[postgis-users] Extending Linestrings

[Leticia]

Julio, THANKS!

 

 

 

  _____  

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Julio
Galindo
Sent: Tuesday, April 17, 2007 1:53 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Extending Linestrings

 

Hi Leticia,

 

I create the following function for your problem, please test it carefully,
I only made two test (at the end):

 

CREATE OR REPLACE FUNCTION ExtendLine(eje_ geometry, bound_ geometry)
RETURNS geometry AS $$
-- version: alfa , by Julio A. Galindo, April 17, 2007:
juliogalindoq at gmail.com  <mailto:juliogalindoq at gmail.com> 
 DECLARE
     b_ geometry = boundary(bound_);
     dist float;
     max_dist float = 0;
     n_points int;
     pto_1 geometry;
     pto_2 geometry;
     first_pto geometry;
     last_pto geometry; 
     u_1 float;
     u_2 float;
     norm float;
     result text = 'LINESTRING(';
 BEGIN
     IF GeometryType(eje_) NOT LIKE 'LINESTRING'
  OR GeometryType(bound_) NOT LIKE 'POLYGON' THEN RETURN NULL; END IF; 
     -- First Search how far is the boundary: (worst case)
     pto_1 := StartPoint(eje_);
     pto_2 := EndPoint(eje_);
     FOR i IN 0..NumPoints(b_)-1 LOOP
  dist := distance(PointN(b_,i),pto_1);
  IF dist > max_dist THEN max_dist := dist; END IF; 
  dist := distance(PointN(b_,i),pto_2);
  IF dist > max_dist THEN max_dist := dist; END IF;
     END LOOP;
     -- Now extent the linestring:
     pto_2 := PointN(eje_,2);
     u_1 := X(pto_2)-X(pto_1); 
     u_2 := Y(pto_2)-Y(pto_1);
     norm := sqrt(u_1^2 + u_2^2);
     first_pto := MakePoint(X(pto_1)-u_1/norm*dist,Y(pto_1)-u_2/norm*dist);
     n_points := nPoints(eje_);
     IF n_points > 2 THEN
  pto_1 := PointN(eje_,n_points-1);
  pto_2 := PointN(eje_,n_points);
  u_1 := X(pto_2)-X(pto_1);
  u_2 := Y(pto_2)-Y(pto_1);
  norm := sqrt(u_1^2 + u_2^2);
     END IF;
     last_pto := MakePoint(X(pto_2)+u_1/norm*dist,Y(pto_2)+u_2/norm*dist); 
     result := result || X(first_pto) || ' ' || Y(first_pto) || ',';
     FOR i IN 1..NumPoints(eje_) LOOP
  result := result || X(PointN(eje_,i)) || ' ' || Y(PointN(eje_,i)) || ','; 
     END LOOP;
     result := result || X(last_pto) || ' ' || Y(last_pto) || ')';
     -- Find the final Linestring:
     b_ := intersection(GeomFromText(result,SRID(eje_)),bound_);
     RETURN b_; 
 END $$
 LANGUAGE plpgsql
 STABLE
 RETURNS NULL ON NULL INPUT;
 

 

Test 1:

 

select asText(extendLine(geomFromText('LINESTRING(5 5, 12
12)',-1),GeomFromText('POLYGON((0 0,15 0,15 20,0 20,0 0))',-1)));

 

             astext              
---------------------------------
 LINESTRING(0 0,5 5,12 12,15 15)
 

The solution is good, because the line is at 45 degress 

 

And the same function can work with a general linestring (many points):

 

select asText(extendLine(geomFromText('LINESTRING(5 5, 12 12, 13
12)',-1),GeomFromText('POLYGON((0 0,15 0,15 20,0 20,0 0))',-1)));
                astext                 
--------------------------------------- 
 LINESTRING(0 0,5 5,12 12,13 12,15 12)

 

Let me know if you improve this simple function.

 

Regards,

 

Julio A. Galindo Q.

Topogen Ltda.

La Paz, Bolivia

 

On 4/16/07, Leticia <lgomez at itba.edu.ar> wrote: 

 

Hi community,

 

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. 

 

Is there a simple method to do this?

 

 

-------------

|           |

      |     |    |

      |     |    |

      |           |

-------------

 

 

I need the other one:

 

 

-------|-----

|     |    |

      |     |    |

      |     |    |

      |     |    |

-------|-----

 

The original linestring is not always vertical u horizontal.

 

 

Thanks in advance,

Leticia

 

 

 


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