DOT MAPPING

DOT MAP DEFINED Dot Map Map in which point symbols of uniform size and value are used to emphasize a spatial pattern

DOT MAP DEFINED Dot Map (cont.) Map in which point symbol frequency illustrates differences in magnitude of an attribute (variable)

DOT MAP DEFINED Dot Map (cont.) Illustrates variations in spatial density

DOT MAP DEFINED Dot Map (cont.) Can have many dots, or just a few

DOT MAP DEFINED Each point symbol can represent One individual feature or phenomenon Called a “Pin Map” Each symbol represents a single golf course Symbols are placed where features are actually located

DOT MAP DEFINED Each point symbol can represent (cont.) Several features or phenomena Called a “Dot Density Map” Each symbol represents 10,000 people Symbols are not placed where features are actually located

DOT MAP DEFINED Earliest dot map of population Frere de Montizon, France (1830) Each dot represents 10,000 people A milestone in thematic mapping that went unnoticed until the 20 th century

DOT MAP DEFINED John Snow’s Dot Map (1854) Used to identify the Broad Street water pump, and halt a cholera epidemic A classic example of the use of a thematic map in service of spatial analysis The first epidemiological map

DOT MAP DEFINED Popular in US Government mapping of agricultural data in the 1930s-1970s.

DOT MAP DEFINED Advantages Can reflect spatial distributions more accurately than other thematic map types Wheat Harvested in Kansas

DOT MAP DEFINED Advantages Reveals non-uniform distributions within enumeration units

DOT MAP DEFINED Advantages Allows the map user to recover original data by counting dots* each equals 4,500 people *Actual value can be greater than the sum, up to the dot value

DOT MAP DEFINED Disadvantages Easy to create inaccurate dot maps, but difficult to produce accurate ones Can depict features as existing in locations where they can’t possibly exist Do people live in lakes? One dot represents 10,000 people

MAP PROJECTIONS and the DOT MAP

MAP PROJECTIONS Equivalent (Equal Area) projections are most appropriate Distortion of relative sizes of areas can mislead the map user Equivalent Conformal

SIZE of POINT SYMBOLS on a PIN MAP

SIZE of POINT SYMBOLS Point symbols need to be as prominent as possible Without obliterating everything else

SIZE of POINT SYMBOLS Denser distributions require smaller symbols In order to limit symbol overlap

SIZE of POINT SYMBOLS Less-dense distributions allow for larger, more sophisticated symbols Pictographic symbols

SIZE of POINT SYMBOLS on a DOT DENSITY MAP

SIZE of POINT SYMBOLS Dots need to be as prominent as possible Without obliterating everything else

SIZE of POINT SYMBOLS Dot size needs to be balanced with dot value (unit value) Each Dot Represents 8,000 People Population, 2005

SIZE of POINT SYMBOLS Dots should coalesce only in the most dense areas Coalescence means overlap Each Dot Represents 8,000 People Population, 2005

SIZE of POINT SYMBOLS Dot size can be calculated using the Nomograph Nomograph is more relevant to traditional, manual cartography Trial-and-error is a better approach for modern, GIS-based cartography

SIZE of POINT SYMBOLS Here, dot value is OK, but size is insufficient Each Dot Represents 8,000 People Population, 2005

SIZE of POINT SYMBOLS Here, dot size is OK, but value is excessive Each Dot Represents 75,000 People Population, 2005

SIZE of POINT SYMBOLS Here, dot size is balanced with dot value Dots are prominent, and coalesce Each Dot Represents 8,000 People Population, 2005

RULES of THUMB for DOT DENSITY MAPPING

RULES OF THUMB Use limiting ancillary attributes Reduces unrealistic placement of dots by limiting where they can be placed Use data from enumeration units that are smaller than your area of interest* Use counties or census tracts when mapping a state Census tracts or block groups when mapping a county Block groups when mapping a city, etc. *Reduces false impressions of density resulting from random placement of dots

RULES OF THUMB Balance dot size with unit value Experiment until dots are clearly visible, not “too big,” and coalesce only in the most dense areas Each Dot Represents 8,000 People Population, 2005