[mkgmap-dev] Small holes in boundary coverage

Tue Mar 27 08:35:04 BST 2012

Hi WanMil,

I found three reasons for these holes: errors in BoundaryQuadTree, "errors"
in java.awt.geom.Area and rounding erros. 

My last patches fixed the errors that I found in BoundaryQuadTree, I also
tried to avoid the problem
with the rounding errors (we are doing the calculations with double
precision, but we save with float
precision). 

I stopped  searching when I saw fewer holes in the result of the performance
branch
than in the result of trunk. 

The errors in java.awt.geom.Area are complex. I think one has different
options to calculate the intersection of two areas a1 and a2:
a) Area x = new Area(a1); x.intersect(a2)
b) Area x = new Area(a2); x.intersect(a1)
c) Area x = new Area(a1); x.add(a2);x.subtract(a1);x.subtract(a2);

In an ideal world I would expect to get exactly the same result for these
three calculations, but sometimes the real results are not equal. The same
problem occurs when we add the areas again in the BoundaryCoverageUtil.

Maybe we can get rid of a few more of the holes if we manage to do all
calculations with doubles, but probably even doubles will not solve all
problems.

The question is if we should invest time for that. I think the holes are
purely cosmetic, if you really manage to query BoundaryQuadTree for a point
that lies within such a hole, it will return the result for a point that is
next to it, I think this is good enough.

What do you mean?

Gerd

WanMil wrote
> 
> Hi Gerd,
> 
> thanks for the patch. I have commited it although I still see the 
> problem. I am not sure if the holes are at the same place but there are 
> some of them which I cannot explain with wrong boundary data.
> 
> Can you please recheck that?
> 
> Thanks!
> WanMil
> 
>> Hi WanMil,
>>
>> attached is a fix for this problem.
>>
>> Please, can you review if the UnusedElementsRemoverHook is still useful?
>> With my test data, it is slowing down mkgmap a little bit and I also see
>> a different result for one tile in the UK when I disable it.
>>
>> Gerd
>>
>>  > Date: Thu, 15 Mar 2012 21:11:02 +0100
>>  > From: wmgcnfg@
>>  > To: mkgmap-dev at .org
>>  > Subject: [mkgmap-dev] Small holes in boundary coverage
>>  >
>>  > Hi,
>>  >
>>  > I have compiled world bounds using the performance branch. They can be
>>  > downloaded from http://www.navmaps.eu/wanmil/bounds_perf_20120313.zip.
>>  >
>>  > I have checked the coverage of different admin_levels with the
>>  > BoundaryCoverageUtil. The result for admin_level=2 can be downloaded
>>  > from http://files.mkgmap.org.uk/detail/61.
>>  >
>>  > Some countries are missing (USA, Canada, some african countries). I
>>  > assume that their boundaries were/are corrupt.
>>  >
>>  > The more interesting thing is that there are small holes throughout
>> the
>>  > world in admin_level=2. They seem to be created by the bounds
>>  > preparation and should not be there. Gerd, can you please have a look
>> on it?
>>  >
>>  > Thanks!
>>  > WanMil
>>  > _______________________________________________
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>>  > mkgmap-dev at .org
>>  > http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
>>
>>
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