[postgis-users] How can I create a point at a known distance & bearing from a point in Postgis?

[Brent Wood]

Hi,

I
 have a dataset with start & end points that we are represtnting as 
linestrings. Many records do not have endpoints - so we are takimg the 
median bearling for comparable startpoints & creating an arbitrary 
endpoint.

While it is easy to get a bearing for a known endpoint 
with ST_Azimuth(), is there a simple way in Postgis to get the point at a
 defined distance & bearing? I've perused the docs & nothing 
jumped out as an obvious solution.


Thanks,

  Brent Wood


________________________________
 From: Simon Greener <simon at spatialdbadvisor.com>
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org> 
Sent: Thursday, April 11, 2013 6:57 PM
Subject: Re: [postgis-users] Finding closet intersect point along direction of line
 


 
Adrian,

That's excellent.

regards
Simon

On Thu, 11 Apr 2013 16:51:14 +1000, <adrian.kitchingman at dse.vic.gov.au> wrote:


Thanks for the code Simon. After a few
tweaks I got it working perfectly in PostGIS2.
>
>Here's the updated code.
>
>Adrian
>
>CREATE OR REPLACE FUNCTION ST_Extend(eje_
geometry, bound_ geometry)
>RETURNS geometry
>AS $$
>-- Use Case: I need to "extend"
a linestring in another one. The new linestring must be
>-- obtained such as their new extremes
are the intersection of it with a
>-- polygon that contains it. The original
linestring is composed of only a
>-- linear segment.
>-- version: alfa , by Julio A. Galindo,
April 17, 2007: juliogalindoq at gmail.com
>DECLARE
>      b_ geometry = st_boundary(bound_);
>      dist float;
>      max_dist float
= 0;
>      n_points int;
>      pto_1 geometry;
>      pto_2 geometry;
>      first_pto geometry;
>      last_pto geometry;
>      u_1 float;
>      u_2 float;
>      norm float;
>      result text = 'LINESTRING(';
>BEGIN
>      IF  st_GeometryType(eje_)
  NOT LIKE 'ST_LineString' OR
>           st_GeometryType(bound_)
NOT LIKE 'ST_Polygon' THEN
>         RETURN
NULL;
>      END IF;
>      -- First Search
how far is the boundary: (worst case)
>      pto_1 := st_StartPoint(eje_);
>      pto_2 := st_EndPoint(eje_);
>      FOR i IN 1..st_NumPoints(b_)-1
LOOP
>         dist
:= st_distance(st_PointN(b_,i),pto_1);
>         IF
dist > max_dist THEN max_dist := dist; --END IF;
>           
max_dist := dist;
>         END
IF;
>         dist
:= st_distance(st_PointN(b_,i),pto_2);
>         IF
dist > max_dist THEN max_dist := dist; --END IF;
>           
max_dist := dist;
>         END
IF;
>      END LOOP;
>      -- Now extent the
linestring:
>      pto_2 := st_PointN(eje_,2);
>      u_1 := st_X(pto_2)-st_X(pto_1);
>      u_2 := st_Y(pto_2)-st_Y(pto_1);
>      norm := sqrt(u_1^2
+ u_2^2);
>      first_pto := st_MakePoint(st_X(pto_1)-u_1/norm*dist,st_Y(pto_1)-u_2/norm*dist);
>      n_points := st_nPoints(eje_);
>      IF n_points >
2 THEN
>         pto_1
:= st_PointN(eje_,n_points-1);
>         pto_2
:= st_PointN(eje_,n_points);
>         u_1
  := st_X(pto_2)-st_X(pto_1);
>         u_2
  := st_Y(pto_2)-st_Y(pto_1);
>         norm
 := sqrt(u_1^2 + u_2^2);
>      END IF;
>      last_pto := st_MakePoint(st_X(pto_2)+u_1/norm*dist,st_Y(pto_2)+u_2/norm*dist);
>      result   :=
result || st_X(first_pto) || ' ' || st_Y(first_pto) || ',';
>      FOR i IN 1..st_NumPoints(eje_)
LOOP
>          result
:= result || st_X(st_PointN(eje_,i)) || ' ' || st_Y(st_PointN(eje_,i))
|| ',';
>      END LOOP;
>      result := result
|| st_X(last_pto) || ' ' || st_Y(last_pto) || ')';
>      -- Find the final
Linestring:
>      b_ := st_intersection(st_GeomFromText(result,st_SRID(eje_)),bound_);
>      RETURN b_;
>  END $$
>  LANGUAGE plpgsql
>  STABLE
>  RETURNS NULL ON NULL INPUT;
>
>
>
>
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-- 
Holder of "2011 Oracle Spatial Excellence Award for Education and Research."
SpatialDB Advice and Design, Solutions Architecture and Programming,
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