[OSM-talk] Maximum segment length?

Lars Aronsson lars at aronsson.se
Thu Mar 22 01:10:52 GMT 2007

Scott Walde wrote:

> But, this causes me to ask the question... how long can a 
> segment be?  Around here, highways can go on in a straight line 
> for many, many kilometers.  (One problem stretch was about 
> 50km.)

This is related to the use of Bezier curves and map projections.  
When you have a line segment, you say node A and B should be 
connected.  But how?  If the two nodes are at the same latitude, 
should the connecting road run at a constant latitude?  That means 
it will look straight on a Mercator projector map, but on the real 
Earth surface it will be a long, smooth curve (unless that 
latitude is the Equator).  For reasonably short segments (say 500 
metres), the difference will be far smaller than the GPS accuracy 
(5 metres), so it doesn't matter to us.  But how much off can the 
middle of a 50 km segment be?  Do you even know how to compute 
that?  Or should you stick to shorter segments?

We once (April 2006) had (and maybe we still have?) somebody (Jörg 
Osterman) who checked the weekly dump for long segments (there 
were plenty in Russia), so that we could go in and add nodes to 
make segments shorter.

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

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