[postgis-users] PostGIS Capabilities

[Jong S. Lee]

Tyler Mitchell wrote:

> This takes me back to the question of how you define "on the line".  I 
> would bet (others can verify) that if the bridge point did actually 
> fall exactly on the road line vector (highly unlikely in my opinion) 
> then this should work. 

I agree with you opinion. However, the bridge data (point shape file) is 
created based on the highway data.  The highway data has LRS (linear 
referencing system) to locate the point (or event) on the highway.  The 
bridge data is created via LRS.  Therefore, it should be exactly on the 
highway.  But, I am not sure. I can't trust ESRI product 100 %.  If I 
assume that ESRI product makes the right bridge data on the highway,  
the precision of the coordinates might be changed when I converted 
shapefile to PostGIS db table.  Still I suspect the ESRI product, too.

> > For your information, the operation with "intersecting" in ArcGIS 
> worked.
>
> Interesting.  I then assume that the ArcGIS buffers your points and 
> then looks for the intersections.  It's really a matter of scale 
> which.  Zoom in as far as you possibly can and tell me if the point 
> and line directly overlap.  I assume that they do not!  This brings up 
> the question of whether or not ArcGIS "should" work or not.  But we're 
> not taught to question ;)

Actually, ArcGIS give you the option field about buffer distance.  I 
used 0 value for the buffer distance.  And, in ArcGIS, even if I zoom in 
as far as possible, the points (bridge) are on the line. 

> In my opinion, the user should have to think about a search radius 
> between features like this and not default to software to guess.  

YES!! it is very useful tip for me.  Thank you again.

-- 
Jong Sung Lee, Graduate Research Assistant
Department of Urban and Regional Planning
University of Illinois at Urbana-Champaign
111 Temple Hoyne Buell Hall
611 East Loredo Taft Dr.
Champaign, IL  61820

Tel:(217)244-6564 
E-mail:jonglee1 at uiuc.edu