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Article Excerpt
Introduction
Mapmaking at scales between those that correspond to available database resolutions can involve changes to the display (such as symbol changes), changes to feature geometry (data generalization), or both. These modifications are referred to (respectively) as cartographic generalization and statistical or model generalization by Brassel and Weibel (1988) and later by Weibel (1995). Changes to display and geometry are in some instances co-dependent, meaning that modifications of one type can impact subsequent changes to the other. We argue that in previous literature, changes to geometry have overshadowed display change for mapping at multiple scales; and in some situations, this increases the mapmaking workload. Cartographic managers intending to balance or optimize the workload are thus faced with a variety of decisions, including:
* Which compiled-data resolution will be used for maps at any given display scale;
* At what point(s) in a multi-scale progression will map symbols be changed;
* At what point(s) in a multi-scale progression will data geometry be modified to suit smaller scale presentation; and
* At what point(s) in a multi-scale progression must new data compilations be introduced.
From a data processing standpoint, symbol modification is often less intensive than geometry modification, and thus changing symbols can reduce overall workloads for the map designer. Figure 1 shows a simplified example of a few coastal features to provide graphical distinctions between display and geometry modification. The large map (Figure 1a) is shown at 20 percent of original size, with modifications to suit presentation at this scale in maps b1 to e1 in Figure 1. Maps b2 to e2 are enlarged versions of these 20-percent maps showing more clearly the quality of the different solutions. Map 1b shows no modification in symbols or geometry. In contrast, map 1e shows modifications to both, with less change in symbols than in map 1c and less change in geometry than in map 1d. For example, roads are reduced less in width in 1e than b and displaced less in e than d. Overall these smaller changes maintain greater detail in the data and require lower magnitude adjustments, reducing overall workloads.
[FIGURE 1 OMITTED]
The European community has emphasized the need to develop efficient workloads for scale changing and for generalization operations (for example, Cecconi et al. 2002; Bobzien et al. 2005), since many are computationally intensive or require tedious manual refinement. We extend the European research by emphasizing that changes to symbol design often allow scale reduction without other data generalization, especially when the target mapping scales show levels of detail that are similar to the resolution of compiled data. We demonstrate a method of establishing specific display scales at which display modification should be imposed. We also prototype a decision tool called ScaleMaster for multi-scale map design across a small or large range of data resolutions, display scales, and map purposes.
Background and Related Literature
The process of modifying details in spatial data has been a subject of longstanding interest in many disciplines, judging by publications in Cartography, GIScience, and Computer Science literature. Data reduction through generalization can effect varying and sometimes dramatic changes in line length, local coordinate density, topology, and other properties, thus most algorithms are designed to preserve one or more of these characteristics (Cromley and Campbell 1990; Buttenfield 2002). The guiding principles driving this body of work are to reduce visual clutter while preserving clarity and logical consistency. We review the work on generalization to show that the solutions offered to preserve clarity and reduce clutter through changes in symbol design and selection (Brewer 1995; Slocum et al. 1995) have been largely ignored in the multi-resolution mapping literature.
Published reports that describe and evaluate algorithms for cartographic generalization date back forty years at least, with early work focused on simplistic coordinate reduction of linear features and polygon boundaries. Subsequently, the emphasis shifted to "... address the interdependent generalization of related features (i.e., the elements of a topographic map)" (Weibel 1991, p. 172). In the past decade, generalization work has formalized knowledge-based rules (Kilpelainen 1997), context-sensitive algorithms (Jones et al. 1995; Burghardt and Cecconi 2003), data modeling (Buttenfield 1995; Ruas and Lagrange 1995), and software agents (Galanda and Weibel 2002). It is important to note that the majority of recent work in cartographic generalization has taken place in Europe. Chronologic overviews can be found in Brassel and Weibel (1988), Buttenfield and McMaster (1991), McMaster and Shea (1992), Meng (1997), Weibel and Dutton (1999), and Mackaness et al. forthcoming).
Another thread of relevant literature has focused on database support for multi-scale data management and flexible zooming through many scales and for many map purposes (see, for example, Jones et al. 2000). Many European authors work within contexts of a national mapping agency producing databases containing linked multiple representations of detailed, high-resolution objects. These databases are referred to as multi-resolution or multi-representation databases (MRDB) (Kilpelainen 1997; Spaccapietra et al. 2000). Updates to these detailed objects autonomously propagate to all resolutions, reducing the work involved in updating maps (Stoter et al. 2004; Bobzien et al. 2005). Objects are linked through all resolutions to accomplish efficient updates, so the approach emphasizes object-level rather than scale-level representations. Some researchers approach object-level generalization and update using representation stamps (Bailey et al. 2004). "The concept of multiple representations includes both the various representations stored in the database and also those that can be derived from existing representations" (Kilpelainen 1997, p. 14), either by display change (symbol redesign) or geometry change (generalization of detail).
Managing decisions to create consistently high-quality products must be balanced against data processing workloads. This is especially important for on-demand and on-the-fly multi-scale mapping for the web (Torun et al. 2000; Cecconi and Gallanda 2002; Cecconi et al. 2002), mobile devices (Harrie et al. 2002; Hampe et al. 2004), and in-car navigation (Li and Ho 2004; Ulugtekin et al. 2004), in addition to multi-scale database management (Timpf 1998;...
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