[OSM-dev] Simplified street-network

[marcus.wolschon at googlemail.com]

On Wed, 18 Mar 2009 11:30:18 +0000, Andy Deakin <andy.deakin at pcmend.net>
wrote:
>>> Hello everyone,
>>>
>>> did anyone else ever work on simplifying the polylines for
>>> low zoom -rendering or similar purposes?
>>> Any algorithms, websites or books that may help me?
>>>     
>>
>> There's been a small amount of work on this in mapnik-rendering
>> circles, but mainly using postgis' simplify(linestring, integer)
>> function. I don't have info from anyone who's looked into it further
>> from an algorithm point of view.
>>   
> 
> Check out the google maps encoded polylines or vertex reduction
algorithms.
> I (and many others) have written algorithms to say which levels a 
> polyline point should be visible at on google maps.
> 
> e.g. (not mine!)
>
http://facstaff.unca.edu/mcmcclur/GoogleMaps/EncodePolyline/algorithm.html

Thanks.
I compared it to my implementation at 
http://travelingsales.svn.sourceforge.net/viewvc/travelingsales/libosm/src/org/openstreetmap/osm/data/WayHelper.java?view=markup
line 360-403 and it seems to be doing the same thing.
Except that they store more then the final level of detail.

I may add additional attributes to the simplified ways to have multiple
simplification-levels in a single map. (Have each node of a way contain the
number of next nodes to skip in a given zoom-level.
Like key="LODDataSet:skip(" + wayid + ")(" + lod-level + ")" value="3").
This way a single low-detail map may contain multiple levels of detail.
And renders that support it can make use of that without breaking renderers
that do not.

I like that idea. :)

Marcus