Dot Distribution Maps
Encyclopedia
A dot density map is as a map
type that uses a dot symbol to show the presence of a feature or phenomenon
. Dot maps rely on a visual scatter to show spatial pattern.
, care must be taken to ensure that the dot is represented in its correct spatial location. Inaccuracies in the location of the dot can misrepresent the data being mapped. Various methods exist for determining the exact spatial location of a single point, including geocoding
.
patients, may not be available for mapping due to restrictions on access to individuals' medical record
s.
In one-to-many dot distribution maps, the reader must be careful not to interpret the dots as actual locations, as the dots represent aggregate data
and are often arbitrarily placed on a map. Methods of dot placement include by areal unit centroid, random dispersement, and uniform (evenly spaced) placement, among others.
s and fire hydrant
s, respectively.
Dot maps can be symbolized by different pictographic elements, by graduated size of the dot, or by altering the colors of the dots, with respect to hue, intensity, and saturation of color.
Effective display of the dots in a dot distribution map is critical for conveying information to the reader.
, Canada
. It is a multivariate dot map that is essentially meaningless for distinguishing between the variables. It does serve as a good general reference map, showing clustering of stations along the western edge of the represented area. Dot maps are effective as general reference maps.
presence across the United States
for a particular company. Here, that data are represented using six different variables
, symbolized by color.
monk
, Armand Joseph Frère de Montizon (1788 - ????). It is a relatively simple map of population by département (administrative district) in France
and is one of the first known examples of a demographic map for the country. The map is shown in Figure 3.1. Each dot represents 10,000 individuals. The dots are spaced in even rows, the distance between which determined by the population of the department. A table in the map lists the departments by name, population, and prefectural city. The departments were numbered on the map to correspond to the table. The regular spacing of the dots in the map produces a visual display of population density
, as higher population levels within an administrative border exhibit a closer, denser pattern of dots. Since the dots are evenly spaced, it is evident that they do not represent the actual locations of where people live within a department. This is an example of an ecological fallacy
, where a value for an area generalizes all within that area to exhibit that value.
Although Montizon's map was the first thematic dot map published, it did not garner the author fame. Instead, his innovation had no effect on practice for nearly 30 years until the dot distribution map was "reinvented" for map by a Swedish Army
officer, published in 1859. This map was authored by Thure Alexander von Mentzer and shows the population distribution for the Scandinavia
n region. No known reproductions of this map exist.
was a major concern. When a large outbreak occurred in London
in 1854, Dr. John Snow created a dot distribution map that settled a debate between two schools of thought: that cholera is transmitted not through the inhalation of infected air, but through the ingestion of contaminated water or food.
Snow's map of the 1854 Broad Street cholera outbreak
in London was simple and effective in its design. The base map is a simple road network, with few buildings named or depicted. Study area is outlined along the relevant road centerlines. Water pump
s around the neighborhood are symbolized with points and bold, uppercase labels. Cholera deaths are depicted along the road network in their correct locations by address, with quantities measured by parallel tick marks stacked next to the road. The symbology, while simple, is effective for a study of fatal disease. The symbology of the cholera deaths is reminiscent of large Plague events, where bodies are stacked next to the roadway for disposal.
The map showed that a high number of deaths were occurring near a water pump on Broad Street
at Cambridge Street. Snow petitioned the local authorities to remove the pumps handle, which caused dramatic decreases in cholera cases for the immediate area. The map helped the germ theory
of disease transmission supplant miasma theory
as the widely accepted view.
.
Dot distribution maps also have disadvantages. One such disadvantage is that the actual dot placement may be random. That is, there may be no actual phenomenon where the dots are located. Second, the subjective nature of the dot size and spacing could give the map a bias
ed view. Inappropriately sized or spaced dots can skew or distort the message a map attempts to communicate. If the dots are too numerous, it may be difficult for the reader to count the dots. This can cause the map to ineffective in communicating its message.
Solutions to problems in dot maps can be found through the combination of remotely sensed data. Satellite imagery
showing lights at night can give good indication of population density. With such information, clustering of dots to relevant areas could possibly be applied to analyses. Data from the Oak Ridge National Laboratory
's Landscan project is commonly used in this emerging technique.
or geographical societies and associations, but by common individuals from all walks of life. Users of such virtual globe
services as Google Earth
constantly create new thematic map
s quickly and easily. Such services offer a base platform upon which users can display layers as they choose, or even add their own data to the display.
CNN
in 2009 describes a recent map, created using Google Maps
on April 21, 2009 by Pittsburgh biochemist
Harry Niman. The map shows where the 2009 H1N1 swine flu virus
was spreading at the time. Despite the fact that the data shown was not backed by any official agency, the author's creation had "gone viral", surpassing 290,000 web views and 3,000 comments within nine days of its being published online.
Emerging technology
is increasingly making map
ping more mobile. Dot distribution maps, through their simple and effective displays, are becoming the standard for such on-the-fly mapping services.
Map
A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....
type that uses a dot symbol to show the presence of a feature or phenomenon
Phenomenon
A phenomenon , plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'...
. Dot maps rely on a visual scatter to show spatial pattern.
One-to-One
In a one-to-one dot map, each dot represents one single recording of a phenomenon. Because the location of the dot corresponds to only one piece of dataData
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
, care must be taken to ensure that the dot is represented in its correct spatial location. Inaccuracies in the location of the dot can misrepresent the data being mapped. Various methods exist for determining the exact spatial location of a single point, including geocoding
Geocoding
Geocoding is the process of finding associated geographic coordinates from other geographic data, such as street addresses, or zip codes...
.
One-to-Many
In a one-to-many, or dot-density map, each dot on the map represents more than one of the phenomena being mapped. The number of data represented by each dot is determined by the map author and may be the product of data availability. Some data, such as the addresses of cancerCancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
patients, may not be available for mapping due to restrictions on access to individuals' medical record
Medical record
The terms medical record, health record, and medical chart are used somewhat interchangeably to describe the systematic documentation of a single patient's medical history and care across time within one particular health care provider's jurisdiction....
s.
In one-to-many dot distribution maps, the reader must be careful not to interpret the dots as actual locations, as the dots represent aggregate data
Aggregate data
In statistics, aggregate data describes data combined from several measurements.In economics, aggregate data or data aggregates describes high-level data that is composed of a multitude or combination of other more individual data....
and are often arbitrarily placed on a map. Methods of dot placement include by areal unit centroid, random dispersement, and uniform (evenly spaced) placement, among others.
Pictographic Symbology
Dot maps may not necessarily display their data using dots. Any number of visual variables may be used to represent the locations of phenomena. An example is Figure 1, a map by John Krygier, which illustrates a sewer and water network. It shows the locations of sewer and water mains, which are symbolized not by dots, but by sewer manholeManhole
A manhole is an opening used to gain access to sewers or other underground structures, usually for maintenance.Manhole may also refer to:* Manhole , a metal band from Los Angeles* The Manhole, a computer game...
s and fire hydrant
Fire hydrant
A fire hydrant , is an active fire protection measure, and a source of water provided in most urban, suburban and rural areas with municipal water service to enable firefighters to tap into the municipal water...
s, respectively.
Dot maps can be symbolized by different pictographic elements, by graduated size of the dot, or by altering the colors of the dots, with respect to hue, intensity, and saturation of color.
Effective display of the dots in a dot distribution map is critical for conveying information to the reader.
Ineffective Use of Symbology
Figure 2.2 shows proposed hydroelectric power stations in British ColumbiaBritish Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. It is a multivariate dot map that is essentially meaningless for distinguishing between the variables. It does serve as a good general reference map, showing clustering of stations along the western edge of the represented area. Dot maps are effective as general reference maps.
Effective Use of Symbology in Dot Maps
Figure 2.3 is an example of a dot map of franchiseFranchising
Franchising is the practice of using another firm's successful business model. The word 'franchise' is of anglo-French derivation - from franc- meaning free, and is used both as a noun and as a verb....
presence across the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
for a particular company. Here, that data are represented using six different variables
Variable (mathematics)
In mathematics, a variable is a value that may change within the scope of a given problem or set of operations. In contrast, a constant is a value that remains unchanged, though often unknown or undetermined. The concepts of constants and variables are fundamental to many areas of mathematics and...
, symbolized by color.
Carte philosophique figurant la population de la France
The first dot distribution map was created by a FranciscanFranciscan
Most Franciscans are members of Roman Catholic religious orders founded by Saint Francis of Assisi. Besides Roman Catholic communities, there are also Old Catholic, Anglican, Lutheran, ecumenical and Non-denominational Franciscan communities....
monk
Monk
A monk is a person who practices religious asceticism, living either alone or with any number of monks, while always maintaining some degree of physical separation from those not sharing the same purpose...
, Armand Joseph Frère de Montizon (1788 - ????). It is a relatively simple map of population by département (administrative district) in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
and is one of the first known examples of a demographic map for the country. The map is shown in Figure 3.1. Each dot represents 10,000 individuals. The dots are spaced in even rows, the distance between which determined by the population of the department. A table in the map lists the departments by name, population, and prefectural city. The departments were numbered on the map to correspond to the table. The regular spacing of the dots in the map produces a visual display of population density
Population density
Population density is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans...
, as higher population levels within an administrative border exhibit a closer, denser pattern of dots. Since the dots are evenly spaced, it is evident that they do not represent the actual locations of where people live within a department. This is an example of an ecological fallacy
Ecological fallacy
An ecological fallacy is a logical fallacy in the interpretation of statistical data in an ecological study, whereby inferences about the nature of specific individuals are based solely upon aggregate statistics collected for the group to which those individuals belong...
, where a value for an area generalizes all within that area to exhibit that value.
Although Montizon's map was the first thematic dot map published, it did not garner the author fame. Instead, his innovation had no effect on practice for nearly 30 years until the dot distribution map was "reinvented" for map by a Swedish Army
Swedish Army
The Swedish Army is one of the oldest standing armies in the world and a branch of the Swedish Armed Forces; it is in charge of land operations. General Sverker Göranson is the Supreme Commander-in-Chief of the Army.- Organization :...
officer, published in 1859. This map was authored by Thure Alexander von Mentzer and shows the population distribution for the Scandinavia
Scandinavia
Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...
n region. No known reproductions of this map exist.
John Snow's Cholera Map
Display of discrete data in the form of points in a map can provide convincing evidence related to medical geography. During the mid-1850s, choleraCholera
Cholera is an infection of the small intestine that is caused by the bacterium Vibrio cholerae. The main symptoms are profuse watery diarrhea and vomiting. Transmission occurs primarily by drinking or eating water or food that has been contaminated by the diarrhea of an infected person or the feces...
was a major concern. When a large outbreak occurred in London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
in 1854, Dr. John Snow created a dot distribution map that settled a debate between two schools of thought: that cholera is transmitted not through the inhalation of infected air, but through the ingestion of contaminated water or food.
Snow's map of the 1854 Broad Street cholera outbreak
1854 Broad Street cholera outbreak
The Broad Street cholera outbreak was a severe outbreak of cholera that occurred near Broad Street in Soho district of London, England in 1854...
in London was simple and effective in its design. The base map is a simple road network, with few buildings named or depicted. Study area is outlined along the relevant road centerlines. Water pump
Water Pump
Water Pump is one of the neighbourhoods of Gulberg Town in Karachi, Sindh, Pakistan. It is near main Water Pump that supplies fresh water to the city of Karachi....
s around the neighborhood are symbolized with points and bold, uppercase labels. Cholera deaths are depicted along the road network in their correct locations by address, with quantities measured by parallel tick marks stacked next to the road. The symbology, while simple, is effective for a study of fatal disease. The symbology of the cholera deaths is reminiscent of large Plague events, where bodies are stacked next to the roadway for disposal.
The map showed that a high number of deaths were occurring near a water pump on Broad Street
Broadwick Street
Broadwick Street is a street in Soho, City of Westminster, London. It runs for 0.18 mile approximately west-east between Marshall Street and Wardour Street, crossing Berwick Street....
at Cambridge Street. Snow petitioned the local authorities to remove the pumps handle, which caused dramatic decreases in cholera cases for the immediate area. The map helped the germ theory
Germ theory of disease
The germ theory of disease, also called the pathogenic theory of medicine, is a theory that proposes that microorganisms are the cause of many diseases...
of disease transmission supplant miasma theory
Miasma theory of disease
The miasma theory held that diseases such as cholera, chlamydia or the Black Death were caused by a miasma , a noxious form of "bad air"....
as the widely accepted view.
Advantages and Disadvantages of Dot Distribution Maps
Dot maps are advantageous when mapping phenomena that change smoothly over a space, as the dot maps will visually match the phenomena. The author is free to choose the dot size and spacing. Further, the author can choose what if any pictographic symbols will be used. These choices aid in effective map design and communicationCommunication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
.
Dot distribution maps also have disadvantages. One such disadvantage is that the actual dot placement may be random. That is, there may be no actual phenomenon where the dots are located. Second, the subjective nature of the dot size and spacing could give the map a bias
Bias
Bias is an inclination to present or hold a partial perspective at the expense of alternatives. Bias can come in many forms.-In judgement and decision making:...
ed view. Inappropriately sized or spaced dots can skew or distort the message a map attempts to communicate. If the dots are too numerous, it may be difficult for the reader to count the dots. This can cause the map to ineffective in communicating its message.
Solutions to problems in dot maps can be found through the combination of remotely sensed data. Satellite imagery
Satellite imagery
Satellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History :The first images from space were taken on sub-orbital flights. The U.S-launched V-2 flight on October 24, 1946 took one image every 1.5 seconds...
showing lights at night can give good indication of population density. With such information, clustering of dots to relevant areas could possibly be applied to analyses. Data from the Oak Ridge National Laboratory
Oak Ridge National Laboratory
Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
's Landscan project is commonly used in this emerging technique.
Dot Maps Today
A large portion of the new maps appearing today are generated not through government agenciesGovernment agency
A government or state agency is a permanent or semi-permanent organization in the machinery of government that is responsible for the oversight and administration of specific functions, such as an intelligence agency. There is a notable variety of agency types...
or geographical societies and associations, but by common individuals from all walks of life. Users of such virtual globe
Virtual globe
A virtual globe is a 3D software model or representation of the Earth or another world. A virtual globe provides the user with the ability to freely move around in the virtual environment by changing the viewing angle and position. Compared to a conventional globe, virtual globes have the...
services as Google Earth
Google Earth
Google Earth is a virtual globe, map and geographical information program that was originally called EarthViewer 3D, and was created by Keyhole, Inc, a Central Intelligence Agency funded company acquired by Google in 2004 . It maps the Earth by the superimposition of images obtained from satellite...
constantly create new thematic map
Thematic map
A thematic map is a type of map or chart especially designed to show a particular theme connected with a specific geographic area. These maps "can portray physical, social, political, cultural, economic, sociological, agricultural, or any other aspects of a city, state, region,nation , or...
s quickly and easily. Such services offer a base platform upon which users can display layers as they choose, or even add their own data to the display.
CNN
CNN
Cable News Network is a U.S. cable news channel founded in 1980 by Ted Turner. Upon its launch, CNN was the first channel to provide 24-hour television news coverage, and the first all-news television channel in the United States...
in 2009 describes a recent map, created using Google Maps
Google Maps
Google Maps is a web mapping service application and technology provided by Google, free , that powers many map-based services, including the Google Maps website, Google Ride Finder, Google Transit, and maps embedded on third-party websites via the Google Maps API...
on April 21, 2009 by Pittsburgh biochemist
Biochemist
Biochemists are scientists who are trained in biochemistry. Typical biochemists study chemical processes and chemical transformations in living organisms. The prefix of "bio" in "biochemist" can be understood as a fusion of "biological chemist."-Role:...
Harry Niman. The map shows where the 2009 H1N1 swine flu virus
2009 flu pandemic
The 2009 flu pandemic was an influenza pandemic, and the second of the two pandemics involving H1N1 influenza virus , albeit in a new version...
was spreading at the time. Despite the fact that the data shown was not backed by any official agency, the author's creation had "gone viral", surpassing 290,000 web views and 3,000 comments within nine days of its being published online.
Emerging technology
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...
is increasingly making map
Map
A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....
ping more mobile. Dot distribution maps, through their simple and effective displays, are becoming the standard for such on-the-fly mapping services.