Good find, Larry.
So there's no magic after all... Too bad, I thought that for the money
Oracle should be stepping up to providing true ellipsoidal computation!
I wonder what they do for very large objects, and ones which span the
poles or data line? Do they just let it return garbage?
Larry Becker wrote:
BTW. I found this about Oracle 11g and buffers:
With geodetic data, this function is supported by approximations, as
explained in Section 6.10.3
<http://download.oracle.com/docs/cd/B28359_01/appdev.111/b28400/sdo_cs_concepts.htm#i894707>.
:
"When these functions are used on data with geodetic coordinates, they
internally perform the operations in an implicitly generated
local-tangent-plane Cartesian coordinate system and then transform the
results to the geodetic coordinate system. For SDO_GEOM.SDO_BUFFER
<http://download.oracle.com/docs/cd/B28359_01/appdev.111/b28400/sdo_objgeom.htm#i856140>,
generated arcs are approximated by line segments before the
back-transform."
I suspect this method only works well for local sized areas, and you
would not want, for instance, to buffer the Mississippi river.
It never hurts to know how the competition does it. :-)
Larry
On Wed, Feb 11, 2009 at 11:48 AM, Stefan Steiniger <[email protected]
<mailto:[email protected]>> wrote:
btw.
I believe deeJUMP uses the degree lib. But I couldn't find it in
this listing:
https://wiki.deegree.org/deegreeWiki/OpenJumpPackages
stefan
Stefan Steiniger wrote:
well..
- Geotools: http://geotools.codehaus.org/Module+Matrix
and
- degree:
http://wald.intevation.org/plugins/scmsvn/viewcvs.php/docs/documentation/crs/?root=deegree
(I hope that is the correct link)
have projection functions
I know this as we, from OpenJUMP, ponder with an
implementation of those - probably the latter one. Meanwhile
we have only this plugin-workaround:
http://www.openjump.org/wiki/show/Working+with+Projections
but to write back to oracle is probably a difficult part too?
stefan
Davis Ford wrote:
Martin / Larry - thanks for the info.
Any tips / pointers / clues on how to do the projection /
re-projection using the JTS API?
Regards,
Davis
On Wed, Feb 11, 2009 at 11:43 AM, Martin Davis
<[email protected] <mailto:[email protected]>>
wrote:
(I'm posting this to the JTS list, since it's really a
JTS question)
This seems to be geodetic month for JTS!
Answers to your questions:
1. JTS is not "coordinate system-aware", and does not
use the SRID in its
calculations. So you can set it or not, and it won't
make any difference
2. JTS assumes that the coordinates of geometries are
located in an infinite
Cartesian (flat) 2D coordinate system. All quantities
(length, area,
distance, angle, etc) are relative in this coordinate
system. So since you
are providing your input in decimal degrees, those are
the units that the
buffer distance must be expressed in. Of course, this
doesn't work all that
well for large distances in a geodetic (ellipsoidal)
coordinate system - the
computed geoemtry will be only an approximation to the
true shape.
What people often do is project their geodetic data to
a local planar
projection, compute there, and then reproject.
There's been a bit of a
thread about this on the JTS list recently. (No code,
though - which would
be nice to have).
As you point out, Oracle appears to do the "right
thing" in this case -
kudos to them. They seem to seamlessly blend geodetic
and planar data
handling, which is certainly nice to have. Maybe one
day JTS will support
this - but it's complex to implement.
Davis Ford wrote:
Hi,
If I have an Oracle table with an SDO_GEOMETRY
column, and I insert a
point into it - example:
MDSYS.SDO_GEOMETRY(2001, 8307, NULL,
MDSYS.SDO_ELEM_INFO_ARRAY(1, 1,
1), MDSYS.SDO_ORDINATE_ARRAY(-82.90755596903085,
42.40409951227155))
Then I can use the SDO_GEOM.SDO_BUFFER function to
get a buffer around
it (and specify the units in miles) ->
// 1.5 miles, 0.5 tolerance
SELECT SDO_GEOM.SDO_BUFFER(a.geometry, 1.5, 0.5,
'unit=mile') geom
FROM MY_TABLE a.id <http://a.id> = 1;
Simple enough, but how do I do the same thing in
JTS? If I try the
following:
Geometry point = new
WKTReader().read("POINT(-82.90755596903085
42.40409951227155)");
// question1: should I set the SRID? Oracle uses
8307 for WGS:84, but
JTS seems to ignore SRID in calculations, is this
true?
// question2: what units does buffer take? I make
the assumption of
meters, but this is wrong
// try converting 1.5 miles to meters
double meters = 2414.016
Geomtry buffer = point.buffer(meters);
This is very wrong since it gives me polygon with
coordinates that are
outside -180/180, -90/90. Do I assume then that
buffer takes radians
I guess?
I'm just trying to accomplish the same thing I can
do in Oracle above with
JTS.
Thanks in advance,
Davis
--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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Refractions Research, Inc.
(250) 383-3022
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