[postgis-devel] ST_Line_Substring

Paul Ramsey pramsey at cleverelephant.ca
Wed Sep 9 11:34:05 PDT 2009


Niklas,

There are at least three different implementations of line substring
hanging around, a couple that are bound to the old functions and one I
did for the elevationbetween functions. The whole conjunction calls
out for a cleaning and consolidation, frankly, but I've been leaving
it alone on the basis of not touching things that are working.

Attach your patch for 3D to a ticket so it doesn't get lost. I've got
reviewing your distance ticket on my list too so don't give up hope on
that.

P.

On Sun, Aug 30, 2009 at 1:17 PM, <nicklas.aven at jordogskog.no> wrote:
> The function ST_Line_Substring is some mixture between 2D and 3D. It
> preserves the z-value but don't use it in calculation. The documentation
> describes this but the note that the function supports 3D I think is false.
>
> Why not make it real 3D so it considers the third dimmension in the
> calculation.
>
> Now this query:
> select st_asewkt(ST_Line_Substring(the_geom,0.2, 0.8))
>  from
> (SELECT ST_GeomFromEWKT('SRID=4269;LINESTRING(1 1 1,1 1 3, 1 1 5, 1 1 8)')
> as the_geom ) a;
> gives the little bit strange result:
> "SRID=4269;LINESTRING(1 1 3,1 1 5)"
>
> I think t would be much better to make it fully 3D-supportive. Then the
> result of this query is:
> "SRID=4269;LINESTRING(1 1 2.4,1 1 3,1 1 5,1 1 6.6)"
>
> I have attached a patch that makes this.
>
> The reason I started to look at this was that I wanted a function that takes
> distances instead of fraction of the total length.
> I hve a working finction that takes the startdistance and end-distance from
> the start of the line to pull out the substring-line. In some cases that is
> more straight on than going via the fractions. Should I post it or will
> there be to many functions too little difference?
> The question is if I should put effort in writing the documentation and
> regression-tests.
>
> Greetings
> Nicklas
>
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