Origin Destination Maps (OD Maps) were developed by the giCentre at City University London and allow an easier way to investigate spatial aspects of flow data. OD Maps have an advantage over more conventional representations of flow data by stopping any occlusion of the data which can make interpretation difficult.
In this example OD Maps work by taking the 32 London Boroughs (City is merged with Westminster) and transforming them to a grid layout that preserves the boroughs approximate geographical location and adjacency (using the After the Flood layout to build a template in Excel.)
You then nest the destinations using the same grid layout within the origin location (which is marked by its initials on each map). Then flow data on commuting and migration within London from the 2011 UK Census is added (using the software easymorph to unpivot the census data) As the flow data in the Census is so skewed towards the origin (basically Tobler’s first law of geography) the colour scaling has a log scale. I’ve run out of time to calcualte the z-score, but I’ll add at a later stage. The tooltip shows the actual flows.