Layers can be optionally grouped into
layer groups. Layers in the same group are displayed together in
the Display Manager.
The visibility for all layers in a group can
be set at the group level. If the group visibility is turned off
then none of the layers in the group will be visible, regardless
of their individual visibility settings. If the group visibility
is turned on, then individual layers within the group can be made
visible or not visible separately.
Layer groups can be nested so a group can contain
other groups. This provides a finer level of control for handling
layer visibility or for grouping layers into legend groups.
an MgLayerGroupCollection object that
contains all the top-level layer groups in the map.
Each layer group in a map must have a unique
name, even if it is nested within another group.