# [mkgmap-dev] Problem with splitter

From Wolfgang Hammel wolfgang.hammel at gmx.de on Sat Mar 17 09:37:32 GMT 2012

```Hi Gerd,

my proposal from yesterday needs some refinement in order to provide
enough information to mkgmap.
I have attached some sketches that might help to explain the need for
that extension.
Consider the different situations in the left and right column.

Situation 1: (left column, top)
we have a multipolygon consisting of 3 ways:
way no. 1: nodes 1, 2, 3, 4
way no. 2: nodes 4, 5, 6, 7, 8, 9, 10
way no. 3: nodes 10, 11, 12, 1

only nodes 2, 3, 11 and 12 are inside of the extended bounding box I was
according to your new algorithm ways no. 1 and 3 will be fully included
in the tile,
that makes nodes 1, 2, 3, 4, 10, 11, 12 to be written to the tile
and ways no. 1 and 3 are written to the tile
way no. 2 is also included in the tile by your new algorithm as it
belongs to a multipolygon, that is included in the tile
but for way no. 2 only the first and last node are written to the tile
(nodes no. 4 and 10, which are already included
through ways no. 1 and 3 respectively)
and way no. 2 will be tagged as an "outside way"

in the second row the shaded areas are the ones the have to be the
output of mkgmap
in situation 1 this makes two subpolygons (hopefully mkgmap can handle
this correctly...)

Situation 2 (right column, top)
we have a multipolygon consisting of 3 ways:
way no. 1: nodes 1, 2, 3, 4 (same as in situation 1)
way no. 2: nodes 4, 5, 10
way no. 3: nodes 10, 11, 12, 1 (same as in situation 1)

again ways 1 and 3 will be included and so are all their nodes
that again makes nodes 1, 2, 3, 4, 10, 11, 12 to be written to the tile
way no. 2 is an outside way and marked as such by an additional tag
for way no. 2 we drop node 5 and write only nodes 4 and 10 to the tile
(these are already included
through ways no. 1 and 3)
So mkgmap has exactly the same data as input as in situation no. 1
but in situation 2 a completely different area has to be te output of
mkgmap, shown
in the right column second row.

This gives the need for more detailed information passed to mkgmap if we
want to omit the
nodes of the outside ways.
I would propose to store the angle between the first and last node of
the outside way
measured against the center of the tile.
This is shown in the third row for both situations. In situation 1, the
outside way circumnavigates the
tile in positive mathematical direction covering +300 degrees on its way
from node 4 to node 10.
In situation 2, the outside way covers a negative angle of -60 degrees
on its way from node 4 to node 10.
So storing that angle as an additional information should give mkgmap
the ability to eventually
reconstruct the area of the multipolygon that falls inside the bounding
rectangle.

In fact it would be sufficient to store a single bit that tells mkgmap
if the outside way passes in
clockwise or counter clockwise direction from the beginning node to the
end node.
But I think this may lead to some errors due to numerical rounding when
both the first and last
node of an outside way have nearly the same angle as measured against
the tile center.
So this is completely avoided when the angle itself and not just the
sign of the angle is written to the tile.

And a final remark:
As a possible optimization a consecutive sequence of outside ways may be
stripped together
into one single outside way. This may be useful if we think of the

I don't know how complicated this will get to be implemented in splitter
and mkgmap,
so these are just my thoughts on a possible solution.

Wolfgang

Am 16.03.2012 12:24, schrieb Gerd Petermann:
> Hi Wolfgang.
>
> thanks for you input. See below..
>
> > Date: Fri, 16 Mar 2012 00:11:58 +0100
> > From: wolfgang.hammel at gmx.de
> > To: mkgmap-dev at lists.mkgmap.org.uk
> > Subject: Re: [mkgmap-dev] Problem with splitter
> >
> > 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
> > algorithm.
> > 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.
>
> Yes, this problem occurs, and my new algorithm can fix that problem
> because it allows to write all points of a way to each tile that it
> belongs to.
>
> > 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
> > have
> > missing nodes.
> > This information could be used by mkgmap to correctly reconstruct the
> > multipolygon.
> > 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
> > boundaries
> > 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.
>
> I like the idea of writing only one new tag instead of many points and
> ways.
> The problem is that it is quite difficult to calculate the original
> shape in
> splitter without blowing up memory.
> I have ideas for this, but it will take a few days to think it over.
>
> Gerd
>
>
>
>
>
>
> >
> > Wolfgang
> >
> > 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
> > > 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
> > >> 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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> > >>>>>> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
> > >>>>>>
> > >>>>> --
> > >>>>> View this message in context:
> > >>>>>
> http://gis.19327.n5.nabble.com/Problem-with-splitter-tp5555886p5558068.html
> > >>>>> Sent from the Mkgmap Development mailing list archive at
> Nabble.com.
> > >>>>> _______________________________________________
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