(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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