I suppose that depends on whether you assume that the person calling for help is looking at the sign when he calls, or whether he's lying on the ground trying to remember the last sign he passed.
If the former, you can put whatever you want on the trail, such as the latitude and longitude.
The latter is more realistic and helpful, I think, and also allows you to minimize sign-clutter. The trick is you want people to remember a sign they may have passed some time ago. Just being an armchair psychologist here, among the things that it seems would make a sign more likely to be remembered are: it's meaningful to the reader, it's different from other signs, it uses words and names that have associations, it has pictures. Example: something like "Joe Jones Hill Ahead," with a picture of a guy wheezing up a hill would be memorable, particularly if Joe Jones is someone of local reknown and the picture looks like him.