I saw this on the net today and I think it’s a great idea.
Personally I use Morse code, But this limits me in two ways.

1. I need a electronic medium to transmit over great distances.
2. The person receiving the message needs to know Morse code.

With his you could easily use a light or any noise maker.

Here is the Wikipedia page on it.

https://en.wikipedia.org/wiki/Tap_code

Here is a link to it over at   Survival Tek   http://survivaltek.com/?p=4617

Or a video by trailkeeper on youtube
http://youtu.be/b-3F_zuQ1Qk