[postgis-users] Is CIRCULARLINESTRING ready for use?

Frank Warmerdam warmerdam at pobox.com
Thu Jul 3 10:48:00 PDT 2008

Bruce Rindahl wrote:
> I also have numerous parcels with circular arcs for testing but you must 
> tell me what the syntax is.  The documentation gives CIRCULARSTRING(0 0, 
> 1 1, 1 0) but does not tell you what the points 0 0 , 1 1  and 1 0 
> mean.  Three point arc?  Start, end, center?  Let me know and I will 
> send you lots of shapes.


Start, End and any other point on the arc that isn't the start or end.
Note necessarily the center.  In the case of a circle the start and
end are the same, but the intermediate point *is* required to be the
arc center (ie. the opposite side of the circle).

> Finally I have to question the support of circularstrings if we don't 
> support ellipses.  The circle of Delaware is an ellipse in every 
> projection except geodetic (?).  We can't correctly project a 
> circularstring to anything and maintain the spatial relationship.  For 
> example in SVG there is no circle command in paths - only ellipses.  A 
> circular arc is defined as an ellipse with both radii the same.

I too am dubious about the use of a circular arc segment instead of
an elliptical arc segment.  However, in this regard PostGIS is just
tracking OGC and ISO standards, so take the argument to them.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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