:Author: Jody Garnett :Author: Micheal Bedward :Thanks: geotools-user list :Version: |release| :License: Create Commons with attribution ******************** Feature Tutorial ******************** .. sectionauthor:: Jody Garnett Welcome ======= Welcome to Geospatial for Java - this workbook is aimed at Java developers who are new to geospatial and would like to get started. .. _quickstarts: ../quickstart/index.html You should have completed one of the GeoTools' Quickstarts_ prior to running through this workbook. We need to be sure that you have an environment to work in with GeoTools jars and all their dependencies. For those using just maven we will start off each section with the dependencies required. This workbook features a new "code first" approach. We have made every effort to make these examples both visual and code centered. We have included some background materials explaining the concepts and ideas in case you are interested. CSV2SHP ======= We are trying a new track for introducing features this year; rather than reading through a shapefile and ripping things apart in an artificial exercise, we are going to start by building a shapefile from scratch so you get to see every last thing that goes into creating features. The tutorial covers the following: * Creating a ``FeatureType``, ``FeatureCollection`` and ``Features`` * Using a ``GeometryFactory`` to build Points * Writing out a Shapefile * Forcing a Projection At the end of the tutorial you will be able to create your own custom shapefiles. Comma Separated Value --------------------- To start with you will need a CSV file. #. Create a text file ``location.csv`` and copy and paste the following locations into it: .. literalinclude:: artifacts/locations.csv #. Or download :download:`locations.csv `. #. Feel free to add other locations to the file such as your home town. Dependencies ------------ Please ensure your ``pom.xml`` includes the following: .. literalinclude:: ./artifacts/pom.xml :language: xml :start-after: :end-before: Note that the jars mentioned above will pull in a host of other dependencies (such as the hsql database driver). Main Application ---------------- 1. Please create the package ``org.geotools.tutorial.feature`` and class ``Csv2Shape.java`` . 2. Copy and paste in the following code: .. literalinclude:: /../src/main/java/org/geotools/tutorial/feature/Csv2Shape.java :language: java :start-after: // start source :end-before: // docs break feature type Now we look at the rest of the main method in sections... Create a FeatureType -------------------- We create a ``FeatureType`` to describe the data that we are importing from the CSV file and writing to a shapefile. Here we use the ``DataUtilities`` convenience class: .. literalinclude:: /../src/main/java/org/geotools/tutorial/feature/Csv2Shape.java :language: java :start-after: // docs break feature type :end-before: // docs break feature collection Create features --------------- We can now read the CSV file and create a feature for each record. Please note the following: * Use of ``GeometryFactory`` to create new Points * Creation of features (``SimpleFeature`` objects) using ``SimpleFeatureBuilder`` .. literalinclude:: /../src/main/java/org/geotools/tutorial/feature/Csv2Shape.java :language: java :start-after: // docs break feature collection :end-before: // docs break new shapefile .. note:: If you have used previous versions of GeoTools you might be used to creating a new ``FeatureCollection`` and using the add method to accumulate features. This usage has now been deprecated and we encourage you to treat ``FeatureCollections`` as immutable views or result sets. Create a shapefile From a FeatureCollection ------------------------------------------- Things to note as we create the shapefile: * Use of ``DataStoreFactory`` with a parameter indicating we want a spatial index * The use of ``createSchema(SimpleFeatureType)`` method to set up the shapefile (We will create the ``getNewShapeFile`` method in the next section) .. literalinclude:: /../src/main/java/org/geotools/tutorial/feature/Csv2Shape.java :language: java :start-after: // docs break new shapefil :end-before: // docs break transaction Write the feature data to the shapefile --------------------------------------- Things to note: * We check that we have read and **write** access by confirming our ``FeatureSource`` object implements the ``FeatureStore`` methods * Take a moment to check how closely the shapefile was able to match our template (the ``SimpleFeatureType`` ``TYPE``). Compare this output to see how they are different. * The ``SimpleFeatureStore`` that we use to do this expects a ``FeatureCollection`` object, so we wrap our list of features in a ``ListFeatureCollection``. * The use of :code:`transaction.commit()` to safely write out the features in one go. .. literalinclude:: /../src/main/java/org/geotools/tutorial/feature/Csv2Shape.java :language: java :start-after: // docs break transaction :end-before: // end main This completes the main method. Prompt for the output shapefile ------------------------------- This method prompts the user for an appropriate shapefile to write out to. The original CSV file is used to determine a good default shapefile name. .. literalinclude:: /../src/main/java/org/geotools/tutorial/feature/Csv2Shape.java :language: java :start-after: // start get shapefile :end-before: // end get shapefile Running the Application ----------------------- .. sidebar:: Show Your Shapefile You might like to see if you can view the new shapefile using the Quickstart.java application from the previous tutorial. When you run this application it will prompt you for: * The location of a CSV file to read; and then * The shapefile to create Things to Try =============== Another way to build a SimpleFeatureType ---------------------------------------- Although the ``DataUtilities`` class used above provided a quick and easy way to build a ``SimpleFeatureType``. For any *real* application you will want to use the more flexible and flexible ``SimpleFeatureTypeBuilder``. Here is how to use ``SimpleFeatureTypeBuilder`` to accomplish the same result: .. literalinclude:: /../src/main/java/org/geotools/tutorial/feature/Csv2Shape.java :language: java :start-after: // start createFeatureType :end-before: // end createFeatureType Note the use of an upper-case constant to hold the ``SimpleFeatureType``. Because the ``SimpleFeatureType`` class is immutable, tracking them as final variables can help you to remember that they cannot be modified once created. With this method our ``SimpleFeatureType`` contains a ``CoordinateReferenceSystem`` so there's no need to call ``forceSchemaCRS`` to generate the ``.prj`` file. Also, we are now limiting the *Name* field to 15 characters. Other things to try ------------------- * Modify the code to read the feature attribute names from the data file header rather than hard-coding them in the application:: LAT, LON, CITY, NUMBER You should be able to use the ``SimpleFeatureTypeBuilder``. .. instructor note: this is the real key to the exercise and why simple feature type builder exists ... allowing you to generate your own shapefile dynamically * Use the ``Geometry`` ``buffer`` method to create circles based on the population size of the each city. .. code-block:: java Polygon polygon = location.buffer( 0.1 ); .. instructor note: the code above is a trick; to see if they can correctly change their feature type to be a polgon * It is easy to write a quick ``CSVReader`` as we have done here; but harder to write a good one that can handle quotation marks correctly. ``JavaCSV`` is an open source library to read CSV files with a variety of options. * To quickly find dependencies you can use the website http://mvnrepository.com/. Sites like this will directly provide you a maven dependency that you can cut and paste into your ``pom.xml``. .. code-block:: xml net.sourceforge.javacsv javacsv 2.0 For a working example of how to use this library visit the http://www.csvreader.com/ website. * Use the same techniques to create shapefiles from data in other structured file formats such as GeoJson * The earth has just passed through a meteor storm – generate 100 circles of different sizes across the globe. Was your town hit? Generating a shapefile from a model or analysis is a common use. .. instructor note: either they display two shapefiles and look to see if their town was hit or they can do a contains test for their town against each circle * Read up about the other ``Geometry`` classes supported by shapefiles: ``MultiLineString`` for linear features and ``MultiPolygon`` for area features and modify this example to work with these. Feature ======== .. sidebar:: Conceptual Feature You can also draw ideas like urban growth or predicted rain fall. A feature is something that can be drawn on a map. The strict definition is that a feature is something in the real world – a feature of the landscape - Mt Everest, the Eiffel Tower, or even something that moves around like your great aunt Alice. Explaining the concept to Java developers is easy - a feature is an Object. Like a java object features can contain some information about the real world thing that they represent. This information is organized into attributes just as in Java information is slotted into fields. .. sidebar:: Key The "feature types" for a map are listed in the map key. .. image:: images/key.png :width: 200 Occasionally you have two features that have a lot in common. You may have the ``LAX`` airport in Los Angeles and the ``SYD`` airport in Sydney. Because these two features have a couple of things in common it is nice to group them together - in Java we would create a ``Class`` called ``Airport``. On a map we will create a ``FeatureType`` called ``Airport``. .. image:: images/airport.png Although it is not a capability supported by Java, early programming languages made use of a prototype system (rather than a class system) that supported lots of "one off" Objects. You will find this situation is fairly common when making maps – since how many Eiffel towers are there? You will also occasionally find the same real world thing represented a couple of different ways (the Eiffel tower can be a landmark or a tower depending on context). Here is a handy cheat sheet: ====================== ============================= Java Geospatial ====================== ============================= ``Object`` ``Feature`` ``Class`` ``FeatureType`` ``Field`` ``Attribute`` ``Method`` ``Operation`` ====================== ============================= The ``Feature`` model is actually a little bit more crazy than us Java programmers are used to since it considers both attribute and operation to be "properties" of a ``Feature``. Perhaps when Java gets closures we may be able to catch up. The really interesting thing for me is that map makers were sorting out all this stuff back in the 1400s and got every bit as geeky as programmers do now. So although we would love to teach them about object oriented programming they already have a rich set of ideas to describe the world. On the bright side, map makers are starting to use UML diagrams. FeatureClass ------------ In GeoTools we have an interface for ``Feature``, ``FeatureType`` and ``Attribute`` provided by the GeoAPI project. In general GeoAPI provides a very strict interface and GeoTools will provide a class. .. image:: images/feature.png It is very common for a ``Feature`` to have only simple ``Attributes`` (``String``, ``Integer``, ``Date`` and so on) rather than references to other ``Features``, or data structures such as ``List``. ``Features`` that meet this requirement are so common we have broken out a sub-class to represent them called ``SimpleFeature``. At the Java level the ``Feature`` API provided by GeoTools is similar to how ``java.util.Map`` is used – it is a Java data structure used to hold information. You can look up attribute values by key; and the list of keys is provided by the ``FeatureType``. Geometry -------- The other difference between an ``Object`` and a ``Feature`` is that a ``Feature`` has some form of location information (if not we would not be able to draw it on a map). The location information is going to be captured by a ``Geometry`` (or shape) that is stored in an attribute. .. image:: images/geometry.png We make use of the JTS Topology Suite (JTS) to represent ``Geometry``. The JTS library provides an excellent implementation of ``Geometry`` – and gets geeky points for having a recursive acronym! JTS is an amazing library and does all the hard graph theory to let you work with geometry in a productive fashion. Here is an example of creating a ``Point`` using the Well-Known-Text (WKT) format. .. code-block:: java GeometryFactory geometryFactory = JTSFactoryFinder.getGeometryFactory( null ); WKTReader reader = new WKTReader( geometryFactory ); Point point = (Point) reader.read("POINT (1 1)"); You can also create a ``Point`` by hand using the ``GeometryFactory`` directly. .. code-block:: java GeometryFactory geometryFactory = JTSFactoryFinder.getGeometryFactory( null ); Coordinate coord = new Coordinate( 1, 1 ); Point point = geometryFactory.createPoint( coord ); DataStore ========= We ran into ``DataStore`` already in our Quickstart. The ``DataStore`` API is used to represent a ``File``, ``Database`` or ``Service`` that has spatial data in it. The API has a couple of moving parts as shown below. .. image:: images/datastore.png The ``FeatureSource`` is used to read features, the sub-class ``FeatureStore`` is used for read/write access. The way to tell if a ``File`` can be written to in GeoTools is to use an ``instanceof`` check. .. code-block:: java String typeNames = dataStore.getTypeNames()[0]; SimpleFeatureSource source = store.getfeatureSource( typeName ); if( source instanceof SimpleFeatureStore){ SimpleFeatureStore store = (SimpleFeatureStore) source; // write access! store.addFeatures( featureCollection ); store.removeFeatures( filter ); // filter is like SQL WHERE store.modifyFeature( attribute, value, filter ); } We decided to handle write access as a sub-class (rather than an ``isWritable`` method) in order to keep methods out of the way unless they could be used.