[postgis-devel] PostGIS 1.1.7

Mark Leslie mark at refractions.net
Wed Dec 13 09:16:03 PST 2006

Markus Schaber wrote:
> Hello Mark,
>> The spec can be downloaded from www.iso.org for the low low price of CHF340.
>> For this release, GEOS doesn't handle them.  The next release should
>> provide the ability to convert the curves into linestrings, pass them to
>> geos functions, then reclaim the curves once returned.
> How do you intend to implement that mapping?
> And how does "reclaim the curves" work when the linestring was
> processed (intersected etc.) in GEOS?
> I ask that because I have the same problem for my PostGIS JTS mappings
> (JDBC and PlJava).

The conversion of curve to linestring will be done simply by
approximating the curve by a given number of segments per quarter
circle.  This is the same approach as is used by the buffer command,
only likely with a higher default number of segments.
The conversion back will be done by examining sets of points and looking
for sequences of segments of equal length with the same angle from the
previous segment.  This way, if the original curve is only partially
included in the final geometry, that bit would be recognized and a new
curve constructed.

>> Eventually GEOS will be upgraded to support them properly.
> So GEOS will finally fork from JTS?

I don't expect so.  This step is still a ways away, but it's expected
that the upgrade will first go into JTS, then be ported back into GEOS.

>> As for the WKT/WKB, full parsing support isn't there yet.
> What's missing there?

The EWKT parser will accept the new types (eg. CIRCULARSTRING(0 0,0 1,1
1)) but the higher dimensional definitions aren't up to the SQL-MM spec
yet.  The parser will accept CIRCULARSTRING(0 0 0, 0 1 3, 1 1 6) for
example, while the spec demands CIRCULARSTRING Z (0 0 0,0 1 3,1 1 6).

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