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Forests disapear in higher zoom levels (lower scal factors). Why?

1
1

Look at the two consequtive zoom levels

https://www.openstreetmap.org/?lat=47.792&lon=-85.794&zoom=9&layers=M

and

https://www.openstreetmap.org/?lat=47.791&lon=-85.793&zoom=10&layers=M

In the level 10 the green (assume forest) on some islans disapear but not on the other land areas. Is there a special reason for that?

asked 27 Feb '13, 22:06

sanser's gravatar image

sanser
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accept rate: 5%

edited 28 Feb '13, 10:16

gormo's gravatar image

gormo
2.9k32660


One Answer:

3

This is an issue in the data, not in the rendering.

Problem : this island is modelized two times.

First time with a single closed way (polygon) tagged with "natural=land":
https://www.openstreetmap.org/browse/way/71638376
and being part of the relation "Lake superior" as "inner" polygon:
https://www.openstreetmap.org/browse/relation/1120169

Second time with several polygons or simple ways:
https://www.openstreetmap.org/browse/way/71638569
https://www.openstreetmap.org/browse/way/71645461
https://www.openstreetmap.org/browse/way/72372507

being part of 3 relations of type "multipolygon" as "outer" and tagged with "natural=wood":
https://www.openstreetmap.org/browse/relation/1117973
https://www.openstreetmap.org/browse/relation/1117939
https://www.openstreetmap.org/browse/relation/1117953

As you can see, the same surface is identified twice as "natural=land" and "natural=wood". Then the renderer will probably select only one of them, depending how the order of the data is processed and the rendering styles which varies at the different zoom levels.

To fix this, a better modeling would be to delete the 3 artificial polygons, keep only the coastline ways, remove the tag "natural=land" in the Lake superior" relation (but keep its "inner" role), and merge the 3 "natural=wood" multipolygon relations into a single one (with the same tags).

answered 28 Feb '13, 10:18

Pieren's gravatar image

Pieren
9.8k2083157
accept rate: 15%

edited 28 Feb '13, 10:19

1

Is any of the QS tools able to detect such problems?

(28 Feb '13, 11:58) mmehl

Source code available on GitHub .