[mkgmap-dev] question regarding MapRoad

Sat Oct 26 16:38:24 BST 2013

Hi Steve,

> 
> Sounds good. I'm not sure what to suggest here. What we have works,
> although it may be sub-optimal in some way. I never had a theory on
> how sub-divisions should be used and I was kind of suprised that the
> first thing I tried seemed to work (this was before routing).

If I got that right, the device has to read all RGN data for all sub divs 
which intersect the selected part of the map when it has to render
the picture that should be displayed. 
So, a subdiv that contains many objects located somewhere in the middle 
and one large object that makes it much bigger in size
might be something that should be avoided, 
as it creates a large subdiv area, overlapping many other, without
containing much info that is useful.
I have no idea what kind of caching the device uses.

Maybe it is useful to detect large objects and place them in their own sub
divisions? With large I mean big dimension.

Another question is whether we shoud try to put as many objects as 
possible into one subdiv. I could imagine that the device is happier 
if it finds more, but smaller subdivs. I think it is worth trying.

Another interesting point: Empty subdivs. 
I don't see many of them in my Garmin map, but mkgmap creates quite a lot.
An empty subdiv is created when we split a subdiv into two equal halves, and all
data is in one half.  It is quite simple to change the current algo to 
split again at a different position so that both parts  contain more or
less the same amount of data.

Gerd

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