[postgis-users] Populating Multipolygon Z as calculation

[Kevin Neufeld]

I did something similar a few years back with LINESTRINGs.  I had a 
large 3D linear dataset that I was constantly fine-tuning and adjusting 
the Z component of.  Rather than always modifying the stored geometry 
(which was a real pain since just rebuilding the index took a couple of 
hours), I extracted the Z components out as a double[] into a separate 
table that I could update as needed.  I wrote a very simple plpgsql 
function that merged the 3 ordinate back into the geometry for viewing 
purposes, but this allowed me to do a bunch of analysis on the Z array 
directly with other simple plpgsql functions, like min/max z values, 
min/max slope, std deviation, waterfall detection algorithms, smoothed 
elevation algorithms, etc.

Although it's old, the merging function might be of some use to you, it 
went something like:
CREATE OR REPLACE FUNCTION adddimension(geometry, double precision[])
   RETURNS geometry AS
'SELECT makeline(
    makepoint(
      x(pointn($1, index)),
      y(pointn($1, index)),
      $2[index]))
  FROM
    (select generate_series(1, numpoints($1)) AS index,
    $1 as the_geom) foo'
   LANGUAGE 'sql' VOLATILE;


Which yields results like:

select asewkt(
   adddimension(
     'LINESTRING(1 1, 2 2)'::geometry,
     ARRAY[3, 4]
   )
)

          asewkt
-------------------------
  LINESTRING(1 1 3,2 2 4)
(1 row)


Cheers,
Kevin


On 4/19/2010 2:19 PM, Chris English wrote:
> Hello,
>
> I've been wrestling around with 3D and have gotten my table/geom to x,y,z
> so that
>
> SELECT ST_AsEwkt(the_geom) FROM <table>
> LIMIT 1;
>
> returns
> "MULTIPOLYGON(((611630.148961496 690526.520745486 0,611619.960661661 
> 690507.134957485 0,
> 611545.79119058 690545.364867903 0,611556.435198162 690565.595798574 
> 0,611630.148961496 690526.520745486 0)))"
>
> In another column are the values I'd like to use for Z, or calculate 
>  Z from.
>
> And then brain death.  What sort of statement would I be using to 
> update the Z part of the XYZ,  as a calculated result
> from another column.
>
> Any help or suggestions greatly appreciated.
>
> Chris
>
>
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