Geography 38:286 Computer Cartography

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Geography 38286Computer Cartography

• Topic 9
• Flow Mapping
• Chapter 12 Dent

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What is Flow Mapping?

• Depicts movement/diffusion of phenomena
• Uses line symbols of varying type or width
• Direction of flow may be indicated with arrows
• Two-way flow indicated with double-headed arrows

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Types of Flow Maps

• Qualitative
• Depict flow of different types of objects
• Use variations in line type (e.g. colour hue)
• Quantitative
• Depict flow of different quantities of objects
• Use variations in line width (not value or
  intensity)
• Width may be proportional or range graded
• Another variation is the quantity and wheres it
  going proportional symbol-flow map

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Flow Line Placement

• Flow lines may be
• Accurately located to show actual path
• Easy if flow is along designated routes
• (e.g. highway, railway, pipelines, etc.)
• Difficult if multiple or unknown flow paths exist
• Stylized to simply show spatial interactions
• Actual path unimportant, unknown, or multiple
  paths of unknown magnitude
• Also called desire line maps

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Spatial Data Characteristics

• Can be used to map flow from or between
• Discrete point locations
• e.g. cities, ports, oil wells, dairy farms, etc.
• Discrete areal units
• aggregate values
• e.g. countries, provinces, other admin. units
• Or along linear features for attributes that flow
  
• e.g. stream discharge, voltage, traffic
• not pipe diameter, road type, stream
• Continuously distributed phenomena do not flow

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Aspatial Data Characteristics

• Depict raw data values
• e.g. barrels of oil, bushels of wheat, etc.
• Derived data do not flow

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When Is It Used?

• Only option when objective is to show
• the movement/flow of something
• spatial interaction (connectivity) between
  locations

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Types of Flow Maps

• Based on direction and pattern of flow
• Radial
• One source to multiple destinations 1-way flow
• Quantitative
• Network
• Interconnectivity of multiple locations 2-way
  flow
• Qualitative or Quantity and where its going
• Distributive
• Multiple sources to selected destinations
• Quantitative

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Types of Flow Maps
Radial
Network
Distributive
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Radial
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Distributive
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Network
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Network
. . . and good example of the proportional
symbol-flow map
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Considerations Line Scaling

• Proportion Line Width
• Width of widest flow line(s) selected first
• Must not cover too much of base map at given
  scale
• Smallest flow line calculated to ensure that it
  will be visible
• If too small, then standard line width can be
  used to represent all lines less than a certain
  value
• Range grading is another option

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Considerations Line Scaling
Proportional
Range Graded
250
500
1000
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Considerations Figure-Ground

• Flow lines should be foremost in visual hierarchy
• Achieved by establishing good figure-ground
  separation
• Solid lines are best
• Translucent lines okay
• Particularly important when base map is complex

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Considerations Figure-Ground
Not So Good
Much Better
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Considerations Line Symbols

• Where lines overlap, smaller lines should overlap
  larger ones
• When lines diverge or converge, width is
  proportionally to value
• For qualitative flow maps, distinction between
  different types of flow lines is essential
• Should not infer difference in value
• Change in hue only

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Considerations Legend Design

• Four common legend designs used
• Units of mapped quantity indicated
• Qualitative flow maps need only identify type of
  phenomena represented

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Considerations Map Projection

• Relative size of land masses or discrete areal
  units is irrelevant
• So equivalent projection not essential
• Conformal projections may be more appropriate
• To maintain form of flow paths
• For improved feature (areal unit) recognition
• For radial or distributive flow maps an azimuthal
  equidistant projection may be most appropriate if
  accurate distance is desired

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