====== GARMIN map checksum gmapbmap.sum ======

On my new **GARMIN eTrex 10** there was a **pre-loaded map** (the quite useless **worldwide basemap**). One of the first thing I did, was to replace it with an [[garmin_etrex_10_maps|home-made OpenStreetMap based map]].

Once connected to the computer using the USB cable, I found the following files on the USB storage:

  * **Garmin/gmapbmap.img**
  * **Garmin/gmapbmap.sum**

The **gmapbmap.img** file is obviously the map file, but what the hell is the **gmapbmap.sum** file instead? The first thing that you think of is a **checksum** to verify the integrity of the file. It is right, but what is the algorithm used to calculate that checksum? I tried several standard methods, like CRC32 or XOR parity, but without success.

It turned out that the file is just **the sum of all the bytes** of the map, packed into the sum file as a **little endian 64 bit integer number**.

It is not clear to me what happen **if the sum will exceed the 64 bit limit** (it can theoretically happen with a map with file size of 16 Mb), so please report to me your cases (map files and sum files) so that I can investigate.

===== Is the map checksum actually used? =====

It seems that **the sum file is not used**, at least by my eTrex 10. Infact I uploaded my home-made map, leaving the original gmapbmap.sum file; the device actually used the map, without any error or warning message.

===== How to calculate or verify the GARMIN sum =====

Here you are a small Python script to calculate or to verify the sum file:

<code python>
#!/usr/bin/python
  
import os.path
import struct
import sys

__author__ = "Niccolo Rigacci"
__copyright__ = "Copyright 2020 Niccolo Rigacci <niccolo@rigacci.org>"
__license__ = "GPLv3-or-later"
__email__ = "niccolo@rigacci.org"
__version__ = "0.1.0"

def usage():
    print('Usage: %s [gmapbmap.img|gmapbmap.sum]' % (os.path.basename(sys.argv[0]),))
    sys.exit(1)

def calculate_sum(mapfile):
    checksum = 0x0
    with open(mapfile, 'rb') as filein:
        while True:
            chunk = filein.read(1)
            if chunk:
                checksum += ord(chunk)
            else:
                break
    return checksum

if len(sys.argv) < 2:
    usage()
arg = sys.argv[1]

if arg.lower().endswith('.img'):
    print('Calculating the GARMIN map sum for file %s...' % (arg,))
    checksum = calculate_sum(arg)
    print("The sum is (big-endian order): 0x%08X" % (checksum,))
    sumfile = arg[:-3] + 'sum'
    if not os.path.exists(sumfile):
        print('Writing sum (in little-endian order) to file %s' % (sumfile,))
        with open(sumfile, 'wb') as fileout:
            fileout.write(struct.pack('<i', checksum))
    sys.exit(0)

elif arg.lower().endswith('.sum'):
    with open(arg, 'rb') as filein:
        buf = filein.read(4)
        checksum = struct.unpack_from('<i', buf)[0]
    print('The GARMIN sum in file %s is (big-endian order): 0x%08X' % (arg, checksum))
    mapfile = arg[:-3] + 'img'
    print('Calculating the sum for map file %s...' % (mapfile,))
    checksum2 = calculate_sum(mapfile)
    if checksum == checksum2:
        print('OK')
        sys.exit(0)
    else:
        print('FAIL')
        sys.exit(1)

else:
    usage()
</code>