> 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 mathematical sense. 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


