DOT
MAPPING
Cartographic Design for GIS (Geog. 340)
Prof. Hugh Howard
American River College
DOT MAP
DEFINED
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
20th 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*
6 dots @ 750 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
One dot
represents
10,000 people
Do people
live in lakes?
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
Conformal
Equivalent
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)
Population, 2005
Each Dot Represents
8,000 People
SIZE of POINT SYMBOLS
•
Dots should coalesce only in the most
dense areas
– Coalescence means overlap
Population, 2005
Each Dot Represents
8,000 People
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
Population, 2005
Each Dot Represents
8,000 People
SIZE of POINT SYMBOLS
•
Here, dot size is OK, but value is
excessive
Population, 2005
Each Dot Represents
75,000 People
SIZE of POINT SYMBOLS
•
Here, dot size is balanced with dot
value
– Dots are prominent, and coalesce
Population, 2005
Each Dot Represents
8,000 People
PLACEMENT of
POINT SYMBOLS
on a PIN MAP
PLACEMENT of SYMBOLS
•
Symbols are placed where features or
phenomena are actually located
Symbols for point features
are placed at the point
Symbols for areal features
are typically placed in the
center of the area
PLACEMENT of SYMBOLS
•
The exact location is sometimes
ambiguous…
PLACEMENT of
POINT SYMBOLS
on a DOT DENSITY MAP
PLACEMENT of SYMBOLS
•
Symbols are allocated to smaller
enumeration units (counties) within a
larger enumeration unit (state)
Cropland, 1997
PLACEMENT of SYMBOLS
•
Symbols are randomly distributed
within enumeration units
– Counties here
Cropland, 1997
Each Dot Represents
5,000 Acres
PLACEMENT of SYMBOLS
•
An infinite number of random
distributions is possible
–
Random placement is not optimal!
Cropland, 1997
Each Dot Represents
5,000 Acres
PLACEMENT of SYMBOLS
•
ArcMap places symbols in a different
location every time the layer is redrawn
– Good for experimentation
– “Fixed” option prevents this
PLACEMENT of SYMBOLS
•
Symbols randomly distributed within
enumeration units
Cropland, 1997
Each Dot Represents
5,000 Acres
PLACEMENT of SYMBOLS
•
Beware of unrealistic distributions!
– Does cropland exist in urban areas?
– Do people live in reservoirs?
Cropland, 1997
Each Dot Represents
5,000 Acres
OmahaCouncil Bluffs
PLACEMENT of SYMBOLS
•
Incorporate Ancillary Attributes
– Ancillary = additional, or supportive
– Used to prevent unrealistic dot placement
•
Limiting ancillary attributes
– Place absolute limits on where dots can be
located (no population on water bodies)
•
Related ancillary attributes
– Place restrictions on where dots can be
located (less population on steeper slopes)
PLACEMENT of SYMBOLS
•
ArcMap allows you to specify limiting
ancillary attributes
– Using the “Masking” option
PLACEMENT of SYMBOLS
•
Here, ancillary attributes are not used
– Cropland dots are unrealistically placed in
urban areas
Cropland, 1997
Each Dot Represents
5,000 Acres
OmahaCouncil Bluffs
PLACEMENT of SYMBOLS
•
Here, the urban areas layer is used as
a limiting ancillary attribute
Cropland, 1997
Each Dot Represents
5,000 Acres
OmahaCouncil Bluffs
PLACEMENT of SYMBOLS
•
Ancillary attributes are often not shown
on the final map
Cropland, 1997
Each Dot Represents
5,000 Acres
OmahaCouncil Bluffs
PLACEMENT of SYMBOLS
•
Multiple ancillary attributes can be
combined into a single map layer
– Using the Union geoprocessing tool
ZONED
INDUSTRIAL
LIMITING
ANCILLARY
ATTRIBUTES
100 People
Water and industrial areas are combined, and used to prevent the
placement of “people” dots in water or industrial neighborhoods
PLACEMENT of SYMBOLS
•
Limiting ancillary attributes can also be
derived from classified satellite imagery
Dots are placed
only on yellow
pixels (cropland)
PLACEMENT of SYMBOLS
•
There have traditionally been three
methods of distributing dots within
enumeration units
– Uniform
– Geographically Weighted
– Geographically Based
PLACEMENT of SYMBOLS
•
Uniform Distribution
– Dots are placed manually to produce a
uniform, but partly random, distribution
Boundaries of
enumeration units
are implied
PLACEMENT of SYMBOLS
•
Geographically Weighted
– Dots are placed manually with attention
paid to adjacent enumeration units
Dots are denser near
enumeration units
with higher overall
values
Boundaries of
enumeration units
are less apparent
PLACEMENT of SYMBOLS
•
Geographically Based
– Uniform, partly random distributions,
limited by ancillary attributes
Boundaries of
enumeration units
are less apparent
Dots not placed in
unrealistic locations
PLACEMENT of SYMBOLS
•
Computer-based methods of dot
placement are less flexible
– Dots are placed within the constraints of
limiting ancillary attributes
– Similar to traditional Geographically Based
PLACEMENT of SYMBOLS
•
Computer-based methods of dot
placement are less flexible (cont.)
– Dots are placed completely at random,
without any uniformity
– Gives false impressions of density
Do lots of people
really live here?
Nobody lives here?
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
Population, 2005
Each Dot Represents
8,000 People
LEGEND DESIGN
LEGEND DESIGN
•
Legends for pin and dot density maps
are typically very simple
– A representative symbol and a definition
– Legend headings are rarely required
LEGEND DESIGN
•
A legend is not required on some pin
maps
– If symbol meaning is self evident
LEGEND DESIGN
•
Include additional information if
necessary
– Legend Heading
– Total for entire area
– Sample densities
DOT
MAPPING
Cartographic Design for GIS (Geog. 340)
Prof. Hugh Howard
American River College