[postgis-users] Line To Path
Martin Davis
mbdavis at refractions.net
Thu May 1 09:07:07 PDT 2008
I would say you want to apply an affine transform to the object. This
is supported in PostGIS, but I don't know the syntax offhand.
Bob Pawley wrote:
> I do have another novice question, if I may.
>
> I have a non-geographical object whose shape is defined by
> coordinates. These coordinates not ony establishe the shape of the
> object but also the objects relative size and its position on the canvas.
>
> Is there a method of using just the shape of the object dynamically
> resizing it and choosing its placement on the canvas?
>
> Bob Pawley
> www.automatingdesign.com <http://www.automatingdesign.com>
>
>
> ----- Original Message -----
> *From:* Obe, Regina <mailto:robe.dnd at cityofboston.gov>
> *To:* PostGIS Users Discussion
> <mailto:postgis-users at postgis.refractions.net>
> *Sent:* Thursday, May 01, 2008 5:19 AM
> *Subject:* RE: [postgis-users] Line To Path
>
> I wouldn't quite call it runnning the function in that table.
> Basically the SQL statement will create a temporary or in memory
> table so to speak. So short answer - yes it is correct - no need
> to create a new table or geometry column. Sometimes you may want
> to if you use it often or you are grouping many geometries since
> the planner has to recalculate each time if it is a dynamic query
> as below or view (a saved dynamic query as Kevin pointed out in
> last post) .
>
>
> Well there should be an alias there otherwise it usually will just
> alias it as something dumb like ST_Union - so let me correct my
> mistake.
> SELECT somefield, ST_Union(the_geom) as newgeom
> FROM sometable
> GROUP BY somefield;
>
> If you wanted to materialize it, I tend to do something like
>
> SELECT somefield, ST_Union(the_geom) as newgeom
> INTO somenewtable
> FROM sometable
> GROUP BY somefield;
>
> A lot of people do
> CREATE TABLE somenewtable As
> SELECT somefield, ST_Union(the_geom) as newgeom
> FROM sometable
> GROUP BY somefield;
>
> But I tend to avoid that second syntax since its not as portable
> as option 1 (from DBMS to DBMS at least the DBMS I tend to deal
> with) and the speed is the same. Granted
> I guess the second version is a bit clearer.
>
> Hope that helps,
> Regina
>
>
> ------------------------------------------------------------------------
> *From:* postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] *On Behalf
> Of *George Silva
> *Sent:* Wednesday, April 30, 2008 10:47 PM
> *To:* PostGIS Users Discussion
> *Subject:* Re: [postgis-users] Line To Path
>
> In the same trailing of that question and the answer, that select
> statement would run the function st_union in that table, without
> the need to create a new table or geometry column?
>
> Sorry to use this post for this, just tought its a quite novice
> question, so more people could use the answer.
>
> Thx
>
> Att.
>
> George
>
> On Wed, Apr 30, 2008 at 7:20 PM, Paragon Corporation <lr at pcorp.us
> <mailto:lr at pcorp.us>> wrote:
>
> Bob,
>
> If I understand you correctly, I think you want to use one of
> the following
>
> SELECT somefield, ST_Union(the_geom)
> FROM sometable
> GROUP BY somefield
>
> So lets say you want to collapse 3 rows into 1 then you just
> need to group
> by some common field.
>
> E.g. if somefield = 1 for your 3 records, then those would get
> rolled into
> the same record.
>
> The above will give you a LINESTRING or MULTILINESTRING. If
> you have all
> LINESTRINGS, then may be more efficient to do this. The below
> will first
> collapse all with common somefield into a MULTILINESTRING and
> then the
> LineMerge will do the best it can to stitch back into a single
> line string.
> This is not possible with completely disjoint linestrings.
>
> SELECT somefield, ST_LineMerge(ST_Collect(the_geom))
> FROM sometable
> GROUP BY somefield
>
>
> If you are using the older version of Postgis, you can just
> take out the ST_
> in the examples I have above.
>
> Hope that helps,
> Regina
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> <mailto:postgis-users-bounces at postgis.refractions.net>
> [mailto:postgis-users-bounces at postgis.refractions.net
> <mailto:postgis-users-bounces at postgis.refractions.net>] On
> Behalf Of Bob
> Pawley
> Sent: Wednesday, April 30, 2008 12:40 PM
> To: PostGIS Users Discussion
> Subject: [postgis-users] Line To Path
>
> Is there a method of converting three lines that require three
> rows into a
> path that occupies a single row??
>
> Bob Pawley
>
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--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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