Changing the map extent

There will be many occasions when you will need to change the map extent. This is frequently the case when you are automating the map production process and need to create many maps of different areas or features. There are a number of ways that the map extent can be changed with arcpy. However, for this recipe, we'll concentrate on using a definition expression to change the extent.

Getting ready

The DataFrame class has an extent property that you can use to set the geographic extent. This is often used in conjunction with the Layer.definitionQuery property that is used to define a definition query for a layer. In this recipe, you will learn how to use these objects and properties to change the map extent.

How to do it...

Follow these steps to learn how to get a list of layers from a map document:

1. Open c:\ArcpyBook\Ch2\Crime_Ch2.mxd with ArcMap.
2. Click on the Python window button from the main ArcMap toolbar.
3. Import the arcpy.mapping module:

   import arcpy.mapping as mapping

4. Reference the currently active document (Crime_Ch2.mxd) and assign the reference to a variable:

   mxd = mapping.MapDocument("CURRENT")

5. Create a for loop that will loop through all the data frames in the map document:

   for df in mapping.ListDataFrames(mxd):

6. Find the data frame called Crime and a specific layer that we'll apply the definition query against:

   if df.name == 'Crime':
     layers = mapping.ListLayers(mxd,'Crime Density by School District',df)

7. Create a for loop that will loop through the layers. There will only be one, but we'll create the loop anyway. In the for loop, create a definition query and set the new extent of the data frame:

   for layer in layers:
     query = '"NAME" = \'Lackland ISD\''
     layer.definitionQuery = query
     df.extent = layer.getExtent()

8. The entire script should appear as follows or you can consult the solution file at c:\ArcpyBook\code\Ch2\ChangeMapExtent.py:
   
9. Save and run the script. The extent of the data view should update so that it visualizes only the features matching the definition expression, as shown in the following screenshot:
   

How it works...

This recipe used a definition query on a layer to update the map extent. Near the end of the script, you created a new variable called query that held the definition expression. The definition expression was set up to find school districts with a name of Lackland ISD. This query string was then applied to the definitionQuery property. Finally, the df.extent property was set to the returned value of layer.getExtent().