[postgis-users] How to add circle to Postgis database?

[Bruce Rindahl]

Ellipses can be made directly using Affine transformations.  The idea is to
create a circle (buffer), distort along the x & y axis (scale), rotate to
the correct angle and then transform to the final location.

 

To mimic your parameters an ellipse is

 

translate(rotate(scale(buffer(makepoint('0','0'),1,slices/4),a,b),alpha),cx,
cy)

 

Note the buffer function has an optional parameter that defines the number
of line segments for a quarter circle (thus the slices/4 - the default is 8)

You can also rotate the ellipse thru an angle alpha (radians).

 

The above should be much faster.

 

Bruce Rindahl

 

  _____  

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of marco
vieira
Sent: Thursday, March 08, 2007 5:54 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] How to add circle to Postgis database?

 

I wrote this function to draw ellipses (or circles) in PG.
Feel free to make changes and send to the list.

Parameters:
cx,cy: Center of ellipse
a: major semi-axis
b: minor semi-axis
slices: number of vertices of polygon that define the ellipse. As big as
this value smoothed is the curve. 

-- Function: make_ellipse(cx float8, cy float8, a float8, b float8, slices
float8)

-- DROP FUNCTION make_ellipse(cx float8, cy float8, a float8, b float8,
slices float8);

CREATE OR REPLACE FUNCTION make_ellipse(cx float8, cy float8, a float8, b
float8, slices float8) 
  RETURNS geometry AS
$BODY$
declare
    cx alias for $1;
    cy alias for $2;
    a alias for $3;
    b alias for $4;
    slices alias for $5;
    i float;
    geom geometry;
begin
    i := 0.0;
    execute 'create table tempo(ii float,the_geom geometry) with oids;';
    WHILE i < 2.0*pi() LOOP
    execute 'insert into tempo select
'||i||'::float,makepoint('||cx+a*cos(i)||','||cy+b*sin(i)||');'; 
    i := i+2.0*pi()/slices;
    END LOOP;
    execute 'select setsrid(makepolygon(makeline(the_geom)),4170) from
tempo;' into geom;
    execute 'drop table tempo;';
    return geom;
end; 
$BODY$
  LANGUAGE 'plpgsql' VOLATILE;
ALTER FUNCTION make_ellipse(cx float8, cy float8, a float8, b float8, slices
float8) OWNER TO marco;
GRANT EXECUTE ON FUNCTION make_ellipse(cx float8, cy float8, a float8, b
float8, slices float8) TO public; 




2007/3/7, Brent Wood <pcreso at pcreso.com>:




OK, I've trimmed all the other content, but my request is slightly off the
main
subject.

Does anyone have any comments/advice on what I am doing in this subject
area?
The discusison so far suggests I could usefully learm something here :-) 


So far I've used a local custom equal area projection (Albers EA from
memory) I
created for my purposes.

I was looking at the recorded start positions for fishing trawls along with
the
distance towed and gear width to derive an estimated swept area held as a 
circle centred on the start point. As I want the circles to represent a
comparable area whether they at lat 20S or 60S, I used an equal area
projection
to create them.

Mu limited understanding was that despite the circles being defined by a 
radius, an equal area was better than equidistant for this.

I'm storing them as lat/long (EPSG:4326 projection with longs 0-360 as they
cross the 180 meridian... sigh...). In this projection the circles display
as 
ellipses, exactly as they should, but this does confuse some around here :-)


Is there a better way to achieve this?

Thanks,

  Brent
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-- 
Marco Vieira
+55 21 9499-6800
e-mail: maovieira at gmail.com 

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