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.