GEO381/550 Lecture note of November 2nd 2004 Proportional Symbol Map

Proportional symbol maps

 

Shows relative magnitudes of phenomena at specific locations

  

Symbol varies its size in proportion to the quantities it represents

Two kinds of proportional symbol maps:

(1) data occur at points (2) data are aggregated at points within areas (Figure 9.2)

 

Demo in ArcView:

(1) data occur at points: Number of hospital beds

   a. choose a graduated symbol map

(2) data aggregated at points within areas: Total population by county

   a. use chart map – properties button - set size field to the field to be mapped

   b. extract centroid, and then make a graduated symbol map from the centroi

When to use proportional symbol maps?

 

I. Phenomenon

Represents the discrete phenomenon with abrupt variation

 

II. What kinds of data

Practically any kind of magnitude data can be used:

Examples:

(1)   tonnage shipped at ports

(2)   retail sales

(3)   total population enumerated by county

(4)   employment enumerated by metropolitan areas

(5)   newspaper circulation enumerated by service areas

(6)   number of workers in asbestos factory

(7)   number of deaths per 100,000 caused by breast cancer

 

III. Variation of data

Should be large so that maps can reveal non-monotonous spatial pattern even though it may be technically accurate (The map should be intuitively appealing)

 

Compare this versus that

Figure 9.13: Monotonous pattern is not good

Figure 9.8 Clustered pattern is good

Symbol form

 

2-Dimensional

 

Compare two maps: circle versus square symbol

 

Circle: compact, visually stable, easy symbol scaling, non-linear perceptual scaling

Square: less compact, less visually stable, linear perceptual scaling

 

3-Dimensional

Visually attractive, space-saving scaling, difficulty of measuring dimension correctly

Table 9.1 Differences in mapped areas of circles and spheres

Proportional symbol scaling

 

How to scale symbol size in proportion to the value to be mapped

Have two kinds: (1) unclassed (2) classed

 

  1. Unclassed
    1. Absolute scaling
    2. Perceptual scaling (apparent-magnitude scaling)
  2. Classed
    1. Range grading

Absolute scaling versus Perceptual scaling

 

When humans read symbol, length is estimated correctly, but area and volume are underestimated according to psychophysical investigations.

Figure 9.5 Apparent-magnitude curves for length, area, and volume

 

Figure 9.6 Absolute and apparent scaling of proportional circles

 

Absolute scaling: area of symbol is directly proportional to the magnitude mapped

Perceptual scaling (apparent-magnitude scaling): area of symbol gets enlarged to compensate for underestimation

 

Example: suppose we have two values, 6400 and 1600, that are to be symbolized and scaled by proportional circles

Absolute scaling will yield 2:1 in terms of radius

Perceptual scaling will yield x:1 in terms of radius where x is bigger than 2 which incorporate correction factor to compensate for the underestimation

 

In ArcView

Set the number of values to the number of classes in proportional symbol map of point layer (Default option for ArcView graduated symbol map is absolute scaling)

Perceptual scaling in ArcGIS: proportional symbols appearance compensation

Range grading

 

Data array is divided into groups

Each circle is easily differentiated from its neighbor (Figure 9.11)

No more than five classes are advisable for this form of scaling

Graphic design issues

 

  1. Avoid symbol overload
    1. Masks other symbols
    2. Using more than two variables are not desirable: confuses map readers
  2. Choose symbol form and scaling methods appropriate to perceptual principles
    1. Circle versus square or 2D versus 3D
    2. Absolute versus apparent scaling methods
    3. Unclassed versus classed scaling methods
  3. When range-graded, the size of symbols should be distinguishable from neighboring symbols
    1. Remember that the purpose of this map is to show the relative magnitude of variables
  4. Data variation should be large to show discriminating patterns
  5. Opaque or transparent symbol? (Figure 9.15)

Demo: pros and cons of each

  1. Map symbols should have contrast to their surroundings
    1. Use dark shade rather than light shade for filling the symboL

 

The above-mentioned guidelines embody the following principles:

  1. Symbols should be made to appear as strong figures in perception
  2. Symbols should be made to stand out from its surroundings
  3. Symbols should be clear and unambiguous in meaning

 

Next time: dot density map (READ CHAPTER 8)