[postgis-users] Buffer generation & drawing problem

[Obe, Regina]

Luis,

I'm not sure if anyone has answered your question, but I didn't see one
so here is my answer.

I think the problem you are having is that you are working in degree
coordinates so when you say buffer 0.1 that actually isn't a constant
radius.

If you can - the best way to fix the problem is to transform your data
to cartesian coordinates and then back, but this may or may not be easy
depending on where you are.  In general its best to keep your data in
some sort of cartesian coordinate (if you are interested in a relatively
small area) since there are not too many functions in postgis as of yet
to deal with geodetic data and then transform back to 4326 as the need
arises.  

Below I will try to give a fairly generic solution that will solve your
problem regardless of where on earth your data is located.

If you look on this page -
http://postgis.refractions.net/support/wiki/index.php?plpgsqlfunctions
You will see a function very useful function called utmzone that takes a
point and returns the SRID of the utmzone it belongs to.  Load that
function.

Now to get real circles first figure out the radius of the circle you
want in meters.  I will choose 100 meters.

SELECT astext(transform(buffer(transform(the_geom,
utmzone(centroid(the_geom))), 100), 4326))
FROM mytable

Note since you are dealing with points you can replace
centroid(the_geom) with the_geom.  The reason I needed to do that is
that the utmzone function only deals with point geometries.

Hope that helps,
Regina

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Luis
Floreani
Sent: Tuesday, October 16, 2007 4:29 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] Buffer generation & drawing problem

Hi, this my situation, I'm using postgis to generate buffers in order
to draw them in google or yahoo maps. The problem is that when they
are drawn, they are elliptical (taller) instead of plain circles.

The column is named 'the_geom', it's a 2D Point and its SRID is 4326.
Here's the SQL:

select astext(buffer(the_geom, 0.1)) from mytable where ...

note: 0.1 is related with the size of the buffer.

here's a typical output:

"POLYGON((-82.600775 27.902917,-82.6026964719596
27.8834079677989,-82.6083870467484 27.8646486567645 ...... etc etc))"

I parse this string in my script language and generate a proper KML
containing all the points of the buffer, but when I open it with
google earth, google maps or even yahoo maps (use another structure of
XML), the buffer is a little elliptical instead of being a simple
perfect circle.

any clue on this?

Thanks in advance,

Luis
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