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[mkgmap-dev] Problem with splitter

From Wolfgang Hammel wolfgang.hammel at gmx.de on Thu Mar 15 23:11:58 GMT 2012

Hi Gerd,

your description of splitter's algorithm is in accordance to what I 
observed when I used some
simple test data.
But beside the mulitpolygon problem there is another issue that results 
from the present
If you consider one single tile, splitter writes a certain node to that 
tile if it is no more than
2000 increments (2^24 incr. <=> 360°) away from the tile's boundary.
I think this is the overlap which leads to a maximum of 4 tiles each 
node is written to.
If we consider a single tile this works like a frame that is 2000 incr. 
wide around the
tile's bounding rectangle and if a certain node falls inside this 
extended bounding rectangle
it is written to the tile.
Now consider a way that has some nodes inside the tile (the original 
tile without the extension)
and some nodes outside the extended tile boundary. All these outer nodes 
will not be written
to the tile. This may lead to a situation where a way for example starts 
inside the tile, has
a node at a certain distance from the tile's original boundary, then the 
leaves the tile without having
any node in the "extended frame" of that tile and finally some nodes 
outside the "extended frame"
where the way ends. So only the first mentioned nodes will be written to 
the tile and mkgmap
will be unable to generate the endnode on the tile boundary as all outer 
nodes are dropped.
However this situation is very unlikely as ways usually have nodes at 
smaller distances.
The width of the "frame" is about 4.77 km in north-south direction and 
3.07 km in east-west direction
at 50° latitude.
Splitters algorithm may lead to corrupted multipolygons with missing 
ways but
also creates corrupted ways with missing nodes, but that is less important
and may very seldom be a real problem.

Yes you are right, writing only the endpoints of the "outside ways" will 
not work.
This could be done if it would be possible to give those ways some 
hidden attribute
(maybe some additional tag, that is not used in the original OSM-data) 
that is
added by splitter and marks those "shortened" ways as outside ways, that 
missing nodes.
This information could be used by mkgmap to correctly reconstruct the 
The exact location of the outside nodes that gives the original shape is 
not needed for
this task by mkgmap.
It is true that just storing the end nodes will give errors as mkgmap 
would assume
a straight line between those end nodes which again may cross the tile 
which the original shape of the multipolygon doesn't. However if mkgmap 
would know
that this is an outside way by means of an additional tag in the outside 
way's data, it
has all the information that is needed to generate the correct data that 
falls inside the tile.
But this approach would also require modification of mkgmap.


Am 15.03.2012 08:51, schrieb GerdP:
> Hi Wolfgang,
> I've described splitters algorithm as it is now here:
> http://gis.19327.n5.nabble.com/Problem-with-splitter-tp5555886p5561551.html
> I am now working on a first approach that looks like this:
> pass 1: calculate tile areas
> pass 2:
> a) for each coord, way: find out all tiles that are "touched", save the info
> in a map
> b) for each relation, find out all tiles that are "touched", and add this
> information to the way map
> (so that each way belonging to a relation will be written to all tiles that
> is touched by the relation)
> pass 3: for each node of each way: add the info from the way map to the
> coords map
> (so that each coord belonging to a way will be written to all tiles that is
> touched by the way)
> pass 4: for each node, way, and relation: write to the output files
> pass 1 and 2 are more or less equal to the current version, only the writing
> is removed.
> Up to now I see no way to reduce the number of points without risking
> errors. My idea regarding
> "saving only the endpoints" will not work, a simple example shows that:
> Think of a relation that contains just two long ways. Each way forms one
> half of an elliptic area. If we only save the endpoints of these two ways
> (which should be identical), we only see two points and it is impossible to
> guess how the original shape looked like. So,
> we would need an algorithm that reduces only those points that are not
> needed, but I see no way
> to do this in splitter because it has to store too much information for
> that.
> So, let's see what happens if we write all points and ways...
> Gerd
> Wolfgang Hammel wrote
>> Hi Gerd,
>> my first thought was also in the direction of option 2)
>> but yes you are right, the administrative boundaries and the coastlines
>> may blow up
>> the tiles a lot and that may probably also increase processing time in
>> mkgmap afterwards.
>> Option 3) would be the most precise one but I don't know anything about
>> splitter's
>> internal structure, and this may be complicated and a lot of work because
>> splitter seems to have no interpretetation of the data it processes up
>> to now.
>> But what about the following option which is a combination of your
>> proposals no. 2) and 3) without the need for an additional file.
>> Enhance splitter in a way that it includes all the ways of a
>> multipolygon that finds its way
>> into the output of a certain tile.
>> But for the ways it would have dropped up to now only the first and last
>> nodes are written
>> to the output. As these ways always have no node inside the tile, this
>> woud give
>> exactly the same data to mkgmap after the polygons have been clipped by
>> the
>> tile's bounding rectangle.
>> The clipping procedure can be done in mkgmap without guessing any
>> missing data.
>> So this would increase the tile size only by a small amout and we have
>> all the data
>> we need in the tile.
>> But in order to give a consistent and correct osm-data file the
>> references to the
>> dropped nodes should be removed from those ways.
>> Otherwise we have the same situation as now where the relation of a
>> multipolygon contains "dead" references
>> to the ways that are not included in the tile's file.
>> As I did not have a look at splitter's code up to now, I'm not sure if
>> this
>> can be easily implemented.
>> By the way:
>> I tried to create a minimal working example where the problem can be
>> reproduced.
>> But this is not finished up to now. What I already know is that
>> splitter's algorithm
>> does not consequently drop all the ways that are outside the tile's
>> boundaries.
>> Maybe you know more details about the criteria splitter uses for
>> dropping ways?
>> Wolfgang
>> Am 13.03.2012 06:59, schrieb GerdP:
>>> Hi Wolfgang,
>>> yes, that' s exactly what happens. I see three ways to solve this
>>> problem:
>>> 1) Enhance the logic in mkgmap that guesses how the missing ways
>>> completed
>>> the multipolygon, e.g. by adding a backtracking algorithmn (this is
>>> already
>>> suggested in the code).
>>> 2) Enhance splitter so that it writes all points and all ways of
>>> multipolygon to each tile.
>>> 3) Enhance splitter to write one extra output file that contains only the
>>> 1st and last point of each way that is part of a multipolygon, and create
>>> a
>>> method in mkgmap that looks for this data when
>>> it doesn't find the way in the normal input. We need only the end points
>>> because we use the data only in cases where we know that they are outside
>>> of
>>> the bounding box. Maybe that can be done with osmfilter as well ?
>>> I did not start coding, but I think option 3) should be easy to do and I
>>> hope it solves most
>>> of the problems. Option 2) looks more difficult and will blow up tile
>>> sizes
>>> and CPU cost both in splitter and mkgmap. Option 1) can be done as well.
>>> Does that sound reasonable?
>>> Gerd
>>> Wolfgang Hammel wrote
>>>> Hi Gerd,
>>>> when I had the problem some time ago, I did some rough checking on
>>>> splitters output.
>>>> What I know so far is, that splitter removes all the ways from a certain
>>>> tile that have no
>>>> node inside this tile.
>>>> The problem arises when a tile boundary divides a multipolyon that
>>>> consist of normally
>>>> a lot of different ways. Tile splitter includes the complete relation
>>>> for that multipolygon
>>>> in the output including all the references to the ways that
>>>> mulitipolygon originally consisted of.
>>>> But as some of the ways are removed from the output, the multipolygon is
>>>> corrupted and
>>>> mkgmap is later no more able to correctly reconstruct the part (or
>>>> parts) of the multipolygon
>>>> that fall inside the tile.
>>>> Wolfgang
>>>> Am 12.03.2012 16:06, schrieb GerdP:
>>>>> Hi Matteo,
>>>>> okay, I am able to reproduce the problem (also without the coastfile
>>>>> parameter).
>>>>> The log shows some warnings for  relation 541757 (the Lago di Como) ,
>>>>> so
>>>>> I
>>>>> should be
>>>>> able to understand what's happening and why it fails.
>>>>> Gerd
>>>>> Matteo Gottardi wrote
>>>>>> 2012/3/12 GerdP&lt;gpetermann_muenchen@&gt;:
>>>>>>> Hi Teo,
>>>>>>> I tried to reproduce the problem with mkgmap trunk version r2248, but
>>>>>>> I
>>>>>>> get
>>>>>>> different results, esp. I don't see this flooding.
>>>>>>> I am using coastlines_europe-120128.osm.pbf, maybe your file is
>>>>>>> older?
>>>>>> Hi Gerd,
>>>>>> my coastlines file was the same as yours, only with a different name
>>>>>> :)
>>>>>> I did some tests. The results were a bit different because of my typ
>>>>>> and style files.
>>>>>> Using no typ file, the default style file and passing only
>>>>>> --generate-sea=multipolygon
>>>>>> --coastlinefile=coastlines_europe-120128.osm.pbf the result look like
>>>>>> this: http://www.gomatteo.net/17.png
>>>>>> PS: I would like to thank all the developers who spends their time
>>>>>> working on this great project, without mkgmap my gpsmap60c would be
>>>>>> useless :)
>>>>>> -- 
>>>>>> * Matteo Gottardi | matgott@
>>>>>> * ICQ UIN 20381372
>>>>>> * Linux - the choice of a GNU generation
>>>>>> * GPG Fingerprint:
>>>>>> * B9EE 108F 52C8 D50C B667 B1F2 AB56 8A01 BA3D 36A1
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