[mkgmap-dev] DEM file and the block-size option

Sat Jan 6 20:27:53 GMT 2018

Hi Andrzej,

for MDR mkgmap sets the blocksize to 16384 if not set otherwise with the block-size option:
params.setBlockSize(args.get("block-size", 16384));

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Andrzej Popowski <popej at poczta.onet.pl>
Gesendet: Samstag, 6. Januar 2018 20:39:12
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] DEM file and the block-size option

Hi Gerd,

I have tested some img, changing block size with GMapTool. These are
results, first original file size, then size delta for blocks 1k, 2k and 4k:

14.8k --  +3.6k  +9.9k  +22.0k
109k  --  +2.6k  +7.7k  +16.9k
1M    --  +0.0k  +5.2k  +17.7k
2M    --  -1.6k  -0.5k  +10.7k
3M    --  -3.6k  -3.6k   +2.6k
4M    --  -5.7k  -4.7k   -0.5k
8M    -- -14.3k -18.4k  -16.4k

Looks like 2MB is a tile size, above which block size 2048 can be
better. I would change default. You could think about optimization, if
you are interested, but gain won't be big.

Side notes:
Block size for overview map is probably always 512, can be problematic
for very big maps with DEM.

There is something wrong with FAT in MDR. FAT shouldn't be bigger than
about 140kB. Mkgmap creates MDR with FAT of 3.2MB, almost all empty.

--
Best regards,
Andrzej
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