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

Wed Feb 17 20:57:06 GMT 2010

Hi WanMil,

> Regarding your questions I reply with some other questions....:
> * Why do you split large multipolygons to smaller ones?

I am expecting problems with some tools. Let us assume that Lake Saimaa  
is consists of 10000 ways with an average 300 nodes per way. If it were  
a single multipolygon, splitter would have to preserve it in all map  
tiles that contain it. Also, if someone makes a change to the coastline  
(for example, finds out that a half-island actually is an isolated  
island) he will have to download and upload the entire multipolygon.  
Furthermore, some bays or areas of the lake could have names.

I think that it could be useful to define multipolygons for certain sea  
coastlines too. For example, Suomenlahti, Saaristomeri, Selkämeri,  
Perämeri are Finnish names of parts of the Baltic sea around Finland.  
These could be defined as multipolygons, but it would feel wrong to tag  
the border between the parts with natural=coastline, as there is no  
clear border in reality ("a line drawn in the water").

> * 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).

Would it be hard to merge adjacent areas in mkgmap? You can identify  
them by the fact that all points of the the connecting lines are shared  
and the lines run in opposite directions.

> * Another idea: why do you want to tag the artificial lines?

Do you suggest using blank lines in role=outer? That would work, sure.

	Marko