# [mkgmap-dev] Problem with splitter

From Gerd Petermann gpetermann_muenchen at hotmail.com on Sun Mar 18 06:14:48 GMT 2012

```Hi Wolfgang,

I think your solution is too simple. Using your graphik, imagine node 6 would lie somewhere inside the bbox.
Or, for a more complex case, imagine a zig-zag line going in and out (sorry, I have no experience creating graphics).
In these cases, you have multiple angles, and it will be required to store the angle and the point(s) to which it belongs.
I fear this is too complicated.

Gerd

Date: Sat, 17 Mar 2012 10:37:32 +0100
From: wolfgang.hammel at gmx.de
To: mkgmap-dev at lists.mkgmap.org.uk
Subject: Re: [mkgmap-dev] Problem with splitter

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

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

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

> 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

> > 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

> >> 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

> >>

> >> 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

> >> 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

> >>> 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

> >>>>>>> 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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> >>>>>> mkgmap-dev at .org

> >>>>>> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev

> >>>>>>

> >>>>> --

> >>>>> View this message in context:

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mailing list archive at Nabble.com.

> >>>>>
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