Geometry Boost List: [ggl] is_simple is_valid

Subject: [ggl] is_simple is_valid
From: Barend Gehrels (Barend.Gehrels)
Date: 2009-10-03 05:24:34

Hi,

I'm looking at self-intersections to implement the function is_simple 
(based on blocks we already have). But I'm what confused about the name.

According to OGC:  IsSimple  (): Integer Returns 1 (TRUE) if this 
geometric object has no anomalous geometric points, such as self 
intersection or self tangency. The description of each instantiable 
geometric class will include the specific conditions that cause an 
instance of that class to be classified  as not simple.

JTS says: Valid polygonal geometries are simple by definition, so 
isSimple trivially returns true.  
(http://www.vividsolutions.com/JTS/javadoc/com/vividsolutions/jts/geom/Geometry.html#isSimple() 
)

The PostGIS implementation give TRUE back to all self-intersecting 
polygons that I try. The function isvalid returns false for 
self-intersecting polygons.

SQL Server throws for issimple at self-intersections:
System.ArgumentException: 24144: This operation cannot be completed 
because the instance is not valid. Use MakeValid to convert the instance 
to a valid instance. Note that MakeValid may cause the points of a 
geometry instance to shift slightly.
System.ArgumentException:
   at Microsoft.SqlServer.Types.SqlGeometry.ThrowIfInvalid()
   at Microsoft.SqlServer.Types.SqlGeometry.STIsSimple()
.
So it is often called isvalid (or is_valid). But I cannot find that in 
OGC specs (looked a.o. in 06-103r3 (still candidate?))

So I probably overlook something somewhere...

Regards, Barend