private static char checkZone(final char inZone) {
        final char zone = Character.toUpperCase(inZone);

        if (zone < 'A' || zone > 'Z') {
            throw new IllegalArgumentException("Invalid UTMPoint zone letter: " + zone);
        }

        return zone;
    }
Method that provides a check for UTM zone letters. Returns an uppercase
version of any valid letter passed in, 'A' .. 'Z'.

@throws IllegalArgumentException if zone letter is invalid.
main/src/main/java/cgeo/geocaching/location/UTMPoint.java