If all you really want is to be able to test whether points lie within a
given distance of another geometry, then certainly the mathforum
approach will work.
If you need the actual buffer geometry for some reason (e.g. display)
then it doesn't help, I don't think.
Davis Ford wrote:
FYI: the app we're building needs to support Oracle 9i +
I was testing with Oracle 10g using the SDO_BUFFER function. When we
tested against 9i, it broke. We looked the issue up and it appears
that there was a bug in 9i with SDO_BUFFER that was fixed in 10g and
apparently "it is too complex to backport" (their words).
So, there you have it.
Now, I am forced to do the buffer calculation in Java anyway :)
I have used Geotools long ago, so I am semi-familiar with it and I
will probably try that first. First a dumb question...is this a naive
solution to the same problem:
http://mathforum.org/library/drmath/view/66987.html
...that is to say, couldn't I use the recommendation there to get a
reasonable approximation of the buffer calculation from geographic
coordinates, or am I missing something fundamental?
Regards,
Davis
On Wed, Feb 11, 2009 at 12:39 PM, Stefan Steiniger <[email protected]> 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]>
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 = 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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--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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