Well, I'm slightly embarassed to admit that for the case of an envelope
for a horizontal or vertical line, JTS actually returns an invalid
polygon. (For point envelopes however it does return a Point geometry).
There's a couple of reasons for this. One is that the OGC SFS seems to
imply that when returning envelopes as a Geometry, a Polygon should be
returned. The other is simply that the original JTS behaviour was to
always return a Polygon, and when the code was enhanced to return Points
for zero-area envelopes, the linear case was omitted.
In its defence I'll point out that in my experience working with JTS it
is *very* rare to need to obtain a Geometry as an envelope. Usually
when doing "Envelopey" kinds of things it is much faster, simpler, and
more robust to simply work with actual Envelope objects.
Charlie Savage wrote:
Paul,
Hmmm... for an linear envelope would returning a linestring not make
more sense?
That does raise another question.
select astext(envelope('LINESTRING(0 0, 0 10)'))
"POLYGON((0 0,0 10,0 10,0 0,0 0))"
select valid(envelope('LINESTRING(0 0, 0 10)'))
false
Should envelope should do the same? What does JTS/GEOS do in this case?
Charlie
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Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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