Lecture 6 Flashcards
Maps
A diagram or collection of data showing the spatial distribution of something or the relative positions of its components
Used in :
Representation of geographic data
Genetics
Mathematics
Thematic Maps
Thematic maps, the area of our interest, are representations of attribute data (quantitative and qualitative) on a base map
Thematic maps display a theme that can be a number of phenomena, such as social, political, economic, or cultural issues, with the purpose of revealing patterns and frequencies in the geography where they occur.
Map Design
Visualizing data with maps involves making decisions in three basic areas:
Projection
Scale
Symbolization
Projection
Map projections are mathematical transformations of the curved three-dimensional surface of the globe onto a flat, two-dimensional plane.
There are three developable surfaces and hence three kinds of map grid:
Plane: azimuthal
Cylinder: Cylindrical
Cone: Conic
All map projections involve transformations that result in distortions of one or more of the geometric properties of angles, areas, shapes, distances, and directions.
Distortion increases with the distances from the point or line of contact—tangent or secant—between the developable surface and the globe
Projection - Distortion
Projection of three dimensional space on a two dimensional surface causes distortion. In the next few slides we will look at a few projections and the distortion induced by them.
Mercator projection Equal-area cylindricalprojection Mollweide projection Robinson projection Sinusoidal projection
Mercator Projection
TheMercator projectionis conformal. Areas and shapes vary with latitude, especially away from the Equator, reaching extreme distortions in the polar regions. All indicatrices are circles as there are no angular distortion.
Equal-Area Cylindrical Projection
Theequal-area cylindricalprojectionpreserves area. Shapes are distorted from north to south in middle latitudes and from east to west in extreme latitudes.
Mollweide Projection
In theMollweide projection,shapes decrease in the north–south scale in the high latitudes and increase in the low latitudes, with the opposite happening in the east–west direction.
Robinson Projection
In theRobinson projection,all points have some level of shape and area distortion. Both properties are nearly right in middle latitudes.
Sinusoidal Projection
Thesinusoidal projectionpreserves area, such that areas on the map are proportional to same areas on the Earth. Shapes are obliquely distorted away from the central meridian and near the poles.
Projection: Another Approach
Considering that all projections will cause distortions, when using whole world maps, “recentering” the projection to favor parts of the globe is usual. Rather than using the usual European-centered projection, this 1851 map depicting volcanic activity around the world by Traugott Bromme is centered on Asia. Delaney expounds, “The large yellow circle around Indonesia and part of Australia shows the destructive reach of Mount Tambora’s explosive eruption on April 11, 1815. Its magnitude has been given a 7 on today’s Volcanic Explosivity Index, the highest rating of any volcanic eruption since the Lake Taupo (New Zealand) eruption circa AD 180.” The map was part of a companion volume to Humboldt’sKosmos. Color encodes categorical data, with red dots standing for eruptions, green circles for volcanic regions, and colored lines for ranges.
Scale
Themap scalerefers to the degree of reduction of the map.
It is the ratio between a distance on the map and the corresponding distance on the Earth
Factors to be considered to select the map scale:
Objective of the map
Intended output
Visual Encoding/Symbolization
Visual encodingis the process of matching the phenomena to be visualized, which is provided by the dataset (data scale and attributes), to the most suitable type of representation (graphical elements and visual properties).
For encoding we need to look at the three aspects:
Data
Titles and legends of map
Variables of visual encoding
Graphical Method
There are six graphical methods used primarily in thematic maps for representing all sorts of qualitative and quantitative data:
- Dot distribution maps
- Graduated symbol maps
- Isometric and isopleth maps
- Flow and network maps
- Choropleth maps
- Area anddistance cartograms
Dot Distribution Maps
Dot distribution maps aim at revealing the spatial distribution of phenomena using the basic element of a point as the visual mark.
Graduated Symbol Maps
Graduated symbol maps use the visual variable of size to proportionally represent magnitudes of thematic discrete data
Choropleth Maps
Choropleth maps are perhaps the most popular technique for representing statistical data using area symbols. Choropleth maps typically display data that have been aggregated by administrative units (the area symbols), and the values have been normalized (e.g., densities, ratios, averages).
Isometric and Isopleth Maps
Isarithmic maps represent real or abstract three-dimensional surfaces by depicting continuous phenomena. There are two kinds of lines of equal value used to demarcate continuous surfaces on the map:
• Isometric lines show distribution of values that can be referenced to points.
• Isopleth lines show distribution of values that cannot be referenced to points.
Flow and Network Maps
Flow and network maps portray linear phenomena that most often involve movement and connection between points: origins and destinations
Area and Distance Cartogram
Area cartograms were devised with this purpose of revealing spatial-geographic patterns. They use the spatial variables in the map for depicting population data according to a thematic variable. To allow identification of the known geographic spaces, most area cartograms make use of algorithms that retain as closely as possible the geographic space in the transformed map space.
Distance cartograms use the relationships in land distance to depict thematic data in the map
Area and Distance Cartogram
As opposed to traditional maps, in which space is used to depict space, cartograms distort the shape of geographic regions to encode another variable into the spatial area.
There are different types of cartograms, and the one used in “A Map of Olympic Medals” is called a Dorling cartogram. The technique represents geographic space as nonoverlapping circles. The map was designed by Lee Byron, Amanda Cox, and Matthew Ericso, and published as an interactive map at theNew York Timesonline in 2012 for occasion of the London Olympic Games. The screenshots show the results for 2012. Size represents the number of medals that countries won in the Olympic Games. Color encodes the continents.
