1. Direct flow mapping
   1. / Tobler 1987
   2. Long history
   3. Produces maps familiar to many users
   4. Doesn't scale well
   5. Filtering can help
      1. / Tobler 1987
      2. / FlowMapLayout 2005 (selecting a dest or origin)
      3. Requires intelligent selection or dynamic interaction
      4. Difficult to provide a visual overview
2. OD Matrix
   1. Spatial properties are lost
   2. Better usability than node-link views (Ghoniem 2004)
   3. Except for 'path finding' and similar geo tasks
   4. / Marble 1997
      1. OD matrix as 3D surface: highly questionable
   5. Sorting/reordering the matrix
      1. May make clusters more apparent
      2. ways of
         1. Clustering strongly connected nodes
            1. / Jarvis 1973
         2. Revealing spatial clusters
            1. / Andrienko 2008, Guo 2006, 2007
         3. Preserving spatial proximity relations
            1. / Marble 1997
      3. Fails to preserve spatial configuration
         1. to overcome
            1. Link to a map
            2. / Guo 2006,2007
3. Summarizing
   1. Aggregation of flows of high density ("bundling")
      1. / Cui 2008
      2. / Holten 2009
      3. Spatial distortion may result in loss of detail
      4. The problem of long trajectories occluding short ones still retains
   2. Flow density surfaces
      1. / Rae 2009
      2. Density does not indicate the density of origin and destination -> arbitrary patterns
      3. Suitability depends on the importance of the paths of the trajectories.
   3. Regionalization
      1. / Guo 2009
4. Alternatives preserving some of the spatial props
   1. Space-filling Morton ordering
      1. / Marble 1997
   2. Aggregation into identifiable spatial regions
      1. / Andrienko 2008
   3. Limited in the ability to integrate other geo data
5. OD Map
   1. / Wood 2010
   2. Eliminates the cluttering=> Scalable
   3. Retains much of the geo space
   4. Aggregation => Potential loss of detail
   5. Problems with long/thin regions
   6. Aliasing when grid cells don't reflect geo structure