jython + the JTS geometry library

My favorite library for dealing with two dimensional points, lines, and polygons is Polygon by Jörg Rädler.  In a Java/jython environment, though, that's not available.  This led me to try my hand at JTS.  

Greg Corradini has a post that highlights JTS use with jython and geoscript.  His overview was very helpful, but I wanted to get to know JTS more thoroughly.

The first thing I came up against when using JTS was the documentation.  It's all javadocs.  This isn't necessarily a bad thing, but, along with the library's java-centric design, it takes a little getting used to.  If concepts like abstract class, iterface, extending, and implementing don't immediately ring a bell to the user, use of the javadocs will make them clear over time.

That shouldn't discourage us, though.  Let's fire up the interpreter and try something simple.

Jython 2.5.2 (Release_2_5_2:7206, Mar 2 2011, 23:12:06)
[OpenJDK Client VM (Sun Microsystems Inc.)] on java1.7.0-internal
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.path.append('/home/carl/Downloads/jts/lib/jts-1.11.jar')

I usually store things I'm testing out in my Downloads folder (I'm using KDE on OpenBSD).  This may not be the best practice.  In any case, to import from jts I need access to the jar in my sys.path.

Making a Point object in JTS in a bit complicated.  There are Coordinate objects and there are Point objects.  The Coordinate is a more basic class with less functionality than the Point class.  Point inherits from the abstract class Geometry.  This leads it to have a number of methods, some meaningless, like convexHull.  Shown below is the process required to make a Point object.

>>> from com.vividsolutions.jts.geom import Point
>>> from com.vividsolutions.jts.geom import GeometryFactory
>>> from com.vividsolutions.jts.geom.impl import CoordinateArraySequence
>>> # get CoordinateArraySequence of size one
>>> cas = CoordinateArraySequence(1)
>>> factoryx = GeometryFactory()
>>> ptx = Point(cas, factoryx)

We have a Point object.  A couple explanatory notes:

1) the documentation for the default GeometryFactory reads thus:  "Constructs a GeometryFactory that generates Geometries having a floating PrecisionModel and a spatial-reference ID of 0."  For what I'm doing this is fine.

2) The preferred constructor for the Point object takes a CoordinateArraySequence of length one instead of a single Coordinate object.  This is analogous to a character in Python being a string of length one.

We don't know yet what our values are for our Point's coordinates.  Let's have a look.

>>> ptx
POINT (0 0)

OK, zero-zero.  Let's assign something to those coordinates away from the origin.

>>> ptx.getCoordinates()[0].x = 44
>>> ptx.getCoordinates()[0].y = 22
>>> ptx
POINT (44 22)

Even though the Point has only one coordinate, we still need to specify that with the index 0.

Well, that was a lot of work for just one point.  Let's try something more efficient and fun.

>>> cas2 = CoordinateArraySequence(6)
>>> somecoordinates = [(1, 5), (7, 14), (22, 44), (36, 12), (19, 1), (12, 4)]
>>> for numx in range(len(somecoordinates)):
...     cas2.getCoordinate(numx).x = somecoordinates[numx][0]
...     cas2.getCoordinate(numx).y = somecoordinates[numx][1]
...
>>> cas2
((1.0, 5.0, NaN), (7.0, 14.0, NaN), (22.0, 44.0, NaN), (36.0, 12.0, NaN), (19.0, 1.0, NaN), (12.0, 4.0, NaN))
>>> from com.vividsolutions.jts.geom import LineString
>>> ls = LineString(cas2, factoryx)
>>> ch = ls.convexHull()
>>> ch
POLYGON ((19 1, 1 5, 22 44, 36 12, 19 1))
>>> 

LineString is one of the classes that can be instantiated from a CoordinateArraySequence.  I went with that mainly for ease of use.

There is a great deal more to JTS.  This post was mainly for getting started with the library.