[geos-devel] What do GEOSSymDifference and GEOSIntersection (CAPI)
Roger.Bivand at nhh.no
Wed Feb 10 04:46:33 EST 2010
Continuing to try to interface R and GEOS (CAPI), I am puzzled that
GEOSSymDifference() and GEOSIntersection() seem to require input objects
to be "Geometry must be a Point or LineString".
When MULTIPOLYGON or POLYGON objects are degraded to LINEARRING,
GEOSSymDifference() returns MULTILINESTRING and GEOSIntersection() returns
MULTIPOINT objects. Are these functions not intended to take at least
POLYGON inputs, and then return POLYGON output? The cryptic notes in
suggest that they don't:
"intersection (const Geometry *other) const Returns a Geometry
representing the points shared by this Geometry and other."
"symDifference (const Geometry *other) const Returns a set combining the
points in this Geometry not in other, and the points in other not in this
If this is all the functions do by design, how should the POLYGON objects
be reconstructed, since the distinction between exterior and interior
rings has been lost? Is there example code anywhere? The code in OGR
doesn't help here, I'm afraid. Is this the OGC SFS understanding of these
functions - is that the ontological problem that I'm having coming from
I'm running GEOS 3.2.0, built locally on RHEL5 x86_64.
For union, UnionCascaded of a MULTIPOLYGON returns a POLYGON or
MULTIPOLYGON, as one would expect.
Grateful for advice,
PS. the previous thread that I opened is still unresolved, and GEOS in R
(if it is submitted at all), will be published with a big question mark
about precision and scaling with regard to the dissolving of contiguous
For now establishing whether GEOS can do geometry operations on
(multi)-polygons is crucial. GEOS ought to be able to deliver, but I need
to be shown how - example code, proper documentation of the C API (saying
what can be used as arguments to functions as a start without having to
delve through arcane C++ documentation which may or may not apply to the C
API) - I should proceed.
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no
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