Thematic Maps

10 Key excerpts on "Thematic Maps"

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  Encyclopedia of Geography Terms, Themes, and Concepts

  • Reuel R. Hanks(Author)
  • 2011(Publication Date)
  • ABC-CLIO

    (Publisher)

  illustrate the spatial characteristics of a single characteristic or those of a closely related set of phenomena. The purpose of the thematic map is to provide a spatial representation of the distribution of such features and the relationship(s) between them. A map showing the average per capita income of each state in the United States for a given year would be an example of a thematic map. Such a map would indicate the state boundaries like a general map, but the addition of the specific attribute of per capita income, unique to each state, creates a specific theme for the map. The possibilities for thematic mapping are infinite, as a limitless number of spatial phenomena and data exist that might be mapped, and new data are con- stantly being generated. The process of creating a map is complex and requires great skill and care. Frequently the cartographer must decide on a proper scheme of generalization and symbolization of the data that are to be displayed. Information must be classi- fied into categories, because mapping all the variation in the data points would result in a map that would be overly complex and confusing. There are numerous techniques for dividing the data into appropriate classes, and the way this is done often depends on the nature of the data itself, as well as what facets of the informa- tion the cartographer wishes to emphasize. One of the most common approaches when working with statistical data is to produce a choropleth map. On this type of map the data is presented as continuous classes that highlight the magnitude or intensity of the data within the defined boundaries on the map. For example, a map showing the average income of residents in Florida by county would likely have five or six classes ranging from the lowest average county to the highest. Such a map will clearly show where the wealthiest residents live in the state, and where incomes are relatively low.

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  Map Librarianship

  A Guide to Geoliteracy, Map and GIS Resources and Services

  • Susan Elizabeth Ward Aber, Jeremy Aber(Authors)
  • 2016(Publication Date)
  • Chandos Publishing

    (Publisher)

  While all maps are concerned with the spatial layout of phenomena, many maps fall into a different category, known as Thematic Maps. These maps explore specific topics or themes of data. Reference maps exist to tell us where things are, while Thematic Maps exist to communicate a specific message about the world. Thematic Maps use general reference information to frame their messages, but only inasmuch as it is useful for putting thematic information in its appropriate context. For example, a map showing population density per county in the state of Tennessee will include county boundaries, but likely will not show every city, waterway, and road in the state. An overload of information can make things visually confusing, potentially to the point of obscuring the intended message. Therefore, on a thematic map, information not directly related to the message is generally not included. One of the most famous examples of a thematic map is the cholera map based on John Snow’s research during an 1854 outbreak in London, see Fig. 2.1. Snow was convinced that contaminated water was the vector by which the disease was being spread, and his geographic analysis is credited with helping to end the outbreak, as well as giving rise to the field of epidemiology (Vinten-Johansen, 2003). While the map in Fig. 2.1 uses general reference information in the form of London streets, the primary purpose is to present medical data in support of the contaminated water theory. Many Thematic Maps follow this approach, and can be considered tools for answering questions about the nature of the world. A more modern example could be a thematic map exploring poverty rates at the county level in the United States. This map would not only answer questions such as “where does poverty exist?,” but would also act as a tool for confronting the issue

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  Web Cartography

  Map Design for Interactive and Mobile Devices

  • Ian Muehlenhaus(Author)
  • 2013(Publication Date)
  • CRC Press

    (Publisher)

  9 Thematic Visualization

  Introduction

  Thematic cartography at its most basic is simply the graphic symbolization of spatial datasets concerning a particular theme over a base map. There are numerous types of thematic representations, or map styles, that can be used for spatial visualization (e.g., choropleth, dot, proportional symbol). Regardless of which form of representation is employed, all Thematic Maps are created by manipulating visual variables.

  As noted in Chapter 7 , most visual variables are only ideal for representing certain types of data. Thus, one of the first things to remember is that not all thematic representations are suitable for mapping all types of data. Be wary of using visual variables that do not accurately, or intuitively, highlight the nature of the information they are presenting.

  This chapter is devoted to helping people who are new to cartography learn more about the different types of thematic representations available to them. This chapter is also meant to help those who have a background in cartography better understand the idiosyncratic benefits and drawbacks of using different thematic representations in their Web maps. (There are several new hurdles confronting traditional representation techniques.) The next section reviews data and technology issues to consider when designing Thematic Maps for the Web. This is followed by a review of the most popular types of thematic representation. I also discuss the benefits for each type of representation. The chapter concludes with a brief discussion about the advantages and disadvantages of allowing map users to interact with and manipulate the thematic data that are represented.

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  Web Cartography

  Map Design for Interactive and Mobile Devices

  • Ian Muehlenhaus(Author)
  • 2013(Publication Date)
  • CRC Press

    (Publisher)

  143 © 2008 Taylor & Francis Group, LLC 9 Thematic Visualization Introduction Thematic cartography at its most basic is simply the graphic symbolization of spatial datasets concerning a particular theme over a base map. There are numerous types of thematic representations, or map styles, that can be used for spatial visualization (e.g., choropleth, dot, proportional symbol). Regardless of which form of representation is employed, all Thematic Maps are created by manipulating visual variables. As noted in Chapter 7, most visual variables are only ideal for represent-ing certain types of data. Thus, one of the first things to remember is that not all thematic representations are suitable for mapping all types of data. Be wary of using visual variables that do not accurately, or intuitively, highlight the nature of the information they are presenting. This chapter is devoted to helping people who are new to cartography learn more about the different types of thematic representations available to them. This chapter is also meant to help those who have a background in cartography better understand the idiosyncratic benefits and drawbacks of using different thematic representations in their Web maps. (There are several new hurdles confronting traditional representation techniques.) The next section reviews data and technology issues to consider when designing Thematic Maps for the Web. This is followed by a review of the most popular types of thematic representation. I also discuss the benefits for each type of representation. The chapter concludes with a brief discussion about the advantages and disadvantages of allowing map users to interact with and manipulate the thematic data that are represented. Data and Technology Limitations When designing Thematic Maps for the Web, mapmakers are immediately confronted with two hurdles: (1) the nature of the data being mapped and (2) the limitations of the technology used to create the map.

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  GIS

  An Introduction to Mapping Technologies, Second Edition

  • Patrick McHaffie, Sungsoon Hwang, Cassie Follett(Authors)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  A reference map shows as many features as possible as long as the map size allows. A topographic map (Figure 5.1) showing both natural and cultural features belongs to a reference map. In contrast, a thematic map picks a particular theme or variable (e.g., primary language, population density, GDP by country), and depicts the spatial distribution of the variable. A population density map (Figure 5.2) is an example of a thematic map. A reference map works like an encyclopedia, and a thematic map is more like an essay due to its emphasis on a particular topic. In the following, we focus on Thematic Maps, because they can be easily created based on field values from the attribute table in Geographic Information System (GIS). FIGURE 5.2 Choropleth map as a thematic map. (U.S. Census Bureau) In Figure 5.2, people per square mile is depicted as graduated shades at the level of counties (areal units), where the darker shades indicate higher population density. This type of map is called a choropleth map. “Choro” means area (region), and “pleth” means value. Hence, a choropleth map depicts “values assigned or constrained to areas” (e.g., population density by county). A choropleth map is also referred to as a graduated color map in GIS software. The population density map uses insets to show Alaska, Hawaii, and Puerto Rico. These three insets have different map projections and map scales, as indicated by the different scale bars in each of those insets. The legend on the right shows that the population density values are grouped into seven classes. This method of dividing quantitative data into classes is referred to as data classification in GIS and will be discussed in a later section. There are two types of Thematic Maps: qualitative maps and quantitative maps. The former maps categorical data (such as land cover and primary language), and the latter maps quantitative data (such as population density and temperature). Some maps only show the locations of features

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  Manual of Geospatial Science and Technology

  • John Bossler, John D. Bossler, James B. Campbell, Robert B. McMaster, Chris Rizos, John D. Bossler, James B. Campbell, Robert B. McMaster, Chris Rizos(Authors)
  • 2010(Publication Date)
  • CRC Press

    (Publisher)

  Cartography and Visualization 613 including the choropleth, graduated symbol, dot, isarithmic, and dasymetric tech-niques (among several others). This was a revolutionary idea—to spatially illustrate data that had previously only existed in chart form. However, the usefulness of these techniques quickly became apparent, particularly for state governments attempting to make sense of the increasing amounts of data they were collecting about their populations and territories.* As opposed to “dictionaries,” special-purpose maps might best be considered as “geographical essays.” Like a good snapshot, special-purpose visualizations can quickly tell a story without the need for words. By taking tabular data and illustrat-ing it over a map via location, an audience is able to interpret the data more quickly and readily than if they were to look at numbers. However, a special-purpose map’s readability and communicative power as an essay is only as good as its articulation; this is where cartographic design becomes extremely important. The rest of this chapter will review the five key realms of thematic (special purpose) map design that you should pay particular attention to when designing maps—scale, generaliza-tion, symbolization, data classification, and map design. Notice that these five realms combine both design and math (art and science). In order to become a proficient and successful thematic cartographer, you must practice the craft of melding these five realms together seamlessly. 31.3 CARTOGRAPHIC SCALE The reason we project the Earth onto a flat piece of paper is to look at a particular part of the Earth’s surface in greater detail—something that is impossible to do with a globe due to size constraints. Thus, the first thing any cartographer must do is to decide at what scale they want to present the data. Scale is simply the math-ematical relationship between the map distance and the commensurate earth dis-tance.

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  Web Cartography

  • Jan-Menno Kraak, Allan Brown, Jan-Menno Kraak, Allan Brown(Authors)
  • 2003(Publication Date)
  • CRC Press

    (Publisher)

  Traditionally maps are divided into topographic and Thematic Maps. Topographic maps visualise, limited by scale, the Earth’s surface as accurately as possible. This will include for instance infrastructure (e.g. railroads and roads), land use (e.g. vegetation and built-up area), relief, hydrology, geographic names and a reference grid. Figure 5.2 shows a topographic map of the Dutch province of Overijssel. Figure 5.3 A socio-economic thematic map: the population of Overijssel. Thematic Maps as shown in Figures 5.3 and 5.4 represent the distribution of particular themes. Figure 5.3 shows a population map of Overijssel and Figure 5.4 a map of the province’s drainage areas. The first is an example of a socioeconomic map and the second an example of a physical map. As can be noted both Thematic Maps also contain information found in the topographic map, since in order to be able to understand the theme represented one should be able to locate it as well. The amount and nature of topographic information required depend on the map theme. In general a physical map will need more topographic data then most socio-economic maps which normally only need administrative boundaries. The map with drainage areas needs the rivers and canals, Web cartography 56 while relief information is also normally added. Today’s digital environment has diminished the distinction between topographic and Thematic Maps. Often both topographic and Thematic Maps are stored in the databases as layers. Each layer contains data on a particular topic, and the user is able to switch layers on or off at will. Figure 5.4 A physical thematic map: the drainage areas of Overijssel. The design of topographic maps is mostly based on conventions, of which some date back to the nineteenth century. Examples are water in blue, forests in green, major roads in red, urban areas in black, etc.

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  Thematic Mapping From Satellite Imagery: A Guidebook

  • J. Denègre(Author)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  Results of visual-analogue interpretation (see Chapter 3) appear in graphic form (i.e. symbols, lines and surface colours). Digital image interpretation gives a pixel-based map, where all the pixels are coded by colours. A great variety of sciences are using satellite images for thematic mapping, existing applications covering almost one hundred disciplines. Land use/cover and Earth sciences are of primary applications. The International Report on Thematic Mapping from Satellite Imagery (edited by J. Denegre, Elsevier, 1988) gives an overview of applica- tions. Representation methods of thematic data are traditional. Design of satellite cartography products is mainly influenced by the application of the right representation method. They all belong to cartographic data—the main element of selecting right information for satellite cartography. 4.2.3 Elements of cartographic representations Thematic Maps have a well-defined and widespread system of cartographic representa- tion. To express reality, maps are using methods of classification, simplification and symboliza- tion. Classification means to create categories, artificial groups representing characteristic elements. Simplification or generalization is a process of selection, reduction, smoothing and combining data important for the purpose of mapping. Using symbols means to express Design and semiology 65 reality in an abstract way through a process of transformation. Geographic information becomes a point, a line or a surface characterized by a certain size, shape, pattern and colour. From a cartographic point of view, representation methods of Thematic Maps can be distinguished by the graphic appearance (i.e. points, lines, areas), what they express (quality only or quantity as well), whether they show any direction, distribution, frequency, what is the surface like they relate to (point, line, continuous or discontinuous surface, etc.).

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  The Map Reader

  Theories of Mapping Practice and Cartographic Representation

  • Martin Dodge, Rob Kitchin, Chris Perkins, Martin Dodge, Rob Kitchin, Chris Perkins(Authors)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  Elements of Cartography first published in 1953, and running to six subsequent revised editions, elided topographic matters. Instead, thematic mapping based on quantitative data dominates the text. As a result, the distinction into thematic mapping, and topographic survey or general purpose mapping, became reified in the day-to-day practices of cartography as a profession: cartographers were most likely to be trained in the design of the former, not the latter. It is perhaps unsurprising then that most subsequent Anglo-American textbooks have also had very little to say about the design of topographic maps. And perhaps these trends are exacerbated in the real world production of maps, with a gradual retreat from state-funded national surveys in the face of increasing competition from commercialised and globalised map sources such as TeleAtlas (underpinning much of Google Maps coverage). So, maybe what has been termed the ‘blandscape’ of multinationally sourced and internet-served mapping will increasingly supplant the national design imaginary offered by printed topographic products (Kent 2009).

  The profusion of thematic cartography over the last century certainly reflects a changing aesthetic. Examining the timeline of significant data visualisation techniques, constructed by Michael Friendly and his collaborators, one is struck by the diversity of techniques that have been invented across many disciplines (Friendly and Denis 2010). Academic cartographers deploy choropleths, dasymmetric and dot distribution maps, isarithmic maps, proportional symbol maps, and cartograms, along with more novel multivariate geovisualisations encompassing the animated and multimediated data displays (Slocum et al . 2008). However, in practice, very few of these techniques have been deployed very much, or very well. Technological shifts such as desktop mapping packages and online geovisualisation have facilitated an emerging and radically different aesthetic, but paradoxically the same shifts have encouraged the mass profusion of often poorly designed thematic map output, centring around the use of off-the-shelf GI defaults and a limited number of map types.

  Notable amongst these techniques has been the choropleth map. First named in 1938 by J.K. Wright, the technique creates maps that depict an average value for each area. Areas allocated to the same class are shaded the same: data are classified. So the designer can change the number of classes, the classification algorithm and the nature of the shading variation or sequencing (Evans 1977). Many of these issues are related to data generalisation, a fact developed long ago by Jenks (1963, excerpted as Chapter 4). Choropleths have probably been more researched than any other cartographic technique: their inadequacies were well documented by Wright in 1938, and have been extensively researched by academic cartographers in the years since. The technique hides any variation within the spatial frame of each enumeration district and is very often used in an inappropriate manner. An unimaginable number of possible displays may be made from the same data (but all the evidence suggests most users are unaware of this wide range); and all too often the sampling frame, the spatial units themselves, are a given and not available for the user to change.

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  The SAGE Handbook of Geographical Knowledge

  • John A Agnew, David N Livingstone, John A Agnew, David N Livingstone, SAGE Publications Ltd(Authors)
  • 2011(Publication Date)
  • SAGE Publications Ltd

    (Publisher)

  Thus, they were argu-ment-driven constructions less closely linked to an ongoing process of data collection than to the development of theories and data manipulation and analysis. Second, and as a consequence, their increased sophistication was more closely tied to the sciences from which the argument emerged than to the geo-graphical tradition of mapmaking. Though, arguably, geographers ultimately came to conceive of their terrain not so much as the space of the earth as the spaces of relations between particular phenomena in part through the thematic map. Finally, that thematic map-ping became so influential in scientific cir-cles has much to do with both the general availability of reliable maps and new tech-nologies that allowed non-specialists to com-pose and disseminate such maps. Thematic Maps, then, heralded a new relationship between geography and the map in which the map led and geographers followed. Technological change has done much to shape the map and determine its impact over the centuries. More important than trigonometry and its associated optical and observational instrumentation but perhaps as important as the development of differential calculus and probability (the mathematics of change and uncertainty), the technical inno-vation with the greatest impact was the print-ing press. As with books and illustrations, the printing press made the rapid and reliable dissemination of cartographic information possible and created a body of knowledge largely shared by scholars, statesmen, church officials, merchants and the relatively well off. The invention of pulp paper and litho-graphic (‘stone writing’) reproduction in the nineteenth century were additional steps on the path to wider access and dissemination initiated by the printing press.

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