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Suppose I am out in the wilderness and an emergency arises. I have an accurate fix on my location (from GPS, map and compass, astronomical observation, etc...). I manage to get in touch with emergency services and need to tell them where I am.

But there are many different formats I could use to report my position.

  • Latitude and longitude in degrees / minutes / seconds

  • Latitude and longitude in decimal degrees

  • Latitude and longitude in degrees and decimal minutes

  • UTM

(And with any of these systems there is the finer question of which Earth ellipsoid or datum my map or GPS is based on, which could make a difference of a few tens or hundreds of meters in the position; in dense forest this could make it considerably harder for rescuers to find me.)

Obviously I want to avoid any confusion; if I give my position in (say) degrees / minutes / seconds, and a helicopter pilot blindly types it in their GPS which is set to decimal degrees, they'll go to the wrong place (perhaps many kilometers away). UTM is less ambiguous, but will they even know what that is? I may be speaking to a dispatcher in an urban office, perhaps over a noisy and unreliable phone or radio connection, and that's hardly the time for a technical discussion of the fine points of geodesy.

So in short:

  • Is there a particular position format that's preferred by emergency responders?

  • Are dispatchers and responders sufficiently trained to understand the difference between these positions, so that I can use any format if I tell them which one it is?

  • Are there other general "best practices" for this situation?

I suppose the answers may differ between different countries or regions (I'm in the US) or between land and sea emergencies (I'm usually on land).

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Datum differences are not large enough to cause a problem. And seconds is usually more accuracy than needed. It is standard to use DD MM.M for most navigators. –  user6972 Jul 6 at 8:14

4 Answers 4

up vote 7 down vote accepted

I would recommend UTM coordinates; it avoids the formatting uncertainty of lat/long and is better suited for ground operations. (Easy to translate to paper maps, define search areas, and calculate distances.) If you use the WGS84 datum, the numerical portions are also identical with the military grid reference system (MGRS) and the national grid (USNG). In my area the local mountain rescue groups (BAMRU, Tahoe Nordic) use UTM.

That said, aircraft and ships at sea usually prefer latitude and longitude values as the numbers don't "reset" when you cross zones, and they are usually covering much greater distances than people on land. Per the addendum I added at the end, the recommended format is degrees and decimal minutes. The linked PDF suggests reading coordinates as in the following example (for 39° 36.06’N by 76° 51.42’W): “Three nine degrees, three six decimal zero six minutes North by seven six degrees, five one decimal four two minutes West.”

Regardless of coordinate system, have the person on the other end repeat the numbers back to you; this is one set of numbers you don't want to have an error. Writing down your message before communicating will also help you with a poor connection or low batteries as you'll avoiding wasting time trying to remember what else you need to communicate.

Finally, as others mentioned, the dispatcher should be able to handle any coordinate format, and any search teams will be using topo maps and translating the coordinates to their preferred format. (They are usually familiar with the areas they regularly work in.)

Addendum: I suspected that [within the US] UTM was becoming a SAR standard, as a quick Google suggests that to be the case. Further digging indicates the National Search and Rescue Committee has designated the US National Grid (USNG) system as the standard coordinate system for land-based SAR in the United States, and latitude and longitude (in decimal minutes) for aircraft and boats. This does not mean that some (many?) local agencies are no longer using their older preferred systems; full compliance is probably many years away. If you want to know more about USNG, I suggest posting a separate question, but here's a comparison:

  • USNG: 18S UJ 23371 06519

  • MGRS: 18SUJ2337106519

  • UTM: 18 323371 4306519

(Note that USNG and MGRS use a letter pair for the 100km grid squares whereas UTM doesn't; UTM instead has additional digits. The Easting and Northing values always have the same precision: 3x3, 4x4, and so forth. I personally prefer the USNG style.)

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I'd give them whatever my device or map provided me, and let them convert to whatever their devices or maps use. Anyone used to receiving lat/lon coordinates regularly should be able to convert from various formats to whatever they use internally.

You're the one in trouble with limited resources. You're out there with a broken leg, lost, in the cold or dark, or whatever. You're a lot more likely to screw up than someone sitting in a office who is professionally used to dealing with this and probably has equipment that does conversions for him routinely.

You are over-thinking this.

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Of course you are right - when in an emergency situation you shouldn't overthink it. However, I like to view this question as, if I were to prepare a trip, and for each of the formats I have exactly one device. I can only bring one with me, which one should it be? –  Niek Haarman Jul 5 at 9:32
@Niek: Bring whichever device is most suitable for your outing. What format it displays location in wouldn't even be on my list. In the unlikely event that you need to communiate those coordinates to rescuers, they'll deal with whatever you have. Otherwise, you want the most convenient device for your purposes. –  Olin Lathrop Jul 6 at 13:29

Always have a water proof map of the area you're traveling in ( one with coordinates on each side if you can help it ), and a compass you can use to triangulate your position with. You'll be better prepared in an emergency and more confident outdoors in general if you practice triangulating your position until it comes naturally: http://www.rei.com/learn/expert-advice/navigation-basics.html

As a backup, have a reliable GPS, and carry extra ( fresh ) batteries for it, more than you think you will ever need. Obviously having your coordinates in an instant will save you time, but electronics aren't to be trusted with your life.

If you can give your coordinates to dispatchers, you're leaps and bounds ahead of most people that need assistance ( have the dispatcher confirm your coordinates ). Most search and rescue teams have the skill and equipment to translate your coordinates into their preferred format faster than you can even read them.

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I'd never relay solely on a GPS, spare batteries or no. GPS should be a backup/quick option and no more. –  Liam Jul 15 at 15:35
@Liam right you are, updated it to reflect the importance of having a waterproof map and compass. –  hillsons Jul 15 at 18:19

I'd give my position in terms of landmarks ("I'm about five miles up Independence Creek; look for the silver emergency blanket draped over the bushes"). If I had a GPS, I'd also give coordinates in whatever units it displays. If you've ever gone geocaching, you'll know that while coordinates are precise, they don't tell you what the terrain is like or which direction to approach from; landmarks can.

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Land marks are only good if they know what area you are in to start with. Also local names often differ from map names. And different maps might use different names or have multiple landmarks with the same name. In many parts of the world land mark names are too undefined to be universally recognized by rescue teams. This is especially true along shorelines. –  user6972 Jul 6 at 8:17
It's not like your SAR team is in a call centre on the other side of the world. They are in the area and they know the area, too. "I'm on a ridge north of Jack Lake" and then some co-ordinates - this sanity checks the co-ordinates also. –  Kate Gregory Jul 15 at 19:01

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