Thank's Bruce!<br>It is much better. I wrote this function a long time ago but it is still useful.<br><br><div><span class="gmail_quote">2007/3/8, Bruce Rindahl <<a href="mailto:rindahl@lrcwe.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">
rindahl@lrcwe.com</a>>:</span>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div link="blue" vlink="blue" lang="EN-US">
<div>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;">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.</span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;">To mimic your parameters an ellipse is</span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;">translate(rotate(scale(buffer(makepoint('0','0'),1,slices/4),a,b),alpha),cx,cy)</span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;">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)</span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;">You can also rotate the ellipse thru an
angle alpha (radians).</span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;">The above should be much faster.</span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;">Bruce Rindahl</span></font></p>
<p><font color="navy" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></p>
<div>
<div style="text-align: center;" align="center"><font face="Times New Roman" size="3"><span style="font-size: 12pt;">
<hr align="center" size="3" width="100%">
</span></font></div>
<p><b><font face="Tahoma" size="2"><span style="font-size: 10pt; font-family: Tahoma; font-weight: bold;">From:</span></font></b><font face="Tahoma" size="2"><span style="font-size: 10pt; font-family: Tahoma;">
<a href="mailto:postgis-users-bounces@postgis.refractions.net" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">postgis-users-bounces@postgis.refractions.net</a> [mailto:<a href="mailto:postgis-users-bounces@postgis.refractions.net" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">
postgis-users-bounces@postgis.refractions.net</a>]
<b><span style="font-weight: bold;">On Behalf Of </span></b>marco vieira<br>
<b><span style="font-weight: bold;">Sent:</span></b> Thursday, March 08, 2007
5:54 AM<br>
<b><span style="font-weight: bold;">To:</span></b> PostGIS
Users Discussion<br>
<b><span style="font-weight: bold;">Subject:</span></b> Re: [postgis-users] How
to add circle to Postgis database?</span></font></p>
</div><div><span>
<p><font face="Times New Roman" size="3"><span style="font-size: 12pt;"> </span></font></p>
<p style="margin-bottom: 12pt;"><font face="Times New Roman" size="3"><span style="font-size: 12pt;">I wrote this function to
draw ellipses (or circles) in PG.<br>
Feel free to make changes and send to the list.<br>
<br>
Parameters:<br>
cx,cy: Center of ellipse<br>
a: major semi-axis<br>
b: minor semi-axis<br>
slices: number of vertices of polygon that define the ellipse. As big as this
value smoothed is the curve. <br>
<br>
-- Function: make_ellipse(cx float8, cy float8, a float8, b float8, slices
float8)<br>
<br>
-- DROP FUNCTION make_ellipse(cx float8, cy float8, a float8, b float8, slices
float8);<br>
<br>
CREATE OR REPLACE FUNCTION make_ellipse(cx float8, cy float8, a float8, b
float8, slices float8) <br>
RETURNS geometry AS<br>
$BODY$<br>
declare<br>
cx alias for $1;<br>
cy alias for $2;<br>
a alias for $3;<br>
b alias for $4;<br>
slices alias for $5;<br>
i float;<br>
geom geometry;<br>
begin<br>
i := 0.0;<br>
execute 'create table tempo(ii float,the_geom geometry) with
oids;';<br>
WHILE i < 2.0*pi() LOOP<br>
execute 'insert into tempo select
'||i||'::float,makepoint('||cx+a*cos(i)||','||cy+b*sin(i)||');'; <br>
i := i+2.0*pi()/slices;<br>
END LOOP;<br>
execute 'select
setsrid(makepolygon(makeline(the_geom)),4170) from tempo;' into geom;<br>
execute 'drop table tempo;';<br>
return geom;<br>
end; <br>
$BODY$<br>
LANGUAGE 'plpgsql' VOLATILE;<br>
ALTER FUNCTION make_ellipse(cx float8, cy float8, a float8, b float8, slices
float8) OWNER TO marco;<br>
GRANT EXECUTE ON FUNCTION make_ellipse(cx float8, cy float8, a float8, b
float8, slices float8) TO public; <br>
<br>
<br>
</span></font></p>
<div>
<p><span><font face="Times New Roman" size="3"><span style="font-size: 12pt;">2007/3/7, Brent Wood <<a href="mailto:pcreso@pcreso.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">pcreso@pcreso.com
</a>>:</span></font></span></p>
<p><font face="Times New Roman" size="3"><span style="font-size: 12pt;"><br>
<br>
<br>
OK, I've trimmed all the other content, but my request is slightly off the main<br>
subject.<br>
<br>
Does anyone have any comments/advice on what I am doing in this subject area?<br>
The discusison so far suggests I could usefully learm something here :-) <br>
<br>
<br>
So far I've used a local custom equal area projection (Albers EA from memory) I<br>
created for my purposes.<br>
<br>
I was looking at the recorded start positions for fishing trawls along with the<br>
distance towed and gear width to derive an estimated swept area held as a <br>
circle centred on the start point. As I want the circles to represent a<br>
comparable area whether they at lat 20S or 60S, I used an equal area projection<br>
to create them.<br>
<br>
Mu limited understanding was that despite the circles being defined by a <br>
radius, an equal area was better than equidistant for this.<br>
<br>
I'm storing them as lat/long (EPSG:4326 projection with longs 0-360 as they<br>
cross the 180 meridian... sigh...). In this projection the circles display as <br>
ellipses, exactly as they should, but this does confuse some around here :-)<br>
<br>
<br>
Is there a better way to achieve this?<br>
<br>
Thanks,<br>
<br>
Brent<br>
_______________________________________________<br>
postgis-users mailing list <br>
<a href="mailto:postgis-users@postgis.refractions.net" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">postgis-users@postgis.refractions.net</a><br>
<a href="http://postgis.refractions.net/mailman/listinfo/postgis-users" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">http://postgis.refractions.net/mailman/listinfo/postgis-users
</a></span></font></p>
</div>
<p><font face="Times New Roman" size="3"><span style="font-size: 12pt;"><br>
<br clear="all">
<br>
-- <br>
Marco Vieira<br>
+55 21 9499-6800<br>
e-mail: <a href="mailto:maovieira@gmail.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">maovieira@gmail.com</a> </span></font></p>
</span></div></div>
</div>
<br>_______________________________________________<br>postgis-users mailing list<br><a href="mailto:postgis-users@postgis.refractions.net" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">postgis-users@postgis.refractions.net
</a><br><a href="http://postgis.refractions.net/mailman/listinfo/postgis-users" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">http://postgis.refractions.net/mailman/listinfo/postgis-users</a><br>
<br>
</blockquote></div><br><br clear="all"><br>-- <br>Marco Vieira<br>+55 21 9499-6800<br>e-mail: <a href="mailto:maovieira@gmail.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">maovieira@gmail.com
</a>