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GPS Formats: Why Do We Use The Format We Use?


JL_HSTRE
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Why do geocachers and geocache listings use MM DD.DDD instead of Decimal Degrees or Degrees-Minutes-Seconds (DD MM'SS")? (UTM is obviously a more complicated format.) Most government agencies I've dealt with use DD or DMS. Was the format we use the most popular with the GPS hobbyist crowd in 2001?

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Why do geocachers and geocache listings use MM DD.DDD instead of Decimal Degrees or Degrees-Minutes-Seconds (DD MM'SS")? (UTM is obviously a more complicated format.) Most government agencies I've dealt with use DD or DMS. Was the format we use the most popular with the GPS hobbyist crowd in 2001?

 

Well for a start you can specify greater precision, with DD MM.MMM you can specify to 1/1000th of a degree; whereas with DD:MM:SS you can only specify to 1 second, which is 1/60th degree, to get more precision you would need to go down to decimal seconds, so DD:MM:SS.SS which is more cumbersome. Also the maths is simpler akin to the Metric system over the Imperial system, and yes I know that the USA still uses the imperial system but even over there your scientists and engineers use metric ;-)

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I think it's a reasonable compromise between accuracy and user-readability which is probably why GPSs default to it. As MartyBartfast notes, you can specify to a level of a thousandth of a minute, or about 2 yards.

 

Compared to decimal degrees, having the minutes figure there helps in location. Near here (Cambridge, UK), I know going one minute north or south is about a mile, one east or west about 2/3 mile, and that the city is roughly N 52 10-15, E 0 5-10, and I have a reasonable feel for where any location within 20-30 miles is going to be. You can do the same tricks using the first two decimal digits, but it's not quite as intuitive.

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I think it's a reasonable compromise between accuracy and user-readability which is probably why GPSs default to it. As MartyBartfast notes, you can specify to a level of a thousandth of a minute, or about 2 yards.

 

That makes sense to me too. It's worth mentioning that DDM (Degrees Decimal Minutes) is only used for display purposes but when lat/long coordinates are encapsulated in a GPX file DD format is used, and is most likely used internally at GS as it's easier to do coordinate "math" using decimal degrees.

 

 

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... Near here (Cambridge, UK), I know going one minute north or south is about a mile, one east or west about 2/3 mile, and that the city is roughly N 52 10-15, E 0 5-10, and I have a reasonable feel for where any location within 20-30 miles is going to be. You can do the same tricks using the first two decimal digits, but it's not quite as intuitive.

Just to be precise, going one minute N/S everywhere in the world is exactly one nautical mile, about 1.15 miles :rolleyes:

 

E/W is another history :rolleyes:

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Why do geocachers and geocache listings use MM DD.DDD instead of Decimal Degrees or Degrees-Minutes-Seconds (DD MM'SS")? (UTM is obviously a more complicated format.) Most government agencies I've dealt with use DD or DMS. Was the format we use the most popular with the GPS hobbyist crowd in 2001?

 

Well for a start you can specify greater precision, with DD MM.MMM you can specify to 1/1000th of a degree; whereas with DD:MM:SS you can only specify to 1 second, which is 1/60th degree, to get more precision you would need to go down to decimal seconds, so DD:MM:SS.SS which is more cumbersome. Also the maths is simpler akin to the Metric system over the Imperial system, and yes I know that the USA still uses the imperial system but even over there your scientists and engineers use metric ;-)

 

Now, that makes sense. Thanks!

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Well for a start you can specify greater precision, with DD MM.MMM you can specify to 1/1000th of a degree; whereas with DD:MM:SS you can only specify to 1 second, which is 1/60th degree, to get more precision you would need to go down to decimal seconds, so DD:MM:SS.SS which is more cumbersome. Also the maths is simpler akin to the Metric system over the Imperial system, and yes I know that the USA still uses the imperial system but even over there your scientists and engineers use metric ;-)

 

Just a technical correction.

 

DD:MM.MMM specifies to 1/60,000 of a degree.

DD:MM:SS specifies to 1/3,600 of a degree.

DD:MM:SS.SS specifies to 1/360,000 of a degree.

 

Regardless of which of these measurement systems you use, GPS error is greater than the coordinate system error.

 

Austin

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Well for a start you can specify greater precision, with DD MM.MMM you can specify to 1/1000th of a degree; whereas with DD:MM:SS you can only specify to 1 second, which is 1/60th degree, to get more precision you would need to go down to decimal seconds, so DD:MM:SS.SS which is more cumbersome. Also the maths is simpler akin to the Metric system over the Imperial system, and yes I know that the USA still uses the imperial system but even over there your scientists and engineers use metric ;-)

 

Just a technical correction.

 

DD:MM.MMM specifies to 1/60,000 of a degree.

DD:MM:SS specifies to 1/3,600 of a degree.

DD:MM:SS.SS specifies to 1/360,000 of a degree.

 

Regardless of which of these measurement systems you use, GPS error is greater than the coordinate system error.

 

Austin

You're right I didn't mean degree I meant minute.

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I agree with crb11 that it's most likely a good choice between readability and accuracy.

 

Another thing I've noticed is that if I copy/paste the degrees/decimal minutes formatted coordinates into just about anything it will be recognized: Google Earth, DeLorme Street Atlas, the GPSrs I've used (Garmin, TomTom, Magellan), a general web search, and on and on.

 

Yes, there have been exceptions. (We had a Toyota 4Runner for a while that insisted on using DMS, which was really annoying; but then the whole GPSr was really extremely difficult to use compared to anything else.)

 

I don't think the same thing can be said about DMS. If you go into Google Earth and type something like: N36 35 43, W093 59 04 it won't recognize it, but it will recognize the other formats.

 

On the other hand, most things will also recognize decimal degrees (except the blasted GPSr on the 4Runner!), but then it's a lot harder for humans to remember an unbroken string of digits than it is to split it into degrees and decimal minutes. Subjective, I admit. But it's a matter of finding what serves the widest audience. And I'm guessing that most folks would rather read decimal minutes than decimal degrees.

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On the other hand, most things will also recognize decimal degrees (except the blasted GPSr on the 4Runner!), but then it's a lot harder for humans to remember an unbroken string of digits than it is to split it into degrees and decimal minutes.

 

I think that goes to the part of readability of the coordinate format and using decimal degrees may also increase the likelihood of a typo error.

 

One thing to be care of with decimal degrees is the use of negative numbers.

 

If you don't specify the hemisphere, positions north of the equator are a positive number and a south of the equator it's a negative number. Similarly, Eastern positions are considered to be positive and western positions to be negative. Specifying N, S, E, W or +/- is allowed.

 

 

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Just a technical correction.

 

DD:MM.MMM specifies to 1/60,000 of a degree.

DD:MM:SS specifies to 1/3,600 of a degree.

DD:MM:SS.SS specifies to 1/360,000 of a degree.

 

Regardless of which of these measurement systems you use, GPS error is greater than the coordinate system error.

Just a technical correction.

 

With newer Garmins using Wide Area Augmentation System (WAAS), the average GPS error is less than 3 meters. That's more accurate than 1/3,600 of a degree for all NS degrees and most EW degrees.

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Why do we use degrees, decimal minutes? Because that's what Dave Ulmer used! :laughing:

 

Well, I did it, created the first stash hunt stash and here are

the coordinates:

N 45 17.460

W122 24.800

Now, there's a good chance that he used that format because that's what his GPSr was already set to. As mentioned in Jeremy's posts linked to by Keystone, GPSrs typically ship from the factory set in this format. This is likely the primary reason why we use degrees, decimal minutes.

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Well for a start you can specify greater precision, with DD MM.MMM y

 

But DD.xxxxx is about 60 percent greater in accuracy then MM.xxx and would be so much simplier. But those days are long past for geocaching. But with the current technology is GPSr's, it really doesn't matter because even on the very best day, it's 8-10 feet accurate which well falls in line with DD MM.xxx or DD.xxxxx

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Well for a start you can specify greater precision, with DD MM.MMM y

 

But DD.xxxxx is about 60 percent greater in accuracy then MM.xxx and would be so much simplier. But those days are long past for geocaching. But with the current technology is GPSr's, it really doesn't matter because even on the very best day, it's 8-10 feet accurate which well falls in line with DD MM.xxx or DD.xxxxx

 

With enough significant digits, DD.xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx is as accurate as anything.

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If you go into Google Earth and type something like: N36 35 43, W093 59 04 it won't recognize it...

Really? When I enter those coordinates in that format, Goggle Earth takes me to the SW corner of Missouri.

That's good. That means that GoogEarth fixed it. When I tried that about 6 months ago it kept thinking I was looking for a business or address named N36 35 43, W093 55 04. (Don't think I'd ever name a business that....)

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With enough significant digits, DD.xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx is as accurate as anything.

"I found your cache four inches away from the indicated coordinates, so I moved it to the opposite side of the lamp post."

That's a good one.

 

But from my limited 3 years of caching and owning only 2 of them, it doesn't take being off even 4 inches. Lots of cachers just move the cache for who-knows-what-reason. I don't get it. It was fine where it was!

 

Need to find out a different fill-in-the-blank for:

"______________________________, so I moved it ...."

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If you go into Google Earth and type something like: N36 35 43, W093 59 04 it won't recognize it...

Really? When I enter those coordinates in that format, Goggle Earth takes me to the SW corner of Missouri.

That's good. That means that GoogEarth fixed it. When I tried that about 6 months ago it kept thinking I was looking for a business or address named N36 35 43, W093 55 04. (Don't think I'd ever name a business that....)

Google Earth is switchable -- top line menu: Tools > Options.

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Need to find out a different fill-in-the-blank for:

"______________________________, so I moved it ...."

 

My mother-in-law was with me and she insisted it was wrong, so I moved it.

I was caching with TooMuchWiskey, so I moved it.

I didn't see where my caching partner found it, but it was my turn to put it back so I moved it.

I didn't like walking just off the trail, so I moved it.

I like pine trees better than spruce trees, so I moved it.

I'm late filing my taxes, so I moved it.

 

How's that for you?

 

Austin

Edited by AustinMN
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Now, there's a good chance that he used that format because that's what his GPSr was already set to. As mentioned in Jeremy's posts linked to by Keystone, GPSrs typically ship from the factory set in this format. This is likely the primary reason why we use degrees, decimal minutes.

 

Agreed.

 

Which still begs the question why GPS manufacturers defaulted to that setting 15 years ago, but the 'balance between ease of use and accuracy' thing someone mentioned seems reasonable.

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I started out frustrated that the format was not DD, but I have changed my mind over the years. Certainly DD is the most accurate for a given total number of digits, and it is the internal format in which coordinates are stored.

 

Well, sort of. To do any real calculations with coordinates DD has to be transformed into radians and the calculations done on those values, but let's ignore that for now.

 

I prefer DD MM.MMM because it's easier for my brain to scan than DD.xxxxx . Of course, that could be just that I have gotten used to it, but I can quickly convert a minute of lat into a nautical mile and a minute of lon (in my area) into about .9 miles (statute) and that is good for intuition. Likewise, a single least-significant digit on minutes is about 6 feet in lat and 5 feet in lon in my area, and that is helpful.

 

But if I had been using DD format all this time my intuition might be equally well-tuned for it, I suppose.

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