[postgis-users] make the buffer in 3d

[Obe, Regina]

Ihab,
 
If your cylinder is perpendicular to the x-y plane, then I think you may
be able to think of your cylinder as
 
the 2-d buffer integrated along the z-axis from say the base to base +
radius of interest (lets call the radius = 10)
 
So then your query I think can be simplified to 
 
if your point of interest (which is the center of the base of the
cylinder) is ST_MakePoint(1,2,3) and radius 10  (in this example my
cylinder is 3 units above ground)
 
SELECT pt.gid, pt.the_geom
FROM  ST_DWithin(pt.the_geom, ST_MakePoint(1,2,3),10)
AND ST_Z(pt.the_geom) Between 3 and 13
 
 
If your cylinder is partly below ground, then your 3 and 13 change to
say -8 and 2 or whatever.  So basically between zofinterest + 0  and
zofinterest + 10
 
Now if your cylinder is not perpendicular to x-y plane, then I think you
may still be able to use something similar to the above trick,
but you would need to redefine your axis relative to the azimuth of your
cylinder which I haven't given much thought to at all.
 
Hope that helps,
Regina

________________________________

From: eehab hamzeh [mailto:eehab40 at hotmail.com] 
Sent: Thursday, September 18, 2008 2:57 PM
To: Obe, Regina
Subject: RE: [postgis-users] make the buffer in 3d




Hello Regina,
 
Thanks you for your answer. I need to have a cylinder in the result
(visualize the cylinder). the cylinder will be resulted from the buffer
of point, the z value of the buffer will be same as the buffer radius
(or a value entered from the user)
 
then i need the to select any feature that are within the cylinder. 
 
thanks 
 
Ihab 
 


 

________________________________


> Date: Thu, 18 Sep 2008 10:50:03 -0700
> From: kneufeld at refractions.net
> To: eehab40 at hotmail.com
> Subject: Re: [postgis-users] make the buffer in 3d
> 
> Regina answered this question this morning.
>
http://postgis.refractions.net/pipermail/postgis-users/2008-September/02
1232.html
> 
> Essentially, just test if your points lie within some 3D distance of 
> eachother.
> 
> Did you try her logic?
> 
> I was mistaken in my previous message to you, as distance3D is not yet

> implemented, but length3D is. This is what her query does ... but it 
> only works between two points. If you want to test 3D polygons, you're

> stuck with testing those against a 3D cube (which I showed you how to
do 
> on the list).
> 
> Sorry, I'm not much more help.
> Kevin
> 
> 
> eehab hamzeh wrote:
> > Kevin,
> > 
> > Thanks, i will try to use sphere , what are the function for sphere
in 
> > the postgis
> > 
> > thanks
> > 
> > ihab
> > 
> > 
> >
------------------------------------------------------------------------
> > 
> > > Date: Thu, 18 Sep 2008 10:12:52 -0700
> > > From: kneufeld at refractions.net
> > > To: eehab40 at hotmail.com
> > > Subject: Re: [postgis-users] make the buffer in 3d
> > >
> > > There is no native JTS/GEOS geometry type "cylinder". So, unless
you
> > > are a guru in programming in GEOS and C, I would recommend you
don't
> > > even consider trying to compute the intersection of a 3d polygon
or line
> > > with a new cylinder object of your making.
> > >
> > > If you just wanted to test if a point lies within an abstract
cylinder
> > > object, you might be able to do this PL/pgSQL. A great resource is
the
> > > PostgreSQL website
> > > (http://www.postgresql.org/docs/8.3/static/plpgsql.html). You
could
> > > create a function that accepts a source point, a radius, a height,
and a
> > > candidate point. Then perhaps, using some math, determine if your
> > > candidate point is within the radius of the source point and
within some
> > > ; height of source point.
> > >
> > > ie.
> > > CREATE FUNCTION CylinderContainsPoint(center_pt geometry, radius
double
> > > precision, height double precision, candidate_pt geometry)
> > > RETURNS boolean AS $$
> > > DECLARE
> > > ....
> > > BEGIN
> > > ...
> > > RETURN ...;
> > > END;
> > > $$ LANGUAGE plpgsql;
> > >
> > >
> > > You're sure you need a cylinder, not a sphere? A sphere can be
tested
> > > on distance == 0 instead of testing for intersection, producing
the same
> > > results - this is easy to do with functions already in PostGIS.
> > >
> > > Have fun!
> > > -- Kevin
> > >
> > >
> > > eehab hamzeh wrote:
> > > > Thanks for your replay,
> > > >
> > > > what i need is to find all the object (3d polygon, line with z 
> > value or
> > > > points) that are intersect that cylinder that resulted from the
buffer.
> > > >
> > > > can you please give me the steps that i need to follow... Is
there is
> > > > any source to learn PL SQL?
> > > > How i can insert cylinder in postgis?
> > > > Thanks
> > > > ihab
> > > >
> > > >
> > 
> > 
> >
------------------------------------------------------------------------
> > Explore the seven wonders of the world Learn more! 
> >
<http://search.msn.com/results.aspx?q=7+wonders+world&mkt=en-US&form=QBR
E>



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