A representation is never the same as the thing being represented. The critical trick is to get the abstractions right, to represent the important aspects and not the unimportant. The appropriate type of a representation depends upon the task (Norman 1994).
17.1.1 Map symbols and visual variables
On a map representation, three geometric categories of map symbols and six visual variables are used. Symbols on flat maps are point symbols, line symbols, or area symbols. Most general-purpose maps use combinations of all three, whereas statistical maps, which portray numerical data, commonly rely upon a single type of symbol (Monmonier 1996).
Each of the six visual variables (size, shape, greytone value, texture, orientation and hue) excels in portraying one kind of geographic difference. Shape, texture and hue are effective in showing qualitative differences (e.g. land uses). For quantitative differences, size is more suited to showing variation in amount or count, whereas greytone value is preferred for portraying differences in rate or intensity. Some visual variables (hue or greytone value) are unsuitable for small point symbols or thin line symbols (Monmonier 1996).
Norman (1994) uses the terms additive and substitutive representation:
• Additive representation: If you wish to increase the value, you simply add something extra to the symbol already there. Nothing present has to be changed (e.g. tally marks).
• Substitutive representation: If you wish to increase the value of a previous symbol, you must substitute a new symbol for the previous one (e.g. Arabic numerals).
The proper way to present a map is to use an additive scale (an ordered sequence of density) to represent an additive dimension (rates or intensities) and a substitutive scale (different hues) to represent a substitutive dimension (differences in kind) (Norman 1994.).
17.1.2 Generalisation
Clarity of a map demands geometric generalisation because map symbols usually occupy more space on the map than the features they represent occupy on the ground (Monmonier 1996). For some maps geometric accuracy is less important than linkages, adjacency and relative position (e.g. linear cartograms portraying subway and rapid transit systems).
Content generalisation promotes clarity of purpose or meaning by filtering out details irrelevant to the map’s function or theme. It has two essential elements:
• selection – choosing only relevant features
• classification – recognising similarities among the features so that a single type of symbol can represent a group of similar features (Monmonier 1996).
Occasionally the “template effect” of standardised symbols will misinform the map user by grouping functionally different features. Standard symbols, designed for ready, unambiguous recognition, are common in cartography and promote efficiency in both map production and map use. Difficulties arise when a standard symbol must represent functionally dissimilar elements. Generalised highway intersections are a prime example of how information obscured by the template effect can mislead or inconvenience a map user (Monmonier 1996).
Computers generally play a positive role in map analysis. Particularly promising is the ability to generalise the geometry and content of maps. However, a generalisation program can produce radically different cartographic pictures from a single database, because it can use different sets of weights or priorities to produce different patterns (Monmonier 1996).
17.1.3 Choropleth maps
Choropleth maps portray geographic patterns for regions composed of area units.
Usually two to six greytone symbols represent an equal number of non-overlapping categories for an intensity index such as population density (Monmonier 1996).
A single set of numerical data can yield markedly dissimilar maps. Areal aggregation can, for instance, have a striking effect on the mapped patterns of rates and ratios.
Also by manipulating breaks between categories of a choropleth map, a mapmaker can often create two distinctly different spatial patterns. Classification ought not to subdivide distinct clusters of homogenous data values, and natural breaks between them, if any occur, should be used. Class breaks of particular meaning (e.g. average values) should also be taken into account (Monmonier 1996).
17.1.4 Colours
In the case of a choropleth map, colours can be confusing if not used carefully. The use of a single hue is preferred. A partial spectral scale (e.g. yellow-orange-red) can also be as consistent and convenient. The full-spectral sequence is not recommended, as the spectral hues have no logical ordering in the mind’s eye. A double-ended scale is sometimes useful for maps showing e.g. positive and negative rates of change (Monmonier 1996).
According to Tufte (1984), colour often generates graphical puzzles, which are crypto- graphical mysteries for the viewer to decode. A sure sign of a puzzle is that the graphic must be interpreted through a verbal rather than a visual process. Despite our
experiences with the spectrum in science textbooks and rainbows, the mind’s eye does not readily give a visual ordering to colours, except possibly for red to reflect higher levels than other colours.
Because they do have a natural visual hierarchy, varying shades of grey show varying quantities better than colours. The shades of grey provide an easily comprehended order to the data measures. Central to maintaining clarity in the face of the complex are graphical methods that organise and order the flow of graphical information presented to the eye (Tufte 1984).
Maps using colours to portray differences in kind can benefit from contrasting hues.
For example vegetation maps, road maps, zoning maps and land use maps showing a variety of features can benefit from different hues, provided that somewhat similar hues represent somewhat similar features and radically different hues represent radically different features (Monmonier 1996).
Tufte (1997) also writes about the design strategy of the smallest effective difference:
make all visual distinctions as subtle as possible, but still clear and effective. In designing information, the idea is to use just noticeable differences, visual elements that make a clear difference but no more – contrasts that are definitive, effective and minimal. An example is a map that depicts depth (blue, bathymetric tints) and altitude (tan, hypsometric tints) in colour gradations with a scale “the deeper or the higher, the darker the colour”. To indicate depth, the contour lines can be labelled by numbers, a design that enhances accuracy of reading and nearly eliminates any need to refer back to the legend. In contrast, if the whole rainbow is used to depict depth, the aggressive colours, so unnatural and unquantitative, render the map incoherent.
Minimal distinctions reduce visual clutter. Small contrasts work to enrich the overall visual signal by increasing the number of distinctions that can be made within a single image – small differences allow more differences. In practice, the appropriate size of small contrasts will depend on the context, priority of particular elements, number of differentiations and characteristics of those viewing the image (Tufte 1997).
17.1.5 Narrative graphics of space and time
An especially effective device for enhancing the explanatory power of a map is to add the time dimension to the design of the graphic. One form of time-space graphics is a small multiple. Small multiples resemble the frames of a movie: a series of graphics showing the same combination of variables, indexed by changes in another variable.
The design should remain constant through all the frames, so that attention is devoted entirely to shifts in the data. This kind of graphic is good at showing for example the levels of air pollutants at different times during the day (Tufte 1984).
17.1.6 Maps and urban and regional planning
For presentations, a particularly interesting and forceful graphic is the concept diagram; a schematic, somewhat stylised map intended to demonstrate the general layout and functional relationship of a plan’s main elements. On a concept diagram, the developer or planner uses lines to subdivide space, highlight patterns of movement, and suggest revitalisation of the central city (Monmonier 1996).
Maps are also an important part of an environmental impact statement (EIS). Detailed oversize maps might accompany the EIS in an appendix, to supplement smaller-scale, more generalised maps in the body of the report. Potentially significant sources of error are the transfer of information from the source map to the common base and the
generalisation of these small-scale maps. Additional problems arise when the boundaries and other data are transferred from unrectified aerial photographs (Monmonier 1996).
17.1.7 The power of visualisation
We generally think that a visual map presentation is better than just a table; on the other hand with nice maps you can easily mislead and cheat the public. The same also applies to the scaling effect of graphs and diagrams.
In his book “How to Lie with Maps” Monmonier (1996) (cynically) gives us eleven rules for polishing the cartographic image. The rules implicitly illustrate the power of a planner using maps for presentation as well. Monmonier’s rules are:
1. Be shrewdly selective.
2. Frame strategically.
3. Accentuate the positive.
4. If caught, have a story ready.
5. Minimise the negative.
6. Dazzle with detail.
7. Persuade with pap.
8. Distract with aerial photographs and historical maps.
9. Generalise creatively.
10. Enchant with elegance.
11. When all else fails, try bribery.