[postgis-users] Populating Multipolygon Z as calculation -Merge Function

[Chris English]

This merge function works flawlessly.  I especially like the abstracting

of Z as my z' are pretty well abstract anyway:

Kevin Neufeld wrote the following:

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

And then the Magician's Apprentice steps in:

CREATE OR REPLACE FUNCTION adddimension_z(geometry, double precision[])
	RETURNS geometry AS

'SELECT ST_GeomFromEWKT(      *//I think I'm trying to build a
Multipolygon object so chose this
	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;


SELECT ST_AsEWKT(
	adddimension_z(
		'MULTIPOLYGON(((1 1, 2 2, 3 3,4 4,5 5, 6 6, 7 7,1 1)))'::geometry,
		ARRAY[1,2,3,4,5,6,7,1]
		)
	);

This actually ran but with unexpected results:

st_asewkt

blank-> i.e. nothing

1 row

where I was expecting something like

st_asewkt

MULTIPOLYGON(1 1 1, 2 2 2, 3 3 3, 4 4 4, 5 5 5, 6 6 6, 7 7 7, 1 1 1)

1 Row

I'm frankly surprised it ran for me at all.  But this is the case
where the apprentice

mutters the incantation and finds himself in a room full of brooms or
the dog starts

talking in Polish.


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*>**



-- 
He doesn't fully understand what he thinks he knows about the problem.
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