Words by Peter Levey
The case for using a standardised GPS co-ordinate format
Now that GPSs are in common use, directions to destinations are frequently given using GPS co-ordinates. Unfortunately, there are three main GPS formats in use today, which leads to confusion as to what format is in use, especially when errors in the use of the formats are so common. Examples of the errors are the use of commas instead of points, and points that are missing altogether.
The various formats of giving out a position are all accurate to one degree or another, but there are convincing arguments for standardising on the hddd.ddddd format. From the above, you can already, visually, see why Number 1 is better. It’s a simple format that brooks no confusion. Communication of positional information is the main reason for using Decimal Degrees (hddd.ddddd). Whether it be verbal, SMS, mail, radio communication, written, or any other, the simplest format to use is Decimal Degrees…
Let’s look at the popular formats in use today:
1) hddd.ddddd (Decimal Degrees)
2) hddd mm.mmm (Degrees and Decimal Minutes)
3) hddd mm ss.ss (Degrees, Minutes and Decimal Minutes)
Let’s use an arbitrary point by way of example.
1) S26.11351 E27.88223
2) S26 06.811 E27 52.934
3) S26 06 48.6 E27 52 56.0
Again, you can visually see that decimal degrees is a simpler format. These days SMS messaging is a common means of communicating co-ordinates and there is no doubt thatthe use of Decimal degrees eliminates confusion. There was a case recently in Lesotho where a message for help was sent using a format which wasn’t correctly understood by the receiver; and as a result, help arrived eight hours later than it should have.
Here’s an example of co-ordinates communicated verbally, showing how other formats can lead to confusion.
1) “South Twenty Six Point One One Three Five One, East Twenty Seven Point Eight Eight Two Two Three”
2) “South Twenty Six Zero Six Point Eight One One, East Twenty Seven Fifty Two Point Nine Three Four”
3) “South Twenty Six Zero Six Forty Eight Point Six, East Twenty Seven Fifty Two Fifty Six Point Zero”
All three formats have the same number of digits. But the second and third (Degrees and Decimal Minutes, and Degrees, Minutes and Decimal Minutes) require more punctuation. The more punctuation that is required, especially in verbal communication, the more confusion and error that can creep in. This also holds true when sending co-ordinates via SMS; confusion can arise when points are used instead of spaces, especially when viewing these on a small screen.
Then there’s accuracy. (Yes, you can extend the decimal places out ad infinitum; however, the GPSs and the associated software we typically use limit you to the above.)
1) 100 000 units per degree
2) 60 000 units per degree
3) 36 000 units per degree
On the 26th parallel, 1 degree is equivalent to 100 km. Thus, for
1) 1 metre is your resolution.
2) 1.67 metres is your resolution.
3) 2.78 metres is your resolution.
Then there’s the translation issue. The two formats confused most often are number 2 and 3 (Degrees and Decimal Minutes, and Degrees, Minutes and Decimal Minutes).
Let’s take our arbitrary waypoint again.
1) S26.11351 E27.88223 – pretty simple.
But express this as: 2) S26 11.351 E27 88.223, and you’re 59 km off the mark.
3) S26 11 35.1 E27 88 22.3, and you’re 60 km off the mark.
The obvious conclusion one comes to, is that one should use a GPS format that is simple to communicate, more accurate, and less prone to communication errors. A co-ordinate is 18 words, or 18 words and one pause, or 18 words and two pauses. It’s these pauses which can be very confusing. So, for simplicity, accuracy, safety, and ease of use, Decimal Degrees is the only one to use.
One objection that people raise is, ‘If you use paper maps, then it’s more difficult to use decimal degrees, as the maps are marked in degrees and minutes’. I don’t buy this excuse, as it’s a simple matter to convert from decimal degrees on the extremely rare occasions that this is actually required. Conversions are done by multiplying the decimal fraction of the degrees by 60, as illustrated in the following example:
Decimal Degree: S25.67386 E028.38956 To convert to you multiply .67386 x 60 which gives you 40.4316 and .38956 x 60 which gives 23.3736. Thus, this coordinate expressed in Hdd mm.mmm is S25 40.4316 E028 23.3736. To convert to Hdd mm ss.s, you need only continue the above calculation and multiply the decimal fraction of the minutes to get the ss.sss portion. Thus, .4316 x 60 gives you 25.896 and .3736 x 60 gives you 22.416. So, S25.67386 E028.38956 expressed in Hdd mm ss.s is S25 40 25.9 E28 23 22.4.
ABOUT THE AUTHOR
Former telecoms and IT man, Peter Levey, has been a map lover since he was a Boy Scout. After buying a GPS in the early days, he became a great fan of the technology and in recent times has assisted on the MAPA project which involves mapping Africa’s protected areas – which ties in neatly to another of his great loves, overlanding. Peter is a member of various mapping and overlanding forums and it was here that he first noticed issues arising around the various co-ordinate formats, with data being sent and quoted with the incorrect spacing and punctuation marks. He originally penned this article for the Land Rovers Owners Club newsletter because he’d noticed the same confusion creeping in there.