[postgis-users] sql statements for angles on multilinestrings

[David Blasby]

Thomas Preuth wrote:

> -----------------------------------------------------------------------------------------------------------------
> Hello,
> 
> I have two questions concerning postgis.
> The issue is:
> 
> The column the_geom includes lines (multilinestring) as geometries, which
> define their position by two coordinates. A symbol  should be placed at
> first coordinate. The base of the symbol should be perpendicular to the
> line. Therefore I need the tilt angle of the multilinestring.
> So my questions:
> 1.)How can I extract the first coordinate of the multilinestring with  a
> sql statement?
> 2.)How can I get the tilt angle of the multilinestring (to the horizontal)
> with a sql statement?

1.
     SELECT startpoint(geometryN(the_geom,1)) FROM <table>

2.
     I'm not sure what you exactly you mean by "tilt angle", but you
     can extract the (x,y) coordinates from a linestring like this:

         x(  pointN(  geometryN( the_geom,1), 1))
         y(  pointN(  geometryN( the_geom,1), 1))

         x(  pointN(  geometryN( the_geom,1), 2))
         y(  pointN(  geometryN( the_geom,1), 2))

     Postgresql supports all the trigonometry functions, so it should be
      easy to compute the "tilt angle".

dave