Essentially, his technique and methodology anticipate principles of a geographic information system GIS. Starting with an accurate base map of a London neighborhood which included streets and pump locations, Snow mapped out the incidents of cholera death. The emerging pattern centered around one particular pump on Broad Street. Further investigation of the area revealed the Broad Street pump was near a sewer line carrying the Cholera bacteria.
The primary purpose of a thematic map is to visually portray a non-visual phenomenon, usually the attributes of geographic features e. A good thematic map clearly shows geographic patterns that mirror patterns in the real-world phenomenon.
For example, if a map reader looks at a map of income distribution, he or she should be able to quickly and intuitively identify geographic concentrations of wealth and poverty that would be the same as those seen in the field.
Aesthetics is also an important goal: potential map readers are more likely to look at an attractive map and to spend enough time reading it to understand the patterns in the phenomena being represented.
Because these are in a clear conceptual order of importance to map readers primary theme most important, reference least important , a well-designed thematic map should reflect this order in the visual hierarchy. In addition, knowing the audience is of equal importance. Asking "Who will read the thematic map and for what purpose? A political scientist might prefer having information mapped within clearly delineated county boundaries choropleth maps.
A state biologist could certainly benefit from county boundaries being on a map, but nature seldom falls into such smooth, man-made delineations, in which case a dasymetric map charts the desired information underneath a transparent county boundary map for easy location referencing.
In constructing any type of thematic map or any map for that matter it is understood that location is a key feature. After selecting the physical area to examine, the next step is collecting data sets. Mapmakers must be careful in designing thematic maps that display too much information or suggest phenomena have a correlation when, in fact, they do not.
Qualitative vs. Qualitative map data is in the form of some subject, like vegetation type, and shows the presence or absence of the subject on the map. Quantitative map data is expressed in the form of a numerical value, like population values, or elevation in feet. Cartographers use many methods to create thematic maps. Most of these methods use a particular visual variable to visualize the attributes of geographic features.
The choice of an appropriate technique for a particular dataset depends on the scale of measurement of the attribute, the dimension of the geographic features point, line, region , and the geographic pattern of the topic in question. Choropleth maps are the most commonly used method of thematic mapping. Choropleth maps are particularly suited for charting phenomena that are evenly distributed within each enumeration unit set area. Raw data e. If derived values or normalized data can be obtained from raw data such as population densities , then the choropleth technique can be applied.
Also known as graduated symbols, these maps represent data associated with point locations i. The data is displayed with proportionally sized symbols to graphically represent a realistic difference in occurrence. Size can be a very intuitive visual variable for certain kinds of data: the larger the symbol, the greater the amount of something at a location [7]. This technique is best for ratio-type attributes that represent the total amount of something, in which values less than zero do not occur e.
Proportional Symbol Maps can map single variables or multiple values by varying the design of each symbol. These maps, also known as contour maps , depict smooth continuous fields such as precipitation. They are also well-suited to displaying three-dimensional values such as elevation i. Isometric and isopleth are the two types of isarithmic maps. In both cases, they can effectively show gradual change over space better than methods like choropleth maps. Use precise geolocation data.
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Briney, Amanda. The map may be qualitative in nature e. A thematic map is also called a special-purpose, single-topic, or statistical map. A thematic map focuses on the spatial variability of a specific distribution or theme such as population density or average annual income , whereas a reference map focuses on the location and names of features.
Thematic maps normally include some locational or reference information, such as place names or major water bodies, to help map readers familiarize themselves with the geographic area covered on the map.
All thematic maps are composed of two important elements: a base map and statistical data. Normally, the two are available as digital files, such as a cartographic boundary file and census data.
Desk-top geographic information systems or computer-mapping packages are typically used to generate thematic maps. Two common thematic maps produced at Statistics Canada are dot maps and choropleth 1 maps. The ecumene concept is generally used for dot and choropleth maps, to ensure that the spatial representation of census data is limited to inhabited land. Thematic maps can be used for exploratory spatial data analysis, confirming hypotheses, synthesizing spatial data by revealing patterns and relationships, and data presentation.
Refer to related definitions of cartographic boundary files CBFs ; ecumene and reference map.
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