Classifying Space
A fundamental process in the classification of space is the abstraction of data. MacEachren and Ganter (1990) have describe this as a shifting along a continuum ranging from images to graphics. At one end of this continuum is information in its rawest form: for spatial data, aerial photographs and other remote sensing products (see Figure 2.1, left). Further along the continuum, maps provide an abstraction of images (Figure 2.1, middle). Some of the information that is present in an image is dropped in order to allow symbolization of information that may not be directly perceivable in the image. For example, replacing a dark line through a green area with symbols for a road and a forested area; the road can then be given a label, as well as provide an indication of the number of lanes and access. At the other end of the continuum, graphics allow the use of position in the display to symbolize any variable (Figure 2.1). For example, a distance decay graph uses the X axis to indicate distance from a point in any direction, and the point may not be tied to any one geographic location.
Figure 2.1 The Image to Graphic continuum. Images approximate what we see, graphics provide abstraction of, possibly, invisible relations (MacEachren and Ganter, 1990).
A similar continuum is the range of symbols that can be used in maps and graphics (see Figure 2.2). At the end of the continuum most similar to images are mimetic symbols. At the end of the continuum most similar to graphics are abstract symbols. Except for a few symbols that are toward the mimetic end of this continuum, most of the symbols that are provided with ARC/INFO are abstract--these tend not to have commonly accepted meanings, and thus can be defined by the cartographer as needed.
Figure 2.2 The Image to Graphic continuum applied to symbolization. Symbols can vary from mimetic to abstract. All of these symbols could be used to represent a marina--the audience must be considered in selecting which to use.
The degree of abstraction that should be used is dependent on several factors. Most importantly, the data that needs to be mapped must be effectively shown, with a degree of abstraction that is appropriate to the data. For example, if a distance decay model is developed from a set of sample sites, showing the values measured at individual sites may not convey the distance decay as clearly as a graph. If the site locations are important, a map can be generated with the data and an inset showing the graph can be made using ARCPLOT's graphing tools. As mentioned in Chapter One, the audience must also be considered when determining symbolization abstractness. Highly mimetic symbols can convey a sense of simpleness, which may need to be avoided in order to project an image of authority. A third consideration is the means of display; mimetic symbols can be used to minimize the need for/use of a legend. This can be helpful for slides and overhead displays.
Projections
There are several classes of projections, each with strong points. Equal area projections show the same amount of space on the earth's surface for any given area of the map. Equidistant projections show true distance from a point or along given lines. Conformal projections have a constant scale in every direction from any given point, and because of this latitude and longitude lines meet at right angles. Some projections, such as Robinson's, are not mathematical transformations, but rather, tabular. These projections have been designed for specific purposes (Arthur Robinson developed his projection for world maps that are more visually appealing than others, like the Mercator projection). Other projections include the Mercator and gnomic projections; the Mercator projection shows lines of constant compass directions as straight, and the gnomic projection shows all great circles as straight lines. These two projections are best used for navigation and not for general reference or environmental maps.
Figure 2.3 EPA Region Six shown in Alber's conic equal area projection.
For environmental maps, in general, equal area projections should be used. This insures that symbols, especially those based on size, are not distorted on the basis of the base cartographic data (such as county, or state outlines). For the 48 contiguous states, Alber's conic equal area projection should be used; Alber's conic should also be used for smaller regions that are east-west oriented, such as EPA Region Six (see Figure 2.3). The sinusoidal equal area projection should be used for regions that are north-south in extent, such as EPA Region One (see Figure 2.4). Lambert's azimuthal equal area should be used for areas that have the same extent in all directions from a center point (see Figure 2.5).
Figure 2.4 EPA Region One shown in the sinusodial equal area projection.
Because the distortions of any given projection are scale dependant, the use of any specific projection becomes less critical as the map's scale increases (and thus the area shown becomes smaller). For example, a map of a hazardous waste site may need the Universal Transverse Mercator grid for locations within the site; at this large a scale (1:50,000 or larger) use of the UTM projection is more appropriate than an equal-area projection--there will be little, if any, perceivable distortion of areas regardless of the projection used, as long as the projection is centered on the site.
Figure 2.5 EPA Region Four shown in Lambert's azimuthal equal area projection.
The ARC command PROJECT allows transformation of coverages between projections, as well as realignment of projections. If data is obtained from a national database, the projection (if not included with the database) may be:
Project: projection albers
Project: units meters
Project: parameters
1st standard parallel: 29 30 0
2nd standard parallel: 45 30 0
central meridian: -96 0 0
latitude of projections origin: 23 0 0
false easting (meters): 0
false northing (meters): 0
Because this is designed for the contiguous 48 states, any subset of this data should be reprojected for the subset, even if the output projection is Alber's. In particular, the standard parallels should be chosen such that the new parallels divide the region into three equal east-west bands, and the central meridian select bisects the region. This minimizes distortion away from the parallels and prevents an appearance of the whole map leaning in one direction, which results from an off-center central meridian.
Scale and Generalization
Figure 2.6 Insets allow focusing on detail and give a `big picture.' When possible, place them in otherwise unused space.
As noted above, selection of scale is critical in the selection of an appropriate projection. Scale is also important in the selection of total area to be mapped, and the relation of the area to the data to be mapped. By using a smaller scale, and thus showing more area, the apparent seriousness of a problem will be reduced. This is because the apparent extent of a problem is reduced by showing more of the surrounding area. Zooming in on a site has the opposition effect--the appearance of symbolization over a large part of the display conveys the idea that the problem is everywhere. Use of zoomed in areas, which allow presentation of detail, with a locator map can combine these two extremes (see Figure 2.6). The locator map allows a wide perspective, which, when combined with the large-scale, detailed map, conveys both a micro and a macro reading, as Tufte (1990) suggests is a key component of good presentation graphics.
Space and Time
With the increased processing speed of single user workstations, and the increased flexibility of ARC/INFO, animation of data--both for analysis and presentation--is becoming feasible. Animation has several possible approaches. Change in time can be used to depict existence, attributes, or change in existence or attributes. Change can also be broken into: looking at different parts of a data set, one after the other; looking at a data set that shows variation in time, in time sequence; or use of progression in time to show another data variable (this is comparable to use of an axis of a graph to indicate change in time rather than change in space).
There are two primary methods of creating animations in ARC/INFO, and both require AML programming. The most flexible method is the use of AML driven interactive map composition. This allows the display of an object (for example a point location, such as the population center of the United States) that changes over time. The object can be drawn at one location, erased with the MDELETE command, and redrawn at a new location. The other method of animation is the use of GRID to display changes in area data. The cliche `Raster is Faster' is still true enough to make a difference. Separate grid layers can be generated, each of which shows the change in areal extent of a phenomenon. Each layer can then be draw with successive calls to a display routine.