[OSM-talk] TIGER 101

[Ben Gimpert]

Hi Andy,

As I explained in my previous email, the displacement technique does not
scale.  Yes, "one meter" might work for San Francisco but it doesn't for
rural Montana.  (This is not conjecture.  I've already written this
code, and it doesn't work.)

		Ben


On Thu, 30 Nov 06 @04:55pm, Andy Robinson wrote:
> Ben,
> 
> Thanks for the update. I think the key one to get right is the node reuse.
> Grouping individual street ways parts into a single street is less critical
> because it's a fairly easy task to do by hand if you need it. It's not
> feasible to merge nodes by hand though.
> 
> As you say the data is coarse and therefore its not really a problem to
> consider node combination similarly. However if I look at the San Francisco
> data I don't see node displacement at junctions exceeding 1 metre (in fact
> significantly smaller if not the same lat and lon) so I'm guessing that many
> of the street junctions do have the same lat/lon or at least very close to
> it. Its very unlikely that there is a problem combining points that are
> within that sort of radius anyway.
> 
> Cheers
> 
> Andy
> 
> Andy Robinson
> Andy_J_Robinson at blueyonder.co.uk 
> 
> >-----Original Message-----
> >From: talk-bounces at openstreetmap.org [mailto:talk-
> >bounces at openstreetmap.org] On Behalf Of Ben Gimpert
> >Sent: 30 November 2006 4:36 PM
> >To: talk at openstreetmap.org
> >Subject: Re: [OSM-talk] TIGER 101
> >
> >Hi Andy & Steve,
> >
> >Right, nodes are not being reused.  This is just because of how they are
> >represrented they are represented in the TIGER data itself.  I've posted
> >about this before -- how "5th Avenue" in Manhattan is actually N
> >separate record chains in TIGER, though obviously it's a
> >physically-contiguously street.  And that this occurs also at the node
> >level, where there is no level of abstraction in the TIGER dataset for
> >shared nodes.
> >
> >Take a look at:
> >
> >	http://svn.openstreetmap.org/utils/tiger_import/tiger/tiger.rb
> >
> >...and note how the TIGER files present (only) raw lat/long, at every
> >scale (node, street, point-of-interest).
> >
> >We have to remember that the TIGER data is just a *very* rough first
> >step to a usable streetmap of America.  I'm sure companies like MapQuest
> >have had to spend enormous money and/or effort to clean things up for
> >their driving directions.
> >
> >I wrote some code to intelligently try to merge roads with the same
> >names that share (roughly) an end node-or-two.  This code didn't scale
> >well outside of Manhattan, since FIPS counties can be very strangely
> >shaped and sized.  (See Steve's 'blog post on Gerrymandering for a
> >similar topic...  Heh.)
> >
> >As for re-using nodes, we face a similar problem of scaling:  At what
> >lat/long precision do we consider two points the "same"?  (Say 0.00005
> >of a degree, or what?)  Again, answering this question is hard across
> >the entire (HUGE) country.
> >
> >We might define a formula based upon the smallest rectangle that can
> >cover a county.  Say, (maxCountyLongitude - minCountyLongitude) / 10^5,
> >but this, umm, doesn't work.  (I tried already.)
> >
> >Let me know via email if someone else wants to take a crack at writing
> >code to "merge nodes" (and streets) in the TIGER data.  I myself won't
> >be able to write any more code for OSM since I'm bogged down in other
> >responsibilities.  Especially given OSM's wiki nature, I feel like a
> >routing system will have to have some intelligence about assuming two
> >"nearby" nodes are really the same intersection / bend / whatever.
> >
> >Hope this make sense!
> >
> >		Cheers,
> >		Ben
> >
> >
> >On Thu, 30 Nov 06 @03:20pm, Andy Robinson wrote:
> >> Ben,
> >>
> >> Looking at the San Francisco data newly imported it still appears that
> >ways
> >> are being added without connection to adjacent ways, ie there is no
> >sharing
> >> of common nodes. See JOSM screen dump where I have selected and dragged a
> >> way. It should have tugged the adjacent ways too.
> >>
> >> http://ajr.hopto.org/osm/tiger-nodes.png
> >>
> >> Cheers
> >>
> >> Andy
> >>
> >> Andy Robinson
> >> Andy_J_Robinson at blueyonder.co.uk
> >>
> >> >-----Original Message-----
> >> >From: talk-bounces at openstreetmap.org [mailto:talk-
> >> >bounces at openstreetmap.org] On Behalf Of Ben Gimpert
> >> >Sent: 30 November 2006 2:53 PM
> >> >To: talk at openstreetmap.org
> >> >Subject: [OSM-talk] TIGER 101
> >> >
> >> >Hi OSM,
> >> >
> >> >The TIGER -> OSM import is (again) kicked off and going.
> >> >
> >> >Since there is now disaster recovery logic atop a MySQL tracking
> >> >database, a proper status report is possible.  Unless there are any
> >> >objections, I intend to commit the status report to the OSM SVN
> >> >repository every night at 3am:
> >> >
> >> >		http://svn.openstreetmap.org/utils/tiger_import/status
> >> >
> >> >Dig the tiny-but-increasing numbers for CAlifornia, around our first
> >> >prioritized counties (for Mikel).
> >> >
> >> >		Cheers,
> >> >		Ben
> >> >
> >> >
> >> >_______________________________________________
> >> >talk mailing list
> >> >talk at openstreetmap.org
> >> >http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/talk
> >>
> >
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