GPS coordinates

[thataway]

Greg tells us the USCG uses:

The standard Latitude/Longitude format for
CISAR operations is Degrees, Decimal
Minutes (DD° MM.mm’).

Thus is is best to have our chart plotters set in this format. It makes it easier to both give the information to SAR, with less chance for error, and to then plot the location of an object by entering a waypoint, and determine if this is close to our position by doing a range and bearing of this waypoint on the chart plotter.

I just reviewed the steps necessary on Garmin chart plotters, which is what I use on my boats.

Go to "Home" then to "Configure", to "Units", "position Formats" and then choose:
"hd*M'" as our default. You can also check on this page to be sure that the map datum is WGS 84. (This page has System units (statute, nautical and metric) Position formathddd.ddddd, hddd.mm.mmmm hddd*mm'ss.s" and MGRS military grid system) Time (time format 12/24 & time zone) Heading (auto mag variation, true, grid user mag var), Map Datum and pressure Ref. time (some GPS give barometric pressure readings)

My home location in hd.m' is N30* 25.527' W087.20561' in MGRS is 16R DU 67090 65080 I still personally find it far easier to find on a map with the degrees, minutes and seconds system, but the Government is always right!

 

[Kushtaka]

But the lack of a marine/land/SAR convention is confusing. Often co-ordinates are given to a dispatcher, and they don't understand, or ask the person reporting to clarify how units are used. Heck most boaters don't understand the difference! A few years ago we had a seminar for first responders, the 911 operators, Dispatchers, various charter boat and dive boat skippers, Coast Guard etc, so that everyone in the 3 county area was all on the same page.

Once when the river was threatening to flood out one of my field camps I established an evac LZ with the USCG rescue helo that my crew could reach if camp was inundated. It was pretty important that they be at the exact right spot, so I confirmed that the coordinates were in the WGS84 datum, and nobody knew what I was talking about, and kept confusing that with DMS, decimal degrees and decimal minutes, which as noted above, are not functionally different, simply arithmetic conversions. In the end I had so little confidence that they knew exactly where the LZ was that I had to send them a map with a bullseye on it and several increasingly detailed scales. The camp barely stayed dry and we never evacuated, but it was an eye opening experience.

I've also been on the ground on the open tundra watching a helicopter with my precise coordinates, and a signal mirror on a clear day take 45 minutes to find me.

I like to relay these experiences to people who substitute good preparation and planning with an EPIRB or PLB.

 

[kennharriet]

Here are a few thoughts about coordinate precision. Latitude and Longitude expressed in Degrees, Minutes, decimal minutes is the maritime standard. Most gps units provide precision to 4 places to the right of the decimal.

If we leave aside longitude, as the further from the equator you travel the shorter they become, and use latitude the most a minute can be is a nautical mile. There are 6,076 feet in a nautical mile. So in short the first digit, after the decimal is a tenth representing = 607 feet. The next digit is one hundredth = 60 feet and the third digit a thousand = 6 feet. So the three digit level of precision is extreme.

My point is that 2 places to the right of the decimal is within the accuracy of the satellite signal. Further description than that has little meaning and only adds more digits in a radio transmission, offering more opportunity for human error in the communication.

I work in the wildland firefighting community where aircraft respond to incidents from multiple directions. The standard Lat Long description is also Degrees, Minutes, decimal minutes with the WGS84 datum. Two places to the right of decimal is enough precision to cause concern of mid air collisions so reporting aircraft orbit the coordinates, rather than flying directly to them.