[OSM-talk] Topology (was: OSM the mediocre alternative)

[Frederik Ramm]

Hi,

> Do you have any examples that don't involve a difference in altitude
> or z-index? I can't think of one. :-/

Mh, perhaps if you add logical things like borders, administrative  
areas, airspace and such? For example, an airspace area will usually  
be a polygon defined by a number of points which are given as lat/ 
lon. If one of these points just so happens to be at the same place  
as a road junction, both are nonetheless totally independent of each  
other. And it isn't an altitude thing since there is airspace that  
starts at ground level.

One could of course create logical layers to cordon off these things  
from one another. But thinking this further, you end up at the very  
same point I postulated, that you cannot simply assume that two  
things meeting somewhere in lat/lon do actually meet logically - in  
my description you would have to explicitly select the object you are  
dealing with, and in a layered world you would have to select the  
layer. (And even then: Two bits of airspace meeting somewhere are not  
necessarily logically connected at that point. They may be defined in  
terms of "so-and-so airspace begins where so-and-so airspace ends",  
but they may also be defined in absolute terms so that one stays  
regardless of the other.)

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00.09' E008°23.33'