[postgis-users] Select Point near Polyline Postgis

[anhtin]

hi Rodrigo
Thanks very much reply to me.
With the first script it good.
However when i run the second script: 

SELECT * ,
line_interpolate_point(the_geom,line_locate_point(the_geom,PointFromText('POINT(517651
2121421)', 42102)))
 FROM mainroad ORDER BY Distance(the_geom,PointFromText('POINT(517651
2121421)', 42102)) LIMIT 1 

Suddenly have a exception error :
ERROR: line_locate_point: 1st arg isnt a line
SQL state: XX000

Note: the number 42102 is my SRIDs.

if i run this script:
Select line_locate_point(the_geom,PointFromText('POINT(517651 2121421)',
42102)) from mainroad

it error too

this is structure my table mainroad:
CREATE TABLE mainroad
(
  gid serial NOT NULL,
  name character varying(40),
  id smallint,
  the_geom geometry,
  CONSTRAINT mainroad_pkey PRIMARY KEY (gid),
  CONSTRAINT enforce_dims_the_geom CHECK (ndims(the_geom) = 2),
  CONSTRAINT enforce_geotype_the_geom CHECK (geometrytype(the_geom) =
'MULTILINESTRING'::text OR the_geom IS NULL),
  CONSTRAINT enforce_srid_the_geom CHECK (srid(the_geom) = 42102)
) 
WITHOUT OIDS;
ALTER TABLE mainroad OWNER TO postgres;

Can u show me ???






Rodrigo Martín LÓPEZ GREGORIO-3 wrote:
> 
> Hi anhtin
> 
> To find the nearest polyline from a table to a point you can do this:
> 
> SELECT * FROM mainroad ORDER BY
> Distance(the_geom,PointFromText('POINT(517651 2121421)')) LIMIT 1
> 
> To get the coordinates of the nearest point of that line to your point
> then
> you must do:
> 
> SELECT *,
> line_interpolate_point(the_geom,line_locate_point(the_geom,PointFromText('POINT(517651
> 2121421)'))) FROM mainroad ORDER BY
> Distance(the_geom,PointFromText('POINT(517651 2121421)')) LIMIT 1
> 
> So with the last query you can get all the data (*) of the nearest
> linestring and the point of the that linestring that is nearest to the
> given
> Point.
> 
> The way it works is, first the line_locate_point function get the
> linstring
> and the point and gives you a value between 0 and 1 representing the
> location of the closest point on LineString to the given Point, as a
> fraction of total 2d line length. Then the line_interpolate_point function
> will take the linestring and the value between 0 and 1 and return a Point
> geometry with the location of the nearest point on the linestring. If you
> want to get the X and Y coordinates of that point you can do also
> X(geometry), Y(geometry):
> 
> SELECT *,
> X(line_interpolate_point(the_geom,line_locate_point(the_geom,PointFromText('POINT(517651
> 2121421)')))),
> Y(line_interpolate_point(the_geom,line_locate_point(the_geom,PointFromText('POINT(517651
> 2121421)')))) FROM mainroad ORDER BY
> Distance(the_geom,PointFromText('POINT(517651 2121421)')) LIMIT 1
> 
> Rodrigo.
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
> 
> 

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