[postgis-users] PostGIS Capabilities

[Tyler Mitchell]

Jong, I was also just thinking that if you use LRS to locate your point in 
the first place, can't you take the attribute (i.e. parent road name) and 
store that with the point data.  Then if you looking for all the points on 
that road you can use regular tabular queries instead of spatial queries.  
Does that make sense?

postgis-users-bounces at postgis.refractions.net wrote on 02/20/2004 09:00:51 
AM:

> > 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.
> 
> Just because you used an LRS to generate the points doesn't mean 
> that they will 
> necessarily be located exactly on the line, in a strict 
mathematicalsense. It 
> may not be possible to describe the location on the line exactly 
> using a given 
> number system (whether it's binary or decimal doesn't matter). For 
example, 
> what if you want to describe the position that is 1/3 of the way 
> along the line 
> from 0,0 to 1,2 - you cannot describe this exactly with decimal 
> numbers, in any 
> given precision. So the point that you get will be at, say 0.333333,
> 0.666667 - 
> which is not, in a strict mathematical sense, on the line. It is very 
close, 
> and I suspect that Arcview is realizing that it is very close (as close 
as 
> possible with the numerical precision available) and accepting it as
> exactly on 
> the line.  Whereas, I would expect GEOS to up and tell you that no, it 
is not 
> on the line.
> 
> I could be completely wrong here... but this is my suspicion. 
> Anyways, AFAIK it 
> is best to ensure that there is an actual node in your linestring 
(highway) 
> located at the exact same location as the point feature (bridge) in 
order to 
> ensure that they are and will reamin touching.
> 
> Chris
> 
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