Using the GeoTools feature-pregeneralized module

Warning

The screenshots on this tutorial have not yet been updated for the 2.0.x user interface. But most all the rest of the information should be valid, and the user interface is roughly the same, but a bit more easy to use.

Introduction

This tutorial shows how to use the geotools feature-pregeneralized module in GeoServer. The feature-pregeneralized module is used to improve performance and lower memory usage and IO traffic.

Note

Vector generalization reduces the number of vertices of a geometry for a given purpose. It makes no sense drawing a polygon with 500000 vertices on a screen. A much smaller number of vertices is enough to draw a topological correct picture of the polygon.

This module needs features with already generalized geometries, selecting the best fit geometry on demand.

The full documentation is available in GeoTools Pregeneralized Plugin documentation.

This tutorial will show two possible scenarios, explaining step by step what to do for using this module in GeoServer.

Getting Started

First, find the location of the GEOSERVER_DATA_DIR. This info is contained in the log file when starting GeoServer.:

----------------------------------
- GEOSERVER_DATA_DIR: /home/mcr/geoserver-1.7.x/1.7.x/data/release
----------------------------------

Within this directory, we have to place the shape files. There is already a sub directory data which will be used. Within this sub directory, create a directory streams.

Within GEOSERVER_DATA_DIR/data/streams create another sub directory called 0. ( 0 meaning “no generalized geometries”).

This tutorial is based on a shape file, which you can download from here Streams. Unzip this file into GEOSERVER_DATA_DIR/data/streams/0.

Look for the WEB-INF/lib/ directory of your GeoServer installation. There must be a file called gt-feature-pregeneralized-version-jar. This jar file includes a tool for generalizing shape files. Open a cmd line and execute the following:

cd <GEOSERVER_DATA_DIR>/data/streams/0
java -jar <GEOSERVER_INSTALLATION>/WEB-INF/lib/gt-feature-pregeneralized-{version}.jar generalize 0/streams.shp . 5,10,20,50

You should see the following output:

Shape file            0/streams.shp
Target directory      .
Distances             5,10,20,50
% |################################|

Now there are four additional directories 5.0 , 10.0 , 20.0 , 50.0 . Look at the size of files with the extension shp within these directories, increasing the generalization distance reduces the file size.

Note

The generalized geometries can be stored in additional properties of a feature or the features can be duplicated. Mixed variations are also possible. Since we are working with shape files we have to duplicate the features.

There are two possibilities how we can deploy our generalized shape files.

  1. Deploy hidden (not visible to the user)

  2. Deploy each generalized shape file as a separate GeoServer feature

Hidden Deployment

First we need a XML config file

<?xml version="1.0" encoding="UTF-8"?>
<GeneralizationInfos version="1.0">
      <GeneralizationInfo dataSourceName="file:data/streams/0/streams.shp"  featureName="GenStreams" baseFeatureName="streams" geomPropertyName="the_geom">
              <Generalization dataSourceName="file:data/streams/5.0/streams.shp"  distance="5" featureName="streams" geomPropertyName="the_geom"/>
              <Generalization dataSourceName="file:data/streams/10.0/streams.shp"  distance="10" featureName="streams" geomPropertyName="the_geom"/>
              <Generalization dataSourceName="file:data/streams/20.0/streams.shp"  distance="20" featureName="streams" geomPropertyName="the_geom"/>
              <Generalization dataSourceName="file:data/streams/50.0/streams.shp"  distance="50" featureName="streams" geomPropertyName="the_geom"/>
      </GeneralizationInfo>
</GeneralizationInfos>

Save this file as geninfo_shapefile.xml into GEOSERVER_DATA_DIR/data/streams.

Note

The dataSourceName attribute in the XML config is not interpreted as a name, it could be the URL for a shape file or for a property file containing properties for data store creation (e. g. jdbc connect parameters). Remember, this is a hidden deployment and no names are needed. The only official name is the value of the attribute featureName in the GeneralizationInfo Element.

Start GeoServer and go to Config‣Data‣DataStores‣New and fill in the form

../../_images/createdatastore.png

Press Submit.

The next form you see is

../../_images/editdatastore1.png

Note

RepositoryClassName and GeneralizationInfosProviderClassName have default values which suit for GeoTools, not for GeoServer. Change GeoTools to GeoServer in the package names to instantiate the correct objects for GeoServer. GeneralizationInfosProviderParam could be an URL or a datastore from the GeoServer catalog. A datastore is referenced by using workspacename:datastorename. This makes sense if you have your own implementation for the GeneralizationInfosProvider interface and this implementation reads the infos from a database.

The configuration should look like this

../../_images/editdatastore2.png

Press Submit, afterward a form for the feature type opens.

Alter the Style to line, SRS is 26713 and press the Generate button labeled by Bounding Box.

../../_images/editfeaturestore1.png

Afterward, press Submit, Apply and Save.

Examine the result by pressing “My GeoServer, Demo and Map Preview. In this list there must be an entry topp:GenStreams. Press it and you will see

../../_images/streams.png

Now start zooming in and out and look at the log file of GeoServer. If the deployment is correct you should see something like this:

May 20, 2009 4:53:05 PM org.geotools.data.gen.PreGeneralizedFeatureSource logDistanceInfo
INFO: Using generalizsation: file:data/streams/20.0/streams.shp streams the_geom 20.0
May 20, 2009 4:53:41 PM org.geotools.data.gen.PreGeneralizedFeatureSource logDistanceInfo
INFO: Using generalizsation: file:data/streams/5.0/streams.shp streams the_geom 5.0
May 20, 2009 4:54:08 PM org.geotools.data.gen.PreGeneralizedFeatureSource logDistanceInfo
INFO: Using generalizsation: file:data/streams/5.0/streams.shp streams the_geom 5.0
May 20, 2009 4:54:09 PM org.geotools.data.gen.PreGeneralizedFeatureSource logDistanceInfo
INFO: Using generalizsation: file:data/streams/20.0/streams.shp streams the_geom 20.0

Public Deployment

First we have to configure all our shape files

../../_images/streams_0_ds.png

The Feature Data Set ID for the other shape files is

  1. Streams_5

  2. Streams_10

  3. Streams_20

  4. Streams_50

../../_images/streams_0_ds2.png

The URL needed for the other shape files

  1. file:data/streams/5.0/streams.shp

  2. file:data/streams/10.0/streams.shp

  3. file:data/streams/20.0/streams.shp

  4. file:data/streams/50.0/streams.shp

../../_images/streams_0_ds4.png

Each feature needs an Alias, here it is streams_0. For the other shape files use

  1. streams_5

  2. streams_10

  3. streams_20

  4. streams_50

Check the result by pressing My GeoServer, Demo and Map Preview. You should see your additional layers.

No we need another XML configuration file

<?xml version="1.0" encoding="UTF-8"?>
<GeneralizationInfos version="1.0">
      <GeneralizationInfo dataSourceNameSpace="topp" dataSourceName="Streams_0"  featureName="GenStreams2" baseFeatureName="streams" geomPropertyName="the_geom">
              <Generalization dataSourceNameSpace="topp" dataSourceName="Streams_5"  distance="5" featureName="streams" geomPropertyName="the_geom"/>
              <Generalization dataSourceNameSpace="topp" dataSourceName="Streams_10"  distance="10" featureName="streams" geomPropertyName="the_geom"/>
              <Generalization dataSourceNameSpace="topp" dataSourceName="Streams_20"  distance="20" featureName="streams" geomPropertyName="the_geom"/>
              <Generalization dataSourceNameSpace="topp" dataSourceName="Streams_50"  distance="50" featureName="streams" geomPropertyName="the_geom"/>
      </GeneralizationInfo>
</GeneralizationInfos>

Save this file as geninfo_shapefile2.xml into GEOSERVER_DATA_DIR/data/streams.

Create the pregeneralized datastore

../../_images/createdatastore2.png

Now we use the CatalogRepository class to find our needed data stores

../../_images/editdatastore3.png

Last step

../../_images/editfeaturestore2.png

In the Map Preview you should find topp:GenStreams2 and all other generalizations. Test in the same manner we discussed in the hidden deployment and you should see something like this in the GeoServer log:

May 20, 2009 6:11:06 PM org.geotools.data.gen.PreGeneralizedFeatureSource logDistanceInfo
INFO: Using generalizsation: Streams_20 streams the_geom 20.0
May 20, 2009 6:11:08 PM org.geotools.data.gen.PreGeneralizedFeatureSource logDistanceInfo
INFO: Using generalizsation: Streams_10 streams the_geom 10.0
May 20, 2009 6:11:12 PM org.geotools.data.gen.PreGeneralizedFeatureSource logDistanceInfo
INFO: Using generalizsation: Streams_10 streams the_geom 10.0

Conclusion

This is only a very simple example using shape files. The plugin architecture allows you to get your data and generalizations from anywhere. The used dataset is a very small one, so you will not feel a big difference in response time. Having big geometries (in the sense of many vertices) and creating maps with some different layers will show the difference.

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