Thematic Mapping: Points Flashcards
Thematic Map
Map that portrays the form or function of the spatial distribution of one, or a combination of variables
- Statistical Mapping, map data, manipulate data
Classification of Thematic Maps: Qualitative
- Location
- Boundaries (Administrative etc.)
- Routes (roads maps)
- Chorochromatic (Nominal)
Classification of Thematic Maps: Quantitative
- Isopleths
- Proportional Symbols
- Dot Maps
- Cartograms
- Flow Maps
- Choropleths
NOIR
Nominal, Ordinal, Interval, Ratio
Important Design Criteria
- Points can represent actual points or centroids of areas
- Idea is to show symbols at each location where the symbol tells the audience something about the data value(s) associated with that location
Nominal Scale: Geometric Symbol
Shape (square, triangle, circle)
Nominal Scale: Mimetic symbol
Looks like what it is
- Mime=copy something
- Fishing location has a symbol that looks like a fish and fish hook for example
- Sometimes easy to understand, sometimes difficult to interpret (i.e. not doing their job!)
- Still need legend
Primary design variables
Shape
Hue
Orientation
How can you differentiate relative magnitude of symbols?
- Size (Small, medium, large)
- Value (colour/texture when same size)
- Both (Useful for multiple variables, shaded colours in small, med, large etc.)
Two Options for Thematic mapping of interval or ratio point data
- Generalize data into range grades (implies loss of detail, not ideal)
- Make size of symbol directly proportional to size of each data value (complex, can be many symbols)
Design considerations for ratio scaling
- What symbols to use
- What attribute of the symbol to scale
- What scaling factor to use (can be psychological)
What are 3 deciding variables for symbol selection?
- Symbol complexity (less intricacy when scaling size
- Data Complexity
- Audience
4 steps for generating a mathematical scale for symbols
- Determine radius of largest circle by visual examination of map
- Select values for observed (Vi) and the value of the largest (Vl)
- Calculate using ri=(Vi/Vl)^0.5 x rl
- Draw symbol of calculated radius
- Repeat until desired effect is reached
Ebbinghaus illusion
When a larger circle is seen among smaller circles it appears 10-15% larger
and when the same circle is seen among larger circles it appears smaller
- The effect creates an illusion that the data is different than it actually is
- Effect of illusion can be reduced by drawing internal boundaries
Over & Under estimation of circle sizes
- Overestimate the importance of small circles
- Underestimate the importance of large circles
- Adjust by using a psychological scaling factor
Psychological scaling of circles
- Aids perception
- Change exponent so that bigger looks bigger and smaller looks smaller
- ri=(Vi/Vl)^0.5716 x rl
- Flannery scale is often used
- Psychological scaling factor should be listed in legend (Flannery, directly proportional, etc.)
- Dent has a table that can be used if data doesn’t work but best option is to fix data
Flannery
- Type of psychological scaling factor
- Often used
- Used in ArcGIS appearance compensation
Range-Graded Proportional Symbols
- Use Dent table
- Don’t just select random sizes
- Size choices must be contiguous
Proportional Breakdown of Pie Charts
- Find % of total occupied by breakdown category
- Multiply % by 3.6 (360 degrees/100%)
- Irrespective of size the angle will always be the same
- Try to always start a pie chart with a north line, draw largest sector first
Legend Design
- Nested (multiple proportional symbols inside each other with value inside or with a leader line)
- Linear vertical or horizontal
- Map is key focus so don’t make legend massive
- Don’t put every size circle on legend
Key element of final project
- Don’t leave any empty polygons
- No Data? Find it or represent another way w/o the proportional symbol
- Data must have suitable amount of variability
