[talk-au] Potlatch and unconnected roads

Andrew Loughhead andrew at incanberra.com.au
Sat Dec 15 10:38:27 GMT 2007


Nick Hocking wrote:
> Most of Canberra has been mapped with potlatch and using imagery. I 
> believe that this is going to cause quite a few problems, but one of 
> the bigger problems is that of unconnected roads. It seems that it is 
> very easy to leave roads unconnected when using potlatch and I think 
> that there are hundreds, possibly thousands of them in Canberra alone.
What suburbs?  I've done a fair bit in Canberra, using both JOSM with 
imagery, and with Potlatch, but I am actually pretty sure I haven't made 
that error (much) - as an ex-GIS geek I am painfully aware of those 
kinds of geometry errors.  In a previous working life I've spent many 
days finding and fixing such errors in stream data from 25k topo sheets 
- and that was semi-automated.  In any case I hope that 'thousands' is 
an overestimate. 

Speaking of GIS, one way to detect these would be to export to 
shapefile, and analyse the data in a GIS such as grass or ArcGIS.  In 
ArcGIS and its predecessor product ArcInfo there are a swag of 
tolerances which go towards preventing this type of error in digitizing, 
and a number of commands to find and fix such things too.  But I don't 
have that type of toolkit available, and having never learnt grass I'm 
not in a position to offer this analysis.

Might also be worth saying that this doesn't really have anything to do 
with the use of imagery, and probably not much to do with Potlatch 
versus JOSM either.  Either way its all heads up digitizing, whether 
based on imagery or GPS traces. 

Its also unlikely to be all that special to Canberra - if this kind of 
problem is coming up, perhaps its worth asking on osm-talk.  I see that 
the Validator page on the wike says that crossing ways are already 
detected, and a user called Fröstel 
<http://wiki.openstreetmap.org/index.php/User:Fr%C3%B6stel> has recently 
requested the 'near-miss' detection too. 

cheers
Andrew.





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