Cartography 101

Rachel Franklin

Newcastle University

Source: https://xkcd.com/1688/large/

Goals for today’s session

• Comfort with basic cartographic principles
• Ability to match symbology to data type
• Introduction to map elements

The power of maps

Racial segregation in Detroit, Michigan

Sources: http://www.wired.com/2013/08/how-segregated-is-your-city-this-eye-opening-map-shows-you/?viewall=true; http://demographics.coopercenter.org/DotMap/index.html

Visualizing spatial data

Representing different kinds of information

The way in which we display data on the map needs to match not only the type of data but also the type of feature

• Nominal – labels
• Categorical – different symbol or color for each category
• Ordinal – we want to use color or symbol size to convey the increase in value that goes with each category
  • Graduated or proportional symbol maps.
    
• Numerical – for points and lines, we use lines thickness or symbol size to express variations in value
  • For polygons we use graduated color or choropleth maps
  • Single color that changes in intensity to convey changes in value
  • Sometimes two colors are used

Classification Schemes

Mapping continuous data means we need a meaningful way to create categories – this is called classification

• Similar to choices that are made in generating a histogram

• How we allocate values to categories will affect our final map

  • This means number of categories
  • But also the classification scheme itself

• Ideally you are able to justify your choices (good general advice!)

  • What is the takeaway message you want to convey?
  • Who is your audience?

Types of classification

• Natural breaks (Jenks) – looks at the distribution of values and sets the break points at “natural breaks” in the data
• Equal interval – Creates a set number of classes of equal size
• Fixed interval – Same as equal interval, except you decide the size of the interval and the number of classes depends on that decision (e.g., by 1,000s)
• Quantile (equal count) – puts the same number of features into each class
• Standard deviation – creates categories based on the number of standard deviations away from the mean
• Pretty breaks – Breaks categories at round numbers
• Manual – you choose the break points yourself

QGIS symbology interface

What if we look at the histogram?

Map design

What do we need to think about?

• Map elements – the pieces of a map that make it a finished (and polished product)
  • Individual elements should contribute to conveying information, not detract from it
  • Limit clumping together of elements
  • Limit use of multiple fonts and colors
• Map symbology – the way in which the information the map conveys is portrayed
  • Colors – hue, intensity, and saturation
  • Symbols – size and type
• By necessity, maps are selective of the information they show and information is generalized

Choosing symbols and colors

• Beauty is in the eye of the beholder, but there are some general guidelines!
• Water and land should not be given unusual colors—e.g., water is blue not red
• Increased thickness of lines or larger symbols should match increased value
• Darker or more intense colors usually go with higher values
• If you use a color ramp with two colors, your data should also reflect two extremes in value—for example, in and out migration or voting patterns
• Avoid mixing and matching lots of different patterns on the same map—the eye can’t handle it

For those who like color—Color Brewer!

Source: https://colorbrewer2.org/#type=sequential&scheme=BuGn&n=3

What makes a map?

Cartographic elements

1. Map object
2. Title
3. Neatline
4. Scale bar
5. Legend
6. North arrow
7. Inset map

Types of maps

So many ways to visualize information on a map!

• Thematic Maps – organize and display spatial variation of a single variable
• Choropleth maps – changes in a variable are classified and mapped by some administrative category (e.g. countries or states)
• Dasymetric maps – Uses additional information to allocate value within polygons
• Isoline or contour maps – lines delineate areas of similar value. E.g., air pressure or elevation
• Reference maps – may display lots of different types of information
  • Topological maps or atlases
• Dot maps, picture symbol maps, graduated symbol maps
• Network and flow maps – show direction and magnitude of flows
• Cartograms—a unit’s display area is determined not by its actual area but by an attribute value

Examples

Choropleth map

Source: Census Atlas of the U.S.

Dasymetric map

Source: https://www.nature.com/articles/s41597-022-01603-z

Reference map

Source: https://www.nationsonline.org/oneworld/map/USA/indiana_map.htm

Topological map

Source: https://content.tfl.gov.uk/standard-tube-map.pdf

Dot density map

Source: http://demographics.coopercenter.org/DotMap/index.html

A fun one: Chernoff faces

For more information, check out this post.

Source: https://mapdesign.icaci.org/2014/12/mapcarte-353365-life-in-los-angeles-by-eugene-turner-1977/

Flow map (a famous one!)

Source: https://en.wikipedia.org/wiki/Charles_Joseph_Minard

Cartograms

Source: https://websites.umich.edu/~mejn/election/2016/

Cartograms

Source: https://websites.umich.edu/~mejn/election/2016/

Cartograms: 2020 presidential election comparison

Source: http://www.viewsoftheworld.net/wp-content/uploads/2020/11/USelection2020Cartogram.png

Maps are more than pretty objects!

Questions we can ask about maps and spatial data

• What are the high and low values and where are they?
• Does there appear to be a relationship between location and value?
• Do similar values appear to be located close together, or clustered? Is another spatial pattern discernible?
• Are there characteristics of areas that appear to correspond with particular sets of values? e.g., coastal or inner city.
• Would the pattern change if we change the scale of analysis? For example, what happens to the spatial pattern if we go from counties to states?

Next up: Tutorial 2