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Principles of Cartography

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					Visualisation and Cartography

         Darius Bartlett,
    UCC Geography Department
         What is visualisation?
• The process of putting complex ideas or images
  into minds
• Visualisation of information is of vital
  importance to geography
• The traditional medium for geographic
  visualisation is the map
       Components of visualisation
• Visualisation depends on the interaction of:
  –   A database containing information
  –   The medium used for the display
  –   The human visual system
  –   Processing of received information by the brain
    Cartographic communication
• “Reading” and “communicating” are separate
  and distinct activities
• A good map should invite, persuade, seduce,
  and ultimately should inform.
• Quality of message depends on information
  content, choice of symbol and colour, the
  hierarchy of sizes, etc.
• Poses many challenges
      Cartographic symbolisation
• “One of the most conceptually significant and
  yet least written about aspects of a map is
  symbology” (Godlewska, 1997)
• Cartographic symbology can be analysed on at
  least three levels:
  – cognitive and conceptual significance of symbols;
  – the complex layers of meaning inherent in
    individual symbols and in groups of symbols;
  – the evolution over time of symbols
   Making sure maps are readable
• The locational puzzle:
  – Reader asks “where is it”
  – Location often difficult to determine from maps
• The homogeneity puzzle:
  – The reader asks “what is it?” or “where should I
    look first?”
• The cartographer should anticipate both
  questions
                Key questions
• What is the image (map) supposed to show?
  – Objective or subjective information?
  – Concepts or ideas?
  – Opinions?
• Who is it aimed at?
  – How much map reading skill can be assumed?
  – How much familiarity with the subject can be
    assumed?
             Graphic variables
• Symbols
  – Cartographic primitives: points, lines, areas
  – Classes of symbols correspond to classes of objects
    in reality
  – Visual differences between symbols convey
    information
  – These differences must be perceptible to the reader
    to be of any use
                  Using maps
• Seek to convey information about often very complex
  ideas or phenomena
• Are limited to two dimensions
• Are static - cannot easily show change in time
• Are limited in the way they can show interactions or
  flows
• Are limited by the tools used to construct them
• May give a false impression of accuracy
              Stages in map use
•   Detection
                       Often regarded as
•   Discrimination     “map reading” in the
                       simplest sense
•   Identification
•   Recognition
•   Interpretation
•   Understanding
                    Detection
• Primarily dependent on two factors:
  – The optical system of the eye
  – The receptor system of the retina
• Symbols near limits of resolution are a
  disadvantage if map is to be used under time
  constraints or pressure
• Most maps assume “normal reading distance”
  of ~30 cm.
                     Symbol location
• Where is the symbol
   – on the display?
   – Where does this correspond to
     in the real world?
• The brain uses location of
  symbols to obtain geographic
  information
   – e.g. about spatial relationships
     between objects
   – often calculated “on the fly”
                    Symbol value
• Qualitative differences:
   – Indicated by shape, lightness or darkness of symbol
   – The eye tends to be led by patterns of dark and light
   – By tradition, darker corresponds to “more”
• Quantitative differences
   – By symbol size
   – By symbol colour
           Symbol hue (colour)
• Important aesthetically
• Mostly used to convey qualitative differences
  – Continuous colour gradation is expensive and
    difficult to achieve in printed maps
  – Often easier on computer displays
• Can also be used to show quantitative
  differences
                 What is colour?
• A complex eye-brain
  response to light
• Average person sees six
  “spectral” colours
   – Some colours cover
     greater span of the
     spectrum than others
• Other colours are a mix
  of “spectral” colours
  Perceptual dimensions of colour
• Hue
  – Density of shading
• Lightness
  – brightness of an area relative to that of a comparable
    white area
• Saturation (chroma)
  – Intensity of the colour
                Symbol size
• Conveys quantitative difference
• May denote rank, ratio or ordinal difference
• The brain usually has difficulty extracting
  precise quantities from size differences
• A scale or key is usually needed
                Symbol shape

– Used to differentiate
  object classes
– Symbolic or iconic
                               
– Not all symbols convey
  the same meaning to all      
  readers
               Other factors
• Symbol spacing
• Symbol orientation
               Discrimination
• The ability to distinguish between symbol types
• Depends on
  – Physiological factors
  – Psychological factors
Applied visualisation: the design
       and use of maps
   With very few exceptions, ALL
       maps should contain...

…Title            …source
…Frame            …date
…Legend           …annotation
…Scale            …and, of course, data
…Orientation       relating to the actual
…Co-ordinates      mapped theme(s)
   Topographic vs. thematic maps
• “A thematic map is to a topographic map
  what an essay is to a dictionary” (Dent, 1993).
  – Distinction between the two not always sharp
  – Topographic maps are general maps.
  – Thematic maps
     • serve particular purposes, or illustrate a particular subject
     • use coastlines, boundaries and places as base data, to
       serve as points of reference for the phenomenon being
       mapped
           Topographic Maps
• Originally designed and intended for soldiers,
  military purposes
• Traditionally produced by National Mapping
  Agencies - these often have military origins
• Nowadays the map most readily available to the
  public
            Topographic maps
• General-purpose maps, designed for multiple
  uses
• Traditional scales range from c 1:10 000 - 1:250
  000
• A few agencies (notably British, Irish OS)
  produce maps at larger scale
  – e.g. 1:1250, 1:2500
                     Charts
• Specially designed to serve the needs of
  nautical or aeronautical navigators
• “Maps are to be looked at, while charts are to be
  worked on” (Robinson et al)
• Navigators use charts to plot position, bearings,
  etc.
• The marine equivalent of the topographic map
  is the bathymetric map - not the chart.
                    Road maps
• Robinson et al suggest that:
  – “the familiar road map is really a chart for land
    navigation. It supplies information about such
    factors as routes, distances, road qualities... As well
    as incidental information such as regional names
    and places of interest.”
 Some maps are difficult to classify
• Tourist maps
• City plans
• World Maps (e.g. in Atlases)
       History of thematic mapping
• “Perhaps the type [of map] most neglected
  by historians of cartography until quite
  recently” (Thrower, 1988)
  –   Very few examples known from great antiquity
  –   Upsurge in scientific revolution of the Renaissance
  –   Flourished in 17th Century France
  –   Edmund Halley (Britain, 1656 - 1742)
  –   “Modern” geological maps originated in C.18th
           Types of thematic map

•   Point maps             •   Flow line maps
•   Proportional symbols   •   Isoline maps
•   Located graph maps     •   Gridded maps
•   Area maps              •   Cartograms
    – Choropleth               (anamorphoses)
    – Chorochromatic
                   Point maps
• Simple point maps:
  – All points are identical and have same value
  – Can indicate precise location of objects or
    phenomena
  – Can indicate general distribution of collection of
    objects
• Complex point maps:
  – Different types or sizes of symbols for different
    phenomena
       Proportional symbol maps
• Size of symbol is related by a defined scale to
  the magnitude or other significance of the
  phenomenon being mapped
• May be area, height, diameter, etc. of symbol
• Symbols may be circles, squares, other
  geometric shapes, graphs or even icons
• Graphs can be used to add further (statistical)
  information
                   Area maps
• Choice of geographies
  – Areas based on actual phenomenon
  – Areas based on artificial boundaries
  – Areas based on statistical constructs
• Choice of symbolism
  – Chorochromatic
  – Choropleth
                Flow line maps
• Used to represent movements (of peoples,
  goods, ideas, air masses, etc.).
• Contain three elements:
  – representation of routes
  – representation of magnitude of flow
  – representation of direction (uni-directional or bi-
    directional)
                 Isoline maps
• Based on data obtained for continuous
  phenomena (air temperature, elevation,
  gravitational attraction, magnetic intensity, etc.)
  sampled at point locations
• Can be actual data or generalised (averages over
  time, etc.)
• The objective is to construct a picture of the
  complete distribution from fragmentary
  knowledge
                Gridded maps
• A regular grid is placed over the mapped area
• A value recorded for each grid square and
  expressed by some shading or colour
  convention
• The shading may relate to
  – An average value for the whole area of the cell
  – Max or min value
  – Value at the centre of the cell…
• Grid maps correspond to raster maps in GIS
                Cartograms
• Based on non-metric space
• Areas are scaled according to the magnitude of
  the variable being mapped
• Shape and size is distorted
• Can be very effective for communicating ideas
  and concepts
• See Daniel Dorling’s “New Social Atlas of
  Britain”

				
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