[OSM-dev] Coastlines and water - contact and technical

Ben Supnik bsupnik at xsquawkbox.net
Sun Jul 19 20:29:01 BST 2009


Hi Jon,

Jon Burgess wrote:

> This is one of the things which has to be done but is not really
> documented. The algorithm osm2pgsql uses is to gather together all the
> tags which are related to the polygon feature (e.g. landuse=, natural=
> etc) and then compare those against the outer ways. If the way and
> polygon match then it will exclude the way. If they do not match then
> the way will be included.

Ah right - otherwise the outer way will form a no-holes polygon, which 
is probably wrong..and if the holes don't have their own map rendering 
representation, the fill might show through.

(Not to mention the interpretation would be wrong. :-)

> In particular, there is some legacy data which has no tags on the
> relation but identical tags on both the outer and inner ways . In this
> case, the polygon needs to be rendered with the appropriate tags but
> neither the inner nor outer ways are rendered.

Oy...that data strikes me as really broken. :-(

In particular, defining anything as having "same" or "different" tags 
strikes me as perilous because tags are sets of values...if one of a 
relation's ways has had additional data tagged onto it, is it now 
"different"? :-)

(For example, someone tags the boundary of an island as a walking 
path...the island boundary is tagged as lake because it's the hole in 
the lake and someone wanted to tag everything...are we now different? 
Is the island now wet? :-)

cheers
bne

-- 
Scenery Home Page: http://scenery.x-plane.com/
Scenery blog: http://xplanescenery.blogspot.com/
Plugin SDK: http://www.xsquawkbox.net/xpsdk/
X-Plane Wiki: http://wiki.x-plane.com/
Scenery mailing list: x-plane-scenery at yahoogroups.com
Developer mailing list: x-plane-dev at yahoogroups.com




More information about the dev mailing list