[Openstreetmap] landsat background to the static renderer

Lars Aronsson lars at aronsson.se
Sat Jul 16 19:19:31 BST 2005


Tom Carden wrote:

> There's a limit of 5000 nodes on the static renderer to stop the 
> server from falling over.  Until we have a way to distinguish 
> "main roads" from others, or to cache the images, it's likely 
> that looking at a whole continent will be less than 
> satisfactory.

It is good that we have this problem, because it means we have a 
lot of data.  Growing pain means we are growing.  A working 
solution must be based on our continued growth.

Every street segment has a visible size.  On a map-of-Europe where 
scale=0.1 every pixel is 0.1 degrees or 11 km.  All points within 
a small city becomes a single pixel.  Instead of looking at the 
maybe 400 points that make up the street net of that city, we 
could look at generating one pixel: should it be white or not?  
Each entire map is 700x500 = 350,000 pixels.  This is constant and 
doesn't change with the map scale or amount of data we collect. 
All we need is a fast way to query the database: Is there anything 
within this 11x11 km area?  If this can be answered in 1 
microsecond, then 350,000 such queries can be answered in 0.35 
seconds, which would be a reasonable response time for a 
webserver.  I guess this is a standard GIS problem.  Quad trees?


-- 
  Lars Aronsson (lars at aronsson.se)
  Aronsson Datateknik - http://aronsson.se




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