http://www.nacgeo.com/nacsite/

The Natural Area Coding System (NAC) is developed by Dr. Xinhang Shen of NAC Geographic Products Inc. (Canada).
This code assumes that whole area on earth is rectangle in Mercator projection.
Divide latitude into 30 area and express in base 30 using table below. Then divide it subsequently by 30 again, and repeat until required precision.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0 1 2 3 4 5 6 7 8 9 B C D F G
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
H J K L M N P Q R S T V W X Z
Letter not to be used "A", "E", "I", "O", "U" and "Y"
Divide longitude value in the same way separately.
Longitude part, space, and latitude part consists NAC code.
I’d like to mention that NOT using bowl is a good idea. First, I and O usually trigger errors as 1(one) and 0(zero). Second, sounds of bowl may differ and confusable among various languages.

It is unique that NAC has a option for altitude information.

Sample Code:
8KDB PGFD
8KD8 PGGK H000 (With Altitude option)