[mkgmap-dev] Still problems with lakes

Tue Sep 18 08:43:50 BST 2012

Hi,

just to let everybody know:
I began development on an improved splitter to handle this problem some months ago, but I got stuck after 
my system crashed when installing an SSD (my fault). After reinstalling around 10 times I was really pissed with computers and than I started planning (and doing) two longer bike rides.

I am not sure if I find the energy to start again with splitter, my solution is somewhat half done.
It turned out to be quite complex to collect all needed information without requiring huge amounts of memory or writing large temp files when splitting e.g. Europe .
As a first work aproach I decided to implement multiple read passes, which is quite slow with pbf format, so I also implemented reading (and writing o5m format), 
which makes splitter much faster.

The major open problem: Many polygons cross tiles, but most of them do not need special handling (think of all the houses near tile borders)
You cannot decide that before you know all nodes (not just the ids, but the position) and tags of the polygon.
It is impossible to collect all needed infomation for all polygons in heap, so one needs clever filters and effective data structures to filter
only the really problematic polygons. 
I must confess that I did not find these clever filters after weeks of thinking, so maybe I don't see the forest because of the trees, maybe it is really too complex for me.
A completely different approach came into my mind today:
Instead of implementing a lot of logic to find the polygons, it might already help if we implement a function that allows to specify polygon ids that cause problems if they are incomplete, 
and splitter would simply make sure that those polygons are completely written to all related tiles? 

If someone is willing to help or continue, I can create a patch for a new branch, else maybe the winter will bring me back to the computer ;)

Ciao,
GerdP

> Date: Tue, 18 Sep 2012 08:26:08 +0200
> From: ligfietser at online.nl
> To: mkgmap-dev at lists.mkgmap.org.uk
> Subject: Re: [mkgmap-dev] Still problems with lakes
> 
> It is not only large lakes that are affected, also large forest or other
> multipolygon areas are sometimes broken at the tile borders.
> This need to be fixed in the splitter. Sometimes it helps making the overlap higher (5000 or more).
> But this is just a workaround...
> 
> > Would it be difficult to have precompiled water rather than sea?
> > 
> > It would save messing about with area files.
> > 
> > Regards, Geoff.
> _______________________________________________
> mkgmap-dev mailing list
> mkgmap-dev at lists.mkgmap.org.uk
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