.. _openlayers.layers.cached:

Cached Tiles
============

By default, the WMS layer makes requests for 256 x 256 (pixel) images to fill
your map viewport and beyond. As you pan and zoom around your map, more requests
for images go out to fill the areas you haven't yet visited. While your browser
will cache some requested images, a lot of processing work is typically required
for the server to dynamically render images.

Since tiled layers (such as the WMS layer) make requests for images on a regular
grid, it is possible for the server to cache these image requests and return the
cached result next time you (or someone else) visits the same area - resulting
in better performance all around.


.. _openlayers.layers.cached.xyz:

OpenLayers.Layer.XYZ
--------------------

The Web Map Service specification allows a lot of flexibility in terms of what a
client can request. Without constraints, this makes caching difficult or
impossible in practice.

At the opposite extreme, a service might offer tiles only at a fixed set of zoom
levels and only for a regular grid. These can be generalized as XYZ layers - you
can consider X and Y to indicate the column and row of the grid and Z to
represent the zoom level.


.. _openlayers.layers.cached.osm:

OpenLayers.Layer.OSM
--------------------

The `OpenStreetMap (OSM) <http://www.openstreetmap.org/>`_ project is an effort
to collect and make freely available map data for the world. OSM provides a few
different renderings of their data as cached tile sets. These renderings conform
to the basic :ref:`XYZ grid <openlayers.layers.cached.xyz>` arrangement and can
be used in an OpenLayers map. The ``OpenLayers.Layer.OSM`` constructor accesses
OpenStreetMap tiles.

.. _openlayers.layers.cached.example:

.. rubric:: Tasks

#.  Open the ``map.html`` file from the :ref:`previous section
    <openlayers.layers.wms>` in a text editor and change the map initialization
    code to look like the following:
    
    .. code-block:: html

        <script>
            var geographic = new OpenLayers.Projection("EPSG:4326");
            var mercator = new OpenLayers.Projection("EPSG:900913");
            
            var world = new OpenLayers.Bounds(-180, -89, 180, 89).transform(
                geographic, mercator
            );
            var sydney = new OpenLayers.LonLat(151, -33.9).transform(
                geographic, mercator
            );
            
            var options = {
                projection: mercator,
                units: "m",
                maxExtent: world
            };
            var map = new OpenLayers.Map("map-id", options);
        
            var osm = new OpenLayers.Layer.OSM();
            map.addLayer(osm);
            
            map.setCenter(sydney, 9);
        </script>

#.  In the ``<head>`` of the same document, add a few style declarations for the
    layer attribution.
    
    .. code-block:: html
    
        <style>
            #map-id {
                width: 512px;
                height: 256px;
            }
            .olControlAttribution {
                font-size: 10px;
                bottom: 5px;
                right: 5px;
            }
        </style>

#.  Save your changes, and refresh the page in your browser:
    http://localhost/ol_workshop/map.html

.. figure:: cached1.png
   
    A map with an OpenStreetMap view of Sydney.


A Closer Look
~~~~~~~~~~~~~

Projections
```````````
Review the first 2 lines of the initialization script:

.. code-block:: javascript

    var geographic = new OpenLayers.Projection("EPSG:4326");
    var mercator = new OpenLayers.Projection("EPSG:900913");

Geospatial data can come in any number of coordinate reference systems. One data
set might use geographic coordinates (latitude and longitude) in degrees, and
another might have coordinates in a local projection with units in meters. A
full discussion of coordinate reference systems is beyond the scope of this
module, but it is important to understand the basic concept.

OpenLayers needs to know the coordinate system for your data. We represent this
with an ``OpenLayers.Projection`` object. You create a new projection object by
using a string that represents the coordinate reference system (``"EPSG:4326"``
and ``"EPSG:900913"`` above).

Locations Transformed
`````````````````````

.. code-block:: javascript

    var world = new OpenLayers.Bounds(-180, -89, 180, 89).transform(
        geographic, mercator
    );
    var sydney = new OpenLayers.LonLat(151, -33.9).transform(
        geographic, mercator
    );

The OpenStreetMap tiles that we will be using are in a Mercator projection.
Because of this, we need to set things like the maximum extent and initial center
using Mercator coordinates. Since it is relatively easy to find out the
coordinates for a place like Sydney, Australia in geographic coordinates, we use
the ``transform`` method to turn geographic coordinates into Mercator
coordinates.

.. note::

    You may notice that the world bounds created above do not *really* cover the
    entire world. For now, you'll have to accept that this particular transform
    doesn't behave well at the poles so we adjust the bounds to compensate.

Custom Map Options
``````````````````

.. code-block:: javascript

    var options = {
        projection: mercator,
        units: "m",
        maxExtent: world
    };
    var map = new OpenLayers.Map("map-id", options);

In the :ref:`previous example <openlayers.layers.wms.example>` we used the
default options for our map. In this example, we set some custom map options.
First, we set the map ``projection`` property to the Mercator projection we
created above. Next we set the map ``units`` to ``"m"`` for meters, and finally
we set the ``maxExtent`` to the world (again in Mercator coordinates).

Layer Creation and Map Location
```````````````````````````````

.. code-block:: javascript

    var osm = new OpenLayers.Layer.OSM();
    map.addLayer(osm);

As before, we create a layer and add it to our map. This time, we accept all the
default options for the layer.

.. code-block:: javascript

    map.setCenter(sydney, 9);
    
Finally, we give our map a center (Sydney in Mercator coordinates) and set the
zoom level to ``9``.

Style
`````

.. code-block:: html

    .olControlAttribution {
        font-size: 10px;
        bottom: 5px;
        right: 5px;
    }

A treatment of map controls is also outside the scope of this module, but these
style declarations give you a sneak preview. By default, an
``OpenLayers.Control.Attribution`` control is added to all maps. This lets
layers display attribution information in the map viewport. The declarations
above alter the style of this attribution for our map (notice the small
"CC-By-SA" line at the bottom right of the map).

Having mastered layers with publicly available cached tile sets, let's move on
to working with :ref:`proprietary layers <openlayers.layers.proprietary>`.

