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PostGIS function to connect geometry LINE together?

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(note: the_geom is a geometry value (TYPE: LINESTRING), in this case i random them for readability)







gid | kstart  | kend    | ctrl_sec_no | the_geom | the_sum_geom

626 | 238     | 239     | 120802      | 123456   | NULL

638 | 249     | 250     | 120802      | 234567   | NULL

4037| 239     | 249     | 120802      | 345678   | NULL







[Real Practice Description] just skip this for those who don't mind the purpose







I would like to do 'this' (a set of queries from my past question, link located on the end of this post) for every row in Table B (aka. land_inventory). These two tables are related by 'ctrl_sec_no' (aka. control section number of a road) which means :: in ONE ctrl_sec_no -- 120802 (in fact, it is a road which is equivalent to 3 geometry LINESTRINGs (the_geom) connected together, from kstart 238 (start at kilometre of 238) to kend 250)







[PostGIS question]





the question is how to connect this 3 lines {aka gid(626,638,4037) from the table} together and result in 'the_sum_geom' (initially NULL) by using PostGIS functions (whatever). after that we will use this 'the_sum_geom' to find the POINT on this geometry LINESTRING





( How calculate things from many tables by using a few queries? ).





Again, thanks in advanced


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Comments

  1. Tips For AllFebruary 10, 2012 at 1:38 PM

    The function you are looking for is ST_Union, you need to use it with the aggregate form:

    update mytable set the_sum_geom =
    ( select ST_Union(the_geom) from mytable where ctrl_sec_no = 120802 )
    where ctrl_sec_no = 120802;

    ReplyDelete

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