <markm...@gmail.com> wrote:



In another thread, I briefly touched on the need to use the proper coordinate format when reporting things like landout position. There are three main formats used by GPS. Many are familiar with the traditional "Degrees, minutes, seconds" (DDD.MM.SS), but the default used by most Search and Rescue personnel is "Degrees, decimal degrees" (DDD.dddd). These are definitely NOT interchangeable and can give wildly different locations. The issue is switching from a system using Base 60 (60 minutes to one degree) to a Base 10 (10 or 100 or 1000 0r 10000 units per degree). As an example, I used Google Earth to drop a pin at a point at Moriarty airport. The point is at 34 degrees 59 minutes 38.65 seconds N, 104 degrees 59 minutes 36.68 seconds W. Using the same numbers, I entered them in the "Degrees, decimal degrees" format (34.5938 N, 104.5938 W) and dropped another pin. The points are 35.68 miles apart. See illustraion below. Make sure everybody is on the same page, or you might turn a retrieve into a major problem!

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[Stefan Will]

Or was your point that make sure you know the format before you blindly enter the digits?

[christopher behm]

100%.

I'm a boat captain. I used to work as a navy contractor on the weapons test range.

I had to re-educate the range test managers on their coordinates given to us for waypoints. 

They would not write the coordinates fully marked. Correct example- 33° 45' 32" (dd.mm.ss)

How they did it- 33° 45 32. Sometimes they even added a decimal point, 33° 45.32 an that converts to 33° 45' 19.2"...

Those "smart guys" drove me nuts. 

Then we had the few customers that did everything in DDD.dddddddd. I just needed to do some math to get it loaded into the chart plotter.

Thanks to Mark, for bringing up that SAR uses degrees and decimal of degrees. I'll switch the format of my nav gear to that, for safety. 

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From: rasp...@googlegroups.com <rasp...@googlegroups.com> on behalf of Stefan Will <stefa...@gmail.com>
Sent: Thursday, 11 June 2026 10:34:58
To: rasp...@googlegroups.com <rasp...@googlegroups.com>
Subject: Re: [RAS_Prime] GPS Format differences

 

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[Mark Mocho]

That is indeed the point. Entering DD.MM.SS in DD.dddd format gives large errors as you get to 59 minutes. 34 degrees 59 minutes 59 seconds (34.59.59) is essentially 35.000 degrees, but only 34.596 if entered in the DD.dddd format. In latitude, one degree is 60 nautical miles. So the .404 degree error equals 24.24 nautical miles. Make the same error in longitude and you can get errors of more than 36 miles.