[postgis-users] Usage of new CIRCULARSTRING

[Ken Southerland]

Hi all,

I need to be able to save and load the new CircularString and related 
geometries in my java application.  I have the source code checked out 
and have been poking through it.  I would like to try to extend the jdbc 
code to be able to handle these new geometries but first I have to be 
able to understand them so that I can program it and I can't find any 
documentation to help me with the exception of section 4.1.3 of the 
on-line documentation found at 
http://postgres.enterprisedb.com/82/doc/dbserver/postgis/postgis.html. 
But this has no words of explanation for the examples.

Here are my questions for each of the examples given in this 
documentation.  Can anyone shed light on this?

1)  CIRCULARSTRING(0 0, 1 1, 1 0)
How is this defined?  I understand the the three point minimum and then 
an odd number of points after that.  What I don't know is what the even 
points represent.  For instance, in this example, is (1 1) the location 
of the center of the circle of which the arc is a segment or is it just 
yet another point on the arc?  If the latter, then do we have to 
calculate the arc every time we read the object from the database? Isn't 
this computationally unattractive?  Again if the latter, is there some 
simple way that I can't see yet of creating a java.awt.geom.Arc2D from 
these three points?


2)  COMPOUNDCURVE(CIRCULARSTRING(0 0, 1 1, 1 0),(1 0, 0 1))
Is this an arc with a straight line segment at the end?  What if the 
start of this straight line segment did not match the end of the arc 
segment?  i.e. it didn't start with the coordinate (1 0).  Is that 
allowed?  If so, is there then a gap in the geometry?

3)  CURVEPOLYGON(CIRCULARSTRING(0 0, 4 0, 4 4, 0 4, 0 0),(1 1, 3 3, 3 1, 
1 1))
My best guess is that this is effectively a circle with a triangle in 
the middle of it.  Help.  :)

4)  MULTICURVE((0 0, 5 5),CIRCULARSTRING(4 0, 4 4, 8 4))
How is this different then the COMPOUNDCURVE object above?  Is the 
difference that the straight line segment can be separated from the arc, 
as in the two geometries don't touch?

5)  MULTISURFACE(CURVEPOLYGON(CIRCULARSTRING(0 0, 4 0, 4 4, 0 4, 0 0),(1 
1, 3 3, 3 1, 1 1)),((10 10, 14 12, 11 10, 10 10),(11 11, 11.5 11, 11 
11.5, 11 11)))
I'm guessing I would understand this one just fine once I understood the 
above ones.  Just adding for completeness.  :)

Thanks for your help.

Ken


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Ken Southerland
Senior Consultant
Sam Six, Inc.

503-236-4288
http://www.samsix.com