[mkgmap-dev] bugreport for new splitter

Mon Aug 10 15:14:30 BST 2009

Chris Miller <chris.miller at kbcfp.com> writes:

>> For really old devices that have limited memory you may only be able
>> to load a few tiles at a time and then it would be more flexible to
>> have smaller tiles.  But this is really from before OSM was started,
>> I've never had anyone complain that tiles were too big, in fact quite
>> the opposite.
>>
>> So ideally you want to be able to make the tiles as large as possible.
>> However if you only look at the number of nodes then you find that
>> you have to have it quite low to cope with one particular area
>> when a higher value would have been fine everywhere else.
>> ..Steve
>
> Thanks for the explanation. Is there anywhere you know of where I can read 
> more about what the known limits on tile sizes/content/quantities are? I've 
> seen various comments about a 2025 map segment limit (is a map segment the 
> same as a map tile?), a 2048MB limit (due to 32 bit indexing in the file 
> format and/or file system?) on forums like these:
>
> http://garminoregon.wikispaces.com/message/view/home/10590340
> http://forums.groundspeak.com/GC/index.php?showtopic=170615&st=54#
>
> But it's still not completely clear to me what's going on. In particular, 
> what determines the maximum tile size? You're saying it's not the number 
> of nodes, but perhaps the number of ways? Or is it even more complex than 
> that, ie a factor of the node/way/relation count combined with the number 
> of nodes per way and/or complexity of the ways?
>
> If the exact criteria was known then maybe we can come up with a
> better approach to choosing the area boundaries to split on.

I suspect there are a bunch of limits in the img format and maybe in
Garmin firmware that parses it and if you exceed any of them there is a
problem.  Certainly the ones above exist, but I wouldn't be surprised if
there are more.

For receivers with a 2GB uSD, I think one wants tiles pretty big.  I
have 2009 vintage Garmin proprietary maps, and all of New England is in
2 tiles, and the .img I think are about 25 MB each.  I also have a 2002
or 2003 vintage receiver and proprietary map data, and that has tiles
that are about 1-4MB.  This lets me choose what I want to fit in the 19
MB internal memory.  There are still some devices like that around and
useful, so I can see a demand for ~3 MB tiles.  But, for the 2GB types,
tiles that are more like 25 MB seem better.

There may also be an effect where smaller tiles makes the receiver draw
maps faster, but I think the internal TRE scheme means that isn't true.
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