Floodplains are identified as zones on FEMA Flood Insurance Rate Maps (FIRMs). Flood zones are defined by type, depth, and frequency of flooding. But the shape and nature of a floodplain may also change over time as the main channel of a river naturally migrates through erosion and accretion, impacting how and where excess water may first over top the banks of the river during a flood event. Geologically ancient floodplains are often represented in the landscape by fluvial terraces. These are old floodplains that remain relatively high above the present floodplain and indicate former courses of a stream. I HOPE THIS HELPS YOU :)
In this case the reference point is Atlanta, this is where the family started their motion from. Such that this point may be used to describe their movement. It is sued to describe their movement to Baltimore.
