I just saw this map:
in the new magazine that just came out and have been wondering how this was done. Is there a tool for this or was it done by freehand?
Mapping Center Answer:
The flow lines were digitized and each unique line segment attributed with coal tonnage. The linear data was saved as a shapefile and once added to ArcGIS.com, the width of the lines can be symbolized according to the tonnage attribute. The ports are a separate point shapefile, with attributes that can then be used to populate the popups. In essence, it's just two layers of data as shapefiles, added to ArcGIS.com and symbolized. The ArcGIS Online blog gives you a lot of detail about making web maps using ArcGIS.com.
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