Dot distribution map

A dot distribution map, or dot density map, is a map type that uses a dot symbol to show the presence of a feature or a phenomenon. Dot maps rely on a visual scatter to show spatial pattern. A dot distribution map, or dot density map, is a map type that uses a dot symbol to show the presence of a feature or a phenomenon. Dot maps rely on a visual scatter to show spatial pattern. 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 data, 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. 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 cancer patients, may not be available for mapping due to restrictions on access to individuals' medical records. 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. The first dot distribution map was created by a Franciscan friar, 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. 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 Scandinavian region. No known reproductions of this map exist. Display of discrete data in the form of points in a map can provide convincing evidence related to medical geography. During the mid-1850s, cholera 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 pumps 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. However, Snow's work is derivative of Thomas Shapter's work in Exeter, Devon. 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 pump's 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.