SAR folks - the NAD83/WGS84 Datum seems to be the most common for SAR work. But coordinating SAR ground ops with air ops always seems a bit tricky. We're updating lots of our operational plans this year, we've used UTM coordinates for many things, but Lat/Long for coordination with air medical evacuations and river searches. Suggestions?

Not an answer to the question, but how big is the difference between the various datum? Is it significant from the standpoint of air search?

Martin, UTM is not datum. You message is confusing to me as to what you're asking.

When I plot ground coords I usually use NAD27 because that's what the USGS maps use. The error can be 0.1 mi if coordinates in one datum are entered in another (seen it).
Coordinate systems between UTM and Lat/Lon would best be worked out with the agencies you talk to. My bet is that aviation agencies, and their flight GPS units use Lat/Lon

unimogbert

The difference may be big enough to put a ground crew on the wrong side of an obstacle.
The real issue seems to be the conversion between UTM and Lat/Lon. Not having a common standard is a bad thing in stress situations. This introduces an additional opportunity for making mistakes. The decission is not easy as the ground crew can calculate distances from UTM more easily and experienced members may even know the location from the coordinates. Air crews may prefer the Lat/Lon format.
I´d prefer the Lat/Lon format as it is more continous because there are no zones. With GPSrs being common and mostly reliable the calculations should no be too hard to do. I´d wait until the maps have to be replaced anyway. I guess that it would be expensiv to throw away maps in good condition just for the sake of standardizing.

sorry, i meant to say,
With UTM, what is the most common Datum to use and more importantly, if I want to coordinate lat/long with UTM, do I have to be concerned that the aiviation folks want/need/care about the Datum we're using or do we just say

We need you to look in a radius from (some cordinate) and that's all there is to it, no datum required.

Here's the thing.

UTM is just so sweet for land nav. I can tell you to search a 1,000 meter radius of point X and you don't even need to think - just look at your position, measure out a few units, done - you have your radius and that's it, you go north, you go west, you go east, you go south until the GPS numbers say this, and I don't have to be a mathematics whiz to tell you how far to go.

With Lat/Long it's just a PITA.

To confuse the issue the government is somewhat forcing the National Grid system which is similar to UTM. I wondered myself when/If they would force aviation to National Grid.

I don't do SAR myself, but if you were to tell me to search a 1000 meter radius I'd be more concerned that the missing person might be located 1100 meters from the center coordinate rather than what datum you used to specify that center coordinate. i.e., your judgement to come up with that 1000 meter number might be more questionable than the datum you used to specify the center point. It's not like you're out geocaching looking for a very specific location, you're searching a larger area for a victim that could be almost anywhere. And when you're searching from a moving airplane high in the air, does 0.1 mile datum error really come into play?

I had a huge datum vs coordinate system post typed, then I lost it all... logged back on and see it isn't needed.

UTM is great, and we operate with it more often then not, however the aviation folks (and wildland folks) don't use UTM/UPS. My solution to this when using a GPS is to change my trip computer display to show both UTM and Lat/log (most garmin units allow this) this allows me to give my position immediately when talking to an aircraft.

There is no easy way if they are giving you a coord in lat and long, I just write it down and the swicth my GPS over to Lat/long and enter it... a PITA but I haven't found an easier way.

If no GPS or my GPS is giving me the finger at the moment, I carry a UTM and a Lat/long plotter card for my map. That way I have the ability to figure locations out either way. I draw both my UTM (solid) and Lat/long (wide gap dashed) on my maps.

As for datum, most Aircraft GPS systems can change datum as needed. Not all though, I have encounted a few pilots at wildland fires that were unable to change their Datum on the A/C GPS... almost all had personal GPS with them that they could change.

Aircraft or other ground unit, I always ask "What Datum are you using" when getting coords from someone. You would be supprised how many people never check to make sure their map and gps datum match!

I´m aware of the advantages of UTM. I found that it´s a matter of being to used to a format and having a map with the right grid. When it´s actually about searching a circle with a certain radius the GPSr shows the distance without any calculation. When it´s about searching a rectangle it´s just as easy to say start at point X go 00°01´ instead of 1000m (I know it´s not the same but it doesn´t require much math either). Eventually it all comes down to being used and having the right maps (using Lat/Lon on a UTM grid or vice versa is a PITA for sure). It may also require slight adaptations of the search patterns but it should be feasiblle.
I have my preferences and I´m perfectly comfy when you use what works for you.

My expectation is that the normal default datum is WGS83. But again, you ought to check with your air assets. Their databases ought to be to some standard that the FAA and the manufacturers have agreed upon. (all assuming USA application)

I've had trouble with website visitors not changing their datum and then blaming my coordinates for being wrong. It happens.

unimogbert

My Garmin has WGS 84, no WGS 83. Isn't NAD27 also fairly common in North America?

It seems if you've got crews on the gound using maps, then it is much easier to have everyone using those maps' datum - probably NAD27 on USGS topo maps. If the maps are setup for UTM then that is probably best, otherwise they'll have to use Lat/Long.

It seems much easier for GPS users in the air (or on ground) to convert to whatever datum & coordinate system than for a map user to change.

Just my $0.02.

Ken

BTW, I have sooo much respect for you SAR folks out there. You're heros in my book.

FWIW, i just looked at my maps here and the USGS 7.5m series are with UTM in NAD27 with offset ticks for NAD83 which for SoCal are to the east by about 100m.
Now, the LA sectional chart says at the top "horizontal datum: north american datum of 1983 (equivalent to world geodetic system 1984). Arggg..

this site datum/lat long conversions is pretty slick - you can enter coordinates in one datum and convert to another, lat/long to utm and back, etc... (continental US only, sorry).

There's another site I only have bookmarked at work that gives utm coordinates on either the 1927 or 1983/84 datum (switch back and forth at will) as you move your cursor over 1:24k quad maps or sat. map. I'll try to send that one tomorrow if anyone is interested.


Both can be handy, esp. if say, your crew has collected data on different gps units set to different datums...

Greetings, While I was on active duty and involved in some flight ops, I kept/used USGS Joint Air Ops Charts. These are topographic w/aviation (VFR) info included. I found these extremely useful for overland SAR missions since our AirSta didn't keep anything but VFR and ocean Nav charts on hand. I have made a few special deliveries to AirSta New Orleans and/or Lakefront airport to get these charts into the hands of the Pilots before they left for a SAR mission up north.
These charts have both UTM and Lat/Long ticks and contain considerable detail. I still keep a few of them on hand for backpacking/camping because of the amount of detail (includes dirt roads).
The charted info is USAF Sat derived and may be a few years behind in updates as far as CONUS areas are concerned (we haven't been invaded since 1814) but they are certainly more detailed than anything else you are likely to find and were designed with the purpose you have in mind.
IRT the chart datum to use, I have found that some chart datums are more accurate in some areas versus others. In the Black Sea and much of the Carribbean, the Naval Hydrographic charts have much to be desired, but the British Admiralty charts of the Med (and Black Sea) were on the money IRT the GPS coordinates. NOAA charts were the overall best in U.S. waters and as far out as they were useful (including crossing the N. Atlantic Ocean. Comparison of gps position, known position, and the various datum charted positions will generally tell you which is the best to use in whatever area you are operating in. In CONUS, I find the 1927 datum to be about the overall best when covering a large area in a short amount of time. For a small area, the 1984WGS seems to work best.

And the answer is NO, I did not have the "NORMAL" USCG career path. <img src="/images/graemlins/tongue.gif" alt="" /> <img src="/images/graemlins/grin.gif" alt="" />

http://oregonstate.edu/~reeset/html/scale.html

Measure the distance between gridlines and plug it in to get your map scale.

Here in N. CA we use both NAD84 and 27 depending on the maps.
People lost, tend to be in remote areas that are the last ones
updated by USGS and so we sometimes use the older map data if that is where the search is.

Some planes and copters we have dealt with cannot (or won't)
use UTM. And ground searchers in terrible conditions and dangerous
terrain shouldn't be using mapping methods that slow them down
(Lat/Lon). So we use both. And practice.

We also have use of a communications van which does the math for us.
If we are in a snow storm at night, it can be a bit hard or dangerous to stop
and figure out the other location data, so we simply call the com van with our utm coordinates and they can then speak to the planes using Lat/Lon. We also, of course, give redundant location information (land marks, section corners etc.) The com van will then help us plan a route around avalanche terrain and other obstacles and give us compass bearings etc. even if we can't see where we are going. They also track everyone on
computer and when a team returns they plug in the GPS and download
the route. At least once, we found the missing when the computer map
showed a small hole in our search pattern.

It is nice if your GPS units can quickly toggle back and forth from UTM
to Lat/Lon. We have a lot of older Etrex that take a bit of fiddling to
convert. Since we usually are in hasty teams of 3 or 4, one team member
can leave his GPS unit on Lat/Lon if we need to coordinate directly
with the planes and copters.

There is an advantage to just being able to read 6 numbers off as to your
location (UTM) when the radios are breaking up.

There is no WGS83 that I am aware of, but there is a NAD83.

Ken, it isn't a map user or a GPS user thing. You can't do SAR work with just a GPS, and you shuoldn't do any land nav without a map. The problems lies when the maps on the ground differ from the GPS in the air or other ground units...

WGS84 is the U.S. Military datum, and is the inherent internal datum in the Navstar GPS system. NAD83 is the U.S. Civil Datum. The difference between the two is in the centimeter range. Unless you are doing high precision engineering, you can generally feel free to use the two interchangably.