How to tag artificial cut lines in multipolygons
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 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
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
participants (2)
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Marko Mäkelä -
WanMil