[postgis-devel] ST_Line_Substring

[nicklas.aven at jordogskog.no]

thanks about the distance-calculations don't miss the dist.doc in ticket #231. That might save you some time to see the idea behind the new distance-calculation of polygons and linestrings that don't have intersecting bounding boxes. /Nicklas  

2009-09-09 Paul Ramsey wrote:

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,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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>>
>>
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