Atlas functions allow you to create map books in an automated way.
Atlas uses the features of a table or vector layer
(
Coverage layer
) to create an output for each feature
(
atlas feature
) in the table / layer.
The most common usage is to zoom a map item to the current atlas
feature.
Further use cases include:
a map item showing, for another layer, only features that share the same
attribute as the atlas feature or are within its geometry.
a label or HTML item whose text is replaced as features are iterated over
a table item showing attributes of associated
parent or children
features of the current atlas feature…
For each feature, the output is processed for all pages and items according
to their exports settings.
Use variables for more flexibility
QGIS provides a large panel of functions and
variables
, including atlas related ones,
that you can use to manipulate the layout items, but also the symbology of
the layers, according to atlas status.
Combining these features gives you a lot of flexibility and helps you
easily produce advanced maps.
To enable the generation of an atlas and access atlas parameters, refer to
the
Atlas
panel. This panel contains the following
(see
Fig. 22.66
):
Fig. 22.66
Atlas Panel
Generate an atlas
enables or disables atlas
generation.
Configuration
A
Coverage layer
combo box that allows you to
choose the table or vector layer containing the features to iterate over.
An optional
Hidden coverage layer
that, if checked,
will hide the coverage layer (but not the other layers) during the generation.
An optional
Page name
combo box to specify the name for the
feature page(s).
You can select a field of the coverage layer or set an
expression
.
If this option is empty, QGIS will use an internal ID, according to the
filter and/or the sort order applied to the layer.
An optional
Filter with
text area that allows you
to specify an
expression for filtering features from the coverage layer. If the expression
is not empty, only features that evaluate to
True
will be processed.
An optional
Sort by
that allows you to
sort features of the coverage layer (and the output), using a field of the
coverage layer or an expression.
The sort order (either ascending or descending) is set by the two-state
Sort direction
button that displays an up or a down arrow.
Output
- this is where the output of the atlas can be configured:
An
Output filename expression
textbox that is used to generate
a filename for each atlas feature. It is based on expressions.
is meaningful only for rendering to multiple files.
A
Single file export when possible
that allows you to
force the generation of a single file if this is possible with the chosen output
format (
PDF
, for instance). If this field is checked, the value of the
Output filename expression
field is meaningless.
An
Image export format
drop-down list to select the output format
when using the
Export atlas as Images…
button.
The most common usage of atlas is with the map item, zooming to the current atlas
feature, as iteration goes over the coverage layer. This behavior is set in
the
Controlled by atlas
group properties of the map item. See
Controlled by atlas
for different settings you can apply on the map item.
In order to adapt labels to the feature the atlas iterates over, you can
include expressions. Make sure that you place the expression part
(including functions, fields or variables) between
[%
and
%]
(see
The Label Item
for more details).
For example, for a city layer with fields
CITY_NAME
and
ZIPCODE
,
you could insert this:
The area of [% concat( upper(CITY_NAME), ',', ZIPCODE, ' is ',
format_number($area/1000000, 2) ) %] km2
or, another combination:
The area of [% upper(CITY_NAME)%],[%ZIPCODE%] is
[%format_number($area/1000000,2) %] km2
The information
[% concat( upper(CITY_NAME), ',', ZIPCODE, ' is ', format_number($area/1000000, 2) ) %]
is an expression used inside the label.
Both expressions would result in the following type of label in the
generated atlas:
The area of PARIS,75001 is 1.94 km2
There are several places where you can use a 
Data defined override button to override the selected setting.
This is particularly useful with atlas generation.
See Data defined override setup for more details on this widget.
For the following examples the Regions layer of the QGIS sample
dataset is used and selected as Coverage layer for
the atlas generation.
We assume that it is a single page layout containing a map item and
a label item.
When the height (north-south) of a region extent is greater than its
width (east-west), you should use Portrait instead of Landscape
orientation to optimize the use of paper.
With a Data Defined Override button you can
dynamically set the paper orientation.
Right-click on the page and select Page Properties to open the
panel. We want to set the orientation dynamically, using an expression
depending on the region geometry, so press the button of
field Orientation, select Edit… to open the
Expression string builder dialog and enter the following
expression:
CASE WHEN bounds_width(@atlas_geometry) > bounds_height(@atlas_geometry)
THEN 'Landscape' ELSE 'Portrait' END
Now if you preview the atlas, the paper orients itself
automatically, but item placements may not be ideal. For each Region you need to
reposition the location of the layout items as well. For the map item you can
use the button of its Width property to set it
dynamic using the following expression:
@layout_pagewidth - 20
Likewise, use the button of the Height property to
provide the following expression to constrain map item size:
@layout_pageheight - 20
To ensure the map item is centered in the page, set its Reference
point to the upper left radio button and enter 10 for its X
and Y positions.
Let’s add a title above the map in the center of the page. Select the label
item and set the horizontal alignment to 
Center.
Next move the label to the right position, choose the middle button for
the Reference point, and provide the following expression for
field X:
@layout_pagewidth / 2
For all other layout items you can set the position in a similar way so they
are correctly positioned both for portrait and landscape.
You can also do more tweaks such as customizing the title with
feature attributes (see Customize labels with expression example), changing images,
resizing the number of legend columns number according to page orientation,
The information provided here is an update of the excellent blog (in English
and Portuguese) on the Data Defined Override options
Multiple_format_map_series_using_QGIS_2.6 .
Another example for using data-defined override buttons is the usage of a dynamic
picture. For the following examples we use a geopackage layer containing a BLOB field
called logo with the field type binary (see Creating a new GeoPackage layer).
For every feature there is defined a different picture so that the atlas can iterate
over as described in Preview and generate an atlas.
All you need to do is add a picture in the print layout and go to its
Item properties in the atlas context. There you can find a
data-defined override button in the Image source section of the
Main Properties.
In the following window choose Edit so that the Expression String Builder
opens. From the Fields and values section you can find the BLOB field that
was defined in the geopackage layer. Double-click the field name logo and click OK.
The atlas iterates over the entries in the BLOB field provided that you choose the geopackage
layer as Coverage layer (further instructions you can find in Preview and generate an atlas).
These are just two examples of how you can use some advanced settings with atlas.
Fig. 22.67 Atlas Preview toolbar
Once the atlas settings have been configured, and layout items (map, table,
image…) linked to it, you can create a preview of all the pages by choosing
or clicking the
Preview Atlas icon.
You can then use the arrows to navigate through all the features:
First feature
Previous feature
Next feature
Last feature
You can also use the combo box to select and preview a specific feature.
The combo box shows atlas feature names according to the expression set in the
atlas Page name option.
As for simple compositions, an atlas can be generated in different ways (see
Creating an Output for more information -
just use tools from the menu or toolbar instead of the
menu.
This means that you can directly print your compositions with
.
You can also create a PDF using :
You will be asked for a directory to save all the generated PDF files,
except if the Single file export when possible has been
selected. In that case, you’ll be prompted to give a filename.
With or
tool, you’re also prompted to
select a folder. Each page of each atlas feature composition is exported to
the image file format set in Atlas panel or to SVG.
With multi-page output, an atlas behaves like a layout in that only the
page that contains the General settings will get a world file (for
each feature output).
Print a specific atlas feature
If you want to print or export the composition of only one feature of the atlas,
simply start the preview, select the desired feature in the drop-down list
and click on (or
to any supported file format).
For users with HTML and Javascript knowledge it is possible to operate on
GeoJSON objects and use project defined relations from the QGIS project.
The difference between this approach and using expressions
directly inserted into the HTML is that it gives you a full,
unstructured GeoJSON feature to work with. This means that you can use
existing Javascript libraries and functions that operate on GeoJSON
feature representations.
The following code includes all related child features from the defined relation.
Using the JavaScript setFeature function it allows you to make flexible HTML
which represents relations in whatever format you like (lists, tables, etc).
In the code sample, we create a dynamic bullet list of the related child features.
// Declare the two HTML div elements we will use for the parent feature id
// and information about the children
<div id="parent"></div>
<div id="my_children"></div>
<script type="text/javascript">
function setFeature(feature)
// Show the parent feature's identifier (using its "ID" field)
document.getElementById('parent').innerHTML = feature.properties.ID;
//clear the existing relation contents
document.getElementById('my_children').innerHTML = '';
feature.properties.my_relation.forEach(function(child_feature) {
// for each related child feature, create a list element
// with the feature's name (using its "NAME" field)
var node = document.createElement("li");
node.appendChild(document.createTextNode(child_feature.NAME));
document.getElementById('my_children').appendChild(node);
</script>
During atlas creation there will be an iteration over the coverage layer
containing the parent features. On each page, you will see a bullet list of
the related child features following the parent’s identifier.
