The two types of overview diagrams

Overview diagrams are a way to show the extent of one data frame within another data frame. There are two types of overview diagrams that can be added to a map or map series:

• Fixed background overview diagrams, where the background data remains fixed and the overview extent moves.
• Centered extent overview diagrams, where the overview extent remains centered and the background data moves.

For both fixed background and centered extent overview diagrams, extent rectangles and data frame managers are used to specify the display for the selected area of interest (AOI). However, in general, extent rectangles are particularly useful with fixed background overview diagrams, while data frame managers are more often used with centered extent overview diagrams.

For a fixed background overview diagram, a data frame manager is used to center the background data. For centered extent overview diagrams, the data frame manager is used to center the AOI.

Once a data frame manager is added to the overview diagram, the rotation angle you define will depend on the overview diagram type:

• For fixed background overview diagrams, because the extent displayed in the overview diagram may differ from the extent displayed in the map sheet or basemap, the appropriate rotation angle for the overview diagram data frame may not necessarily be the same as the rotation angle for the basemap or map sheet. When the extent of the map sheet is contained within the display of the overview diagram, a rotation angle specific to the displayed area of interest may be defined for each.
• For centered extent overview diagrams, you can set the same rotation angle for the overview diagram as the map sheet or basemap.

The spatial reference for each type of overview diagram can also be set as follows:

• Fixed background—Because the selected map sheet AOI may be contained within the displayed extent of the fixed background overview diagram, the coordinate system for each may not necessarily be the same. For example, if the overview diagram displays a larger extent that may contain the extent of the map sheet within it, a UTM zone may automatically be calculated for the map sheet while a Lambert projection may be more appropriate for the overview diagram's display of the larger extent.
• Centered extent—You will usually use the same spatial reference coordinate system for both the overview diagram and the map sheet or basemap.

When defining the Scale and Extent dialog box for either overview diagram type, the most commonly used options are Dynamic Scale or Fixed Scale.

Dynamic scale has the following options:

• Do Not Resize Data Frame—This option sets the data frame so that it will change if it is resized interactively; the AOI feature is scaled to fit within the resulting data frame.
• Set Data Frame Size—This option sets the data frame to a fixed height and width. If you attempt to interactively resize it, it will return to the specified size.

Choosing Fixed Scale provides the following options:

• Do Not Resize Data Frame—This option sets the data frame so that it will change if it is resized interactively; the AOI feature is scaled to fit within the resulting data frame.
• Resize Data Frame to Fit Area of Interest—This option will cause the data frame to automatically be resized to fit to the edge of the AOI feature, after any buffer and rotation angles have been calculated.
• Set Data Frame Size—This option sets the data frame to a fixed height and width. If you attempt to interactively resize it, it will return to the specified size.

When defining the Dynamic Scale or Fixed Scale option, you can choose one of the following Unit Types:

• Page—This unit type will size the data frame in page units such as inches or centimeters.
• Map—This unit type will size the data frame based map units such as meters, feet, or decimal degrees. Size is determined based on the number of map units at the current data frame scale.

The Area of Interest dialog box can be used to specify an intersection area of interest for overview diagrams that contain a fixed background, but the fixed background needs to change based on its relative location to the map sheet area of interest. For example, when you make a county atlas, you can use this dialog box to create an overview diagram showing which state the county is in as a fixed background, so that when you switch to a county in a different state, the background data also switches to the new state.

The options include:

• Intersection Area of Interest—This option allows you to select another feature class to use as your area of interest in your overview diagram. The feature or features that are set to your area of interest are determined by how they spatially intersect your map sheet area of interest.
  • Browse—You can browse to the feature class you want to use as your overview diagram intersecting area of interest.
  • Centroid—This option returns the feature that intersects the center of the current map sheet AOI feature.
  • Whole Shape—This option returns the overview diagram AOI features where the whole shape of the map sheet AOI intersects the overview diagram's AOI features.
• Map Sheet Area of Interest—The existing area of interest feature in the basemap will be used for the area of interest feature in the overview diagram. This will keep the data frames synchronized when you switch the current map sheet you are using.

In the Result area of the Area of Interest dialog box, for the Intersection Area of Interest option, you can choose from the following:

• First—This option returns the first AOI feature encountered. With the Centroid option selected, it will return the feature that intersects the center point of the map sheet AOI. With the Whole Shape option it will return the feature based on the first Object ID encountered. In this case, the returned feature may seem to be random.
• Largest Area—This option works with the Whole Shape option and returns the AOI feature where the largest area of the map sheet AOI intersects the overview diagram AOI feature.
• Union of All—This option works with the Whole Shape option and returns a combination of all the overview diagram AOI features that intersect the map sheet AOI.