I agree with the above. Yes, X marks the spot, but actually I think it means like "medical help required" or something more specific than "help!"

SOS SHOULD mean something. Unless you're Canadian (eh!) employed to actually pay attention to those things. Kind of like the paramedics with that actress up in Quebec (but I digress, and as I understand things, she turned them away).

I think if you have time and resources, an SOS would probably be more useful than an X.

I wonder how many people are actually trained to recognize the one-letter ground-to-air communication signals?