Flow maps

http://your.kingcounty.gov/airport/pilots/fly_quiet.stm

Flow maps are used to convey the flow path across a surface and the intensity of the flow. These maps will show the general direction and not the actual path. Flow maps can be used for traffic flow, social network, exchange of goods and services between locations, and in this case migration flow. In the map above the red indicates the south flow and the blue indicates the north flow and the yellow surrounding the flow represents noise sensitive areas.