[postgis-users] Need some help with storing line segments

[Ehud Shabtai]

On 7/19/05, Stephen Woodbridge <woodbri at swoodbridge.com> wrote:
> I'm not sure you can do this in postGIS, but the idea would be to try
> something like:
> 
> Table points: pid, point_geom, attributes, ...
> Table lines: pid_from, pid_to, attributes, ...
> View: select makeline(
>           (select point_geom from points a, lines b where
> a.pid=b.pid_from) as p1),
>           (select point_geom from points a, lines b where
> a.pid=b.pid_to) as p2)) as line_geom, attributes from lines;

This solution seems to require creating a line for every two points.
My lines may have any number of points.
 
> If the view trick doesn't work then you might want to add the line_geom
> directly to the lines table and use a trigger on the points table to
> update the line_geom if the points_geom gets changes based on the pid field.

This is a great idea!

The only question is how can I change a specific point of a line (as
it may contain many points)? As suggested earlier, I could use type-m
points in the line, M being the id of the point. Can I select a
specific point inside a line using the M value? Can I change a point
(update its coordinates) inside a line without creating the whole line
and replacing the old one?

Thanks a lot for the help,

Ehud.

> 
> Ehud Shabtai wrote:
> > Hi,
> >
> > I'm new to postgis and this is my first usage of it. I read the
> > documentation, but I could not find a way to store the data I want (in
> > the way I want it).
> >
> > Here's the story:
> >
> > I got some GPS tracks which I want to store in postgis. Before I store
> > them, I do some processing and build line segments from the separated
> > GPS points. Now, I'd like to store these lines in postgis, but I'd
> > also like to keep attributes for each node of the line (node id, time,
> > direction for next node, etc').
> >
> > The only way I see it is to store each point twice, first as a point
> > with some attributes and then as a line. The problem with this
> > approach is that I'd like to be able to update the coordinates of a
> > specific point, which will result in searching for the line consisting
> > this point and then updating the line.
> >
> > Is there any better approach to sore my data?
> > Can I update a specific point of a line in postgis, or the only way is
> > to get all the points of a line and create a new one?
> >
> > Thanks,
> >
> > Ehud.
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> >
> 
>