The Choropleth Map Slide #2: Choropleth mapping enumeration units

Transcription

1 The Choropleth Map Slide #2: Choropleth mapping is a common technique for representing enumeration data These are maps where enumeration units, such as states or countries, are shaded a particular color depending on that unit's data value. There is not much more to them, and it is this simplicity that makes them popular. Slide #3: Choropleth is derived from the Greek word choros (place), and plethein (to fill) Choropleth mapping has also been called area or shaded mapping Slide #4: Major concerns of the cartographer are: o Data classification o Area symbolization o Legend design

2 Mapping Rationale Slide #5: The choropleth technique is defined by the International Cartographic Association as follows: A method of cartographic representation which employs distinctive color or shading applied to areas other than those bounded by isolines. These are usually statistical or administrative areas. Because this form of mapping is used to depict areas that have boundaries and that have classified or aggregated data (often defined by administrative areas), it is sometimes called enumeration mapping Slide #6: See figure 6.1, p.103, A Typical Choropleth Map Slide #7: Examples of enumeration units used in choropleth maps include: o Countries o States o Provinces o Counties

3 o Census tracts Slide #8: Choropleth mapping may be thought of as a three-dimensional histogram or stepped statistical surface A choropleth map is a planimetric representation of this three-dimensional prism model Slide #9: See figure 6.2, p. 103, The Three- Dimensional In the model, the height of each prism is proportional to the value it represents The planimetric way of looking at the model incorporates area symbols to show the heights of the prisms

4 Appropriateness of Data The choropleth technique should be selected only when the form of data is appropriate Slide #10: Typically, choropleth maps are constructed when data occur to definite enumeration units such as census tracts, counties or states Geographic phenomena that are continuous in nature should not be mapped by the choropleth technique because their distribution are not controlled by political or administrative subdivisions For example, to map average annual temperature by this method would not be appropriate, but to map the number of people per square mile would be Slide #11 Enumeration data can be of two kinds: o Totals o Derived values (rates or ratios)

5 The number of people living in a census tract is a total Average annual income is a rate Slide #12: Traditionally it is not acceptable to map total values when using the choropleth technique This convention is based on sound reasoning In most choropleth mapping situations, the enumeration units are unequal in area The varying size of areas and their mapped values will alter the impression of the distribution Slide #13: See figure 6.4, p. 104 The most important assumption made in choropleth mapping is that the value in the enumeration unit is spread uniformly throughout the unit Slide #14 See figure 6.5, p. 105

6 The choropleth technique is insensitive to changes of the variable that may occur at scales larger than the chosen enumeration unit If the variable is changing within the enumeration unit, the change cannot be detected on the choropleth map If the level of variation is too great within the enumeration units, a smaller enumeration unit level should be considered Slide #15: In choropleth mapping, the boundaries of the polygons or the enumeration units (sometimes called statistical units, chorograms or areal units) have no numerical values associated with them They function only to separate the enumeration areas. This differs from the lines on an isoline map which do have values

7 Legend Design Slide #16: Choropleth legend boxes are usually rectangular in form Need to be large enough to provide a visual anchor for the map reader to correctly interpret the symbolization but not so large as to detract from the map Class ranges are usually placed on the right side of boxes in a vertical presentation and underneath the boxes in the horizontal presentation Continuous and Non-continuous Class Ranges The continuous style is the more traditional style of reporting class ranges although both styles are used extensively

8 Slide #17: See figure 6.9, p. 112 There are no gaps or overlaps in the class ranks Other Legend Hints and Info Slide #18: Values for each class always increase from left to right Both a dash ( ) and the word to (501 to 925) are used in depicting class ranges the word to should be used if there are negative numbers to avoid confusion between the dash and negative sign

9 Symbolization for Choropleth Maps Black & White Mapping Slide #19: B&W maps are lower cost and people with deficiency perceiving color don t have problems with grayscales Slide #20: See figure 6.10, p. 114 Pattern fills were very popular but are rarely used anymore because solid color or grayscale fills give the map a better and less dated appearance

10 Color Map Symbolization Slide #21: Color use in choropleth mapping is becoming fairly standard owing to decreasing costs of color printing and increased use of virtual maps Color gives more symbolization options Also introduces a higher degree of complexity and subjectivity into the design process The choice of hue, color value and color saturation are all important Hue Slide #22: Hue is the name we give to various colors: o Reds o Greens o Blues browns o Red-orange o Etc Each hue has its own wavelength

11 Slide #23: Use of Hue in Area Symbols Color Value 24- Color value is the quality of lightness or darkness of colors Color Saturation Slide #25: Saturation is also call chroma, intensity or purity The vividness of the color If you compare a color to neutral gray with the addition of more and more pigment of a color, it will begin to appear less and less gray finally achieving a full saturation or brilliance Color Ramp

12 Slide #26: Popular with GIS software in which the color variation is automatically applied by the software Most mapping software provide a variety of color ramps that can be used as a starting point How does one choose a good color scheme? o Slide #27: Make sure there s enough differential between the symbolization in the class ranges so that the difference can be clearly seen on the map One simple technique to is to cover up or temporarily delete the legend and ask yourself or someone else if the symbolization suggests more or less for each class range o Slide #27: If a map is a virtual map, be aware that different monitors and graphic cards reproduce color differently. Make sure there s enough visual separation of class ranges so that slight variations in

13 monitors won t change the reader s ability to distinguish classes o Slide #27: Use ColorBrewer* to help select color schemes * o Avoid using qualitative discrete color fills that are normally reserved for categorical data For example, on a four class map, blueyellow-red-green is a horrible sequence for choropleth maps. Such a sequence does not in any way suggest more or less and is inappropriate for continuous quantitative data Adding Other Reference Features to a Map Slide #28: Should you place general reference features such as cities, roads or rivers on the map? Slide #29: See figure 6.11, p. 116

14 If the additions don t clutter the map and if they convey needed info, yes, add them