[mkgmap-dev] How to tag artificial cut lines in multipolygons

Wed Feb 17 20:47:23 GMT 2010

> Hi WanMil and others,
>
> Some time ago, there was discussion that the MultiPolygon code in
> mkgmap should tag the lines that it generates when splitting
> multipolygons to polygons.
>
> I think that there is a genuine need to split large multipolygons to
> smaller multipolygons in the OSM data too. For example, I generated
> multipolygons for the lake Saimaa in the past couple of weeks. I split
> it to maybe 10 or 20 multipolygons, some of them still huge (hundreds
> of ways in the relation, many of them consisting of 500 nodes).
>
> A fellow mapper who generated the natural=coastline for many Finnish
> waters in the first place got upset of the artificial splitting. I
> suggested a solution where the artificial split lines would not be
> given any natural=water or natural=coastline tag, but something else,
> say, artificial=yes.
>
> What do you think? Is there an existing tag that we could use for this?
>
> Is there now a multipolygon around the Canarian Islands? I could not
> find one. I am considering defining some multipolygons with some
> artificial cut lines on the Finnish coast, so that I would get water
> without --generate-sea and could work around the limitations of the
> Geofabrik map extracts by defining the borders of the sea multipolygons
> within the Geofabrik cutting boundary.
>
> 	Marko

Hi Marko,

I am still thinking about the former discussion but I haven't found a 
good solution yet. I don't know the style section very much. Maybe 
that's my problem. I will post a patch for the result of the discussion 
soon, so that you can play with it (it should not be committed...). 
Hopefully you will get more good ideas how to solve the mp tagging problems.

Regarding your questions I reply with some other questions....:
* Why do you split large multipolygons to smaller ones?
* The artificial lines (I don't know a tag for them) are not really 
useful for the mp code. It can handle full polygons only and there is no 
chance to tag a part of a polygon as artificial (and prevent the garmins 
to draw a border line on them).
* Another idea: why do you want to tag the artificial lines?

WanMil