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tgext.geo MapFish Tutorial

Introduction

MapFish is a Web Application Framework for developing Geographic Applications. It has two components, a server component and a client component. The client component comprises a Javascript mapping toolkit based on ExtJS and OpenLayers, whereas the server component is based on the Pylons web framework. The MapFish server uses a set of paster commands for creating controller and model code that make use of the following components for providing a fully editable GIS vector layer functionality.

  • SQLAlchemyGeom : Introduces a new data type (Geometry) in SQLAlchemy
  • Shapely : A python GIS library for manipulation of 2D geospatial geometries
  • GeoJSON : A JSON encoder / decoder for simple GIS features

tgext.geo makes use of modified paster commands that create model and controller code for TG2 apps. The model uses reflection to map a PostGIS table to the created model class. The controller is mounted on the root controller and used for CRUD operations on the GIS database using GeoJSON.

About This Tutorial

In this tutorial we create a TG2 app and use the tgext.geo extension to create a GIS vector layer. We also make use of tw.openlayers to create a map with a WMS base layer and an overlay of vector data retrieved from the tgext.geo backend.

Installation

It is assumed that a fresh virtualenv has been created and TG2 installed following the TurboGears 2.1 Standard Installation. Install tgext.geo using easy_install:

(tg2env)$ easy_install -i http://www.turbogears.org/2.0/downloads/current/index tgext.geo

Make sure that tw.openlayers is installed:

(tg2env)$ easy_install tw.openlayers

MapFish uses a PostGIS backend for storing Geographic data. Install instructions for PostGIS can be found here <http://postgis.refractions.net/documentation/>_. Additionally, we need to install the following:

(tg2env)$ easy_install -i http://www.turbogears.org/2.0/downloads/current/index egenix-mx-base
(tg2env)$ easy_install -i http://www.turbogears.org/2.0/downloads/current/index psycopg2

Creating A New TG2 App

Create a new TG2 app with gis capability use the following paster command:

(tg2env)$ paster quickstart VectorApp --geo
(tg2env)$ cd VectorApp

Create A MapFish Layers Config

Create a MapFish Layers config in the file layers.ini in the project folder and add the necessary configuration. The layers.ini file should have layer definitions as sections, e.g. [mylayer] followed by a series of parameter value pairs in the param=value format. For this example, we use a layer definition like this:

[countries]
singular=country
db=gisdb
table=world_fb
epsg=4326
units=degrees
geomcolumn=the_geom
idcolumn=Integer:gid

In the above example, a layer named countries would be created. The singular param is used for creating the model class with the first letter capitalized. In this case the model class would be Country . The db should be a the PostGIS database and table the table name to be mapped to the model class using database metadata reflection (SQLAlchemy autoload feature). epsg should have the EPSG (European Petroleum Survey Group) code for the desired datum and projection for the geometry data. geomcolumn should have the name of the column containing the geometry data. idcolumn should contain the data type and name of the primary key column.

Creating The Geo Model And Controller

Once the layers.ini file has been created in the project folder, the model and controller can be created by creating a new layer using the following paster command:

(tg2env)$ paster geo-layer countries

where countries is the new controller and should match the layer name defined in the layers.ini file. Now edit the root controller (package/controllers/root.py) to import the new controller and mount it inside the RootController class:

from vectorapp.controllers.countries import CountriesController

class RootController(BaseController):
    countries = CountriesController()

The countries controller should now be accessible at the url location http://<host>:<port>/countries.

Pointing the browser to the above url should show up all objects (records) in the PostGIS table as JSON (GeoJSON).

Displaying The Vector Data As A Layer In An OpenLayers Map

We are now ready to access the vector data from the PostGIS spatial database using the new countries controller. We now need to use the tw.openlayers ToscaWidgets Library to create a map and use the data returned by the countries controller as a vector layer in the map. We also make use of some OpenLayers based javascript code to select features on mouse hover and display them in the sidebar div.

Initialize The Widgets In Controller

The tw.openlayers has library of widgets for creating Map, Layers and Controls using OpenLayers. The following paragraphs show how the layers, controls and the map widgets are initialized. First we need to import the necessary symbols from the ToscaWidgets and the tw.openlayers API:

from tw.api import WidgetsList, js_symbol
from tw.openlayers import Map, GML, WMS, LayerSwitcher, OverviewMap, \
                    MouseToolbar, MousePosition, PanZoomBar, \
                    Permalink, SelectFeature

We create the layers as a WidgetsList which contains several layers. Our data would be fetched into the transportation layer which is defined as a GML (Geography Markup Language) layer. In OpenLayers the GML layer is used to create a vector layer using data obtained in specific vector formats. In this case, the option format: OpenLayers.Format.GeoJSON indicates that our data would be in GeoJSON format. Note the use of js_symbol function used from the ToscaWidgets API. This is useful in passing Javascript symbols to the encapsulated Javascript code. Otherwise the expression would get passed as a string. The url parameter of GML specifies the url to be used to fetch the data. In this case a relative path to the countries controller is specified:

class MyLayers(WidgetsList):
    ol = WMS(name="OpenLayers WMS",
        url=["http://labs.metacarta.com/wms/vmap0"],
        options = {'layers':'basic'})
    nasa = WMS(name="NASA Global Mosaic",
        url=['http://t1.hypercube.telascience.org/cgi-bin/landsat7'],
        options={'layers': 'landsat7'})
    transportation = GML(name="Transportation", url="countries",
        options = {
            "format": js_symbol(" OpenLayers.Format.GeoJSON"),
            "isBaseLayer": False,
            "projection": js_symbol(' new OpenLayers.Projection("EPSG:4326")')
        })

We have to also initialize the required controls as a WidgetsList. Out of these the SelectFeature is the most interesting in this example. Feature selection takes place on hovering the mouse over the feature geometry as specified by the “hover”: True option. The Javascript custom functions show_info() and erase_info() would be called respectively when a feature is selected or unselected:

class MyControls(WidgetsList):
    ls = LayerSwitcher()
    ovm = OverviewMap()
    mtb = MouseToolbar()
    mp = MousePosition()
    pzb = PanZoomBar()
    pl = Permalink()
    sf = SelectFeature(layer_name="Transportation", options={
            "hover": True,
            "onSelect": js_symbol("show_info"),
            "onUnselect": js_symbol("erase_info")})

The Map widget is initialized using the layers and controls initialized earlier:

mymap = Map(id="map", layers=MyLayers(), controls=MyControls(),
                            center=(15,0), zoom=3)

Finally, we use the Map widget inside the controller method to stick it to the template context:

class RootController(BaseController):
    countries = CountriesController()

    @expose('geogrid.templates.index')
    def index(self):
        pylons.c.map = mymap
        return dict(page='index')

Adding The Style Code

The following stylesheet code may be added to suite the map display:

<style>
#map {
    width: 480px;
    height: 480px;
    border: 2px solid #0000ff;
    float: left;
}
</style>

Define The Javascript Functions

We had called two custom Javascript functions on feature select and unselect. These functions could be either included within a pair of <script> tags in the head section of index.html template or included as a file placed at the package/public/javascript folder:

<script type="text/javascript">
    function show_info (feature) {
        $("sb_top").innerHTML = "<BR/><b>Country : " + feature.attributes.country +
              "<BR/></b>Airports : " + feature.attributes.airports +
              "<BR/>Roadways : " + feature.attributes.roadways +
              "<BR/>Railways : " + feature.attributes.railways +
              "<BR/>Waterways : " + feature.attributes.waterways;
    }

    function erase_info(feature) {
        $("sb_top").innerHTML = "<br />Select a country by hovering the mouse over it.";
    }
</script>

Add The Widget In The HTML Body

The template HTML code would be modified to render the map by calling the widget from the template context:

<body>
  ${sidebar_top()}
  ${tmpl_context.map()}
  <div class="clearingdiv" />
  <div class="notice"> Thank you for choosing TurboGears.
  </div>
</body>

See tgext.geo And tw.openlayers In Action

Its time to see tgext.geo and tw.openlayers in action now. Run the paster command to start the local HTTP server:

(tg2env)$ paster serve --reload development.ini

Point your browser to http://localhost:8080 to view the map. Moving the mouse over the countries shows data about the country in the sidebar_top div element.