Thread: Garmin TransAlpin 2012 Pro

The part number contains 12 characters (example 010-D1160-00).
It is coded in binary with 9 Bytes.

So when you have your 9 bytes ( example 10 D4 40 56 14 91 0D 14 41 or more precisely 41 14 0D 91 14 56 40 D4 10 because you have to reverse the bytes order) , this represents 8x9 bits i.e. 72 bits.
Group your 72 bits by 6 : so you have now 12 groups of 6 bits.
Each group of 6 bits give you an integer between 0 and 2^6 - 1 i.e 63 .
Add 32 to these values : you have now 12 integer between 32 and 95 : each of these integers give you a character of the part number (using ASCI coding)

Originally Posted by GPSFranz

So they saved 3 bytes (a 12-character part number coded in 9 bytes).
This looks like a lot of effort to save 3 bytes (in files that usually measure in hundreds of megabytes).
What was the reason for all this effort?
Are there any technical reasons?
Did they believe that their coding scheme would not be discovered?
Or what?
(Scratching my head...)

From what I can see, the main reason is indeed to save space, a 25% saving. By design, Garmin reserved some areas in their img header for future use and expansion. As time goes, these reserved areas gradually become defined and used. The 9-byte part number at offset 48 is just one example. It is obvious that you want to minimize the use of reserved areas as much as possible so there are more rooms left for the future.

Originally Posted by syzygy

From what I can see, the main reason is indeed to save space, a 25% saving. By design, Garmin reserved some areas in their img header for future use and expansion. As time goes, these reserved areas gradually become defined and used. The 9-byte part number at offset 48 is just one example. It is obvious that you want to minimize the use of reserved areas as much as possible so there are more rooms left for the future.

I totally agree with this analyse: Garmin wants to save space, not in the whole img but in the header.