Framing Guidelines for Multi-Scale Map Design Using Databases at Multiple Resolutions

Framing Guidelines for Multi-Scale Map Design Using Databases at Multiple Resolutions Cynthia A. Brewer and Barbara P. Buttenfield ABSTRacT: This paper extends European research on generating cartographic base maps at multiple scales from a single detailed database. Similar to the European work, we emphasize reference maps. In contrast to the Europeans’ focus on data generalization, we emphasize changes to the map display, including symbol design or symbol modification. We also work from databases compiled at multiple resolutions rather than constructing all representations from a single high-resolution database. We report results demonstrating how symbol change combined with selection and elimination of subsets of features can produce maps through almost the entire range of map scales spanning 1:10,000 to 1:5,000,000. We demonstrate a method of establishing specific map display scales at which symbol modification should be imposed. We present a prototype decision tool called ScaleMaster that can guide multi-scale map design across a small or large range of data resolutions, display scales, and map purposes. Introduction • At what point(s) in a multi-scale progression will data geometry be modified to suit smaller scale presentation; and apmaking at scales between those • At what point(s) in a multi-scale progression that correspond to available database must new data compilations be introduced. resolutions can involve changes to From a data processing standpoint, symbol modi- Mthe display (such as symbol changes), changes to fication is often less intensive than geometry modi- feature geometry (data generalization), or both. fication, and thus changing symbols can reduce These modifications are referred to (respec- overall workloads for the map designer. Figure 1 tively) as cartographic generalization and statisti- shows a simplified example of a few coastal features cal or model generalization by Brassel and Weibel to provide graphical distinctions between display (1988) and later by Weibel (1995). Changes to and geometry modification. The large map (Figure display and geometry are in some instances co- 1a) is shown at 20 percent of original size, with dependent, meaning that modifications of one modifications to suit presentation at this scale in type can impact subsequent changes to the other. maps b1 to e1 in Figure1. Maps b2 to e2 are enlarged We argue that in previous literature, changes to versions of these 20-percent maps showing more geometry have overshadowed display change for clearly the quality of the different solutions. Map mapping at multiple scales; and in some situa- 1b shows no modification in symbols or geometry. tions, this increases the mapmaking workload. In contrast, map 1e shows modifications to both, Cartographic managers intending to balance or with less change in symbols than in map 1c and less optimize the workload are thus faced with a vari- change in geometry than in map 1d. For example, ety of decisions, including: roads are reduced less in width in 1e than b and • Which compiled-data resolution will be used displaced less in e than d. Overall these smaller for maps at any given display scale; changes maintain greater detail in the data and • At what point(s) in a multi-scale progression require lower magnitude adjustments, reducing will map symbols be changed; overall workloads. The European community has emphasized the Cynthia A. Brewer, Associate Professor, Department of need to develop efficient workloads for scale chang- Geography, 302 Walker Building, Pennsylvania State University, ing and for generalization operations (for example, University Park, PA 16802. E-mail: <[email protected]>. Tel: Cecconi et al. 2002; Bobzien et al. 2005), since 814-865-5072; Fax: 814-863-7943. Barbara P. Buttenfield, many are computationally intensive or require Professor of Geography, Department of Geography UCB 260, tedious manual refinement. We extend the European University of Colorado, Boulder, CO 80309. E-mail: <babs@ research by emphasizing that changes to symbol colorado.edu>. Tel: 303-492-3618; Fax: 303-492-7501. design often allow scale reduction without other Cartography and Geographic Information Science, Vol. 34, No. 1, 2007, pp. 3-15 Figure 1. Schematized example showing geometry and display modifications applied for reduction to smaller-scale map. data generalization, especially when the target topology, and other properties, thus most algo- mapping scales show levels of detail that are similar rithms are designed to preserve one or more of to the resolution of compiled data. We demon- these characteristics (Cromley and Campbell strate a method of establishing specific display 1990; Buttenfield 2002). The guiding principles scales at which display modification should be driving this body of work are to reduce visual imposed. We also prototype a decision tool called clutter while preserving clarity and logical con- ScaleMaster for multi-scale map design across a sistency. We review the work on generalization to small or large range of data resolutions, display show that the solutions offered to preserve clar- scales, and map purposes. ity and reduce clutter through changes in symbol design and selection (Brewer 1995; Slocum et al. 1995) have been largely ignored in the multi-res- Background olution mapping literature. Published reports that describe and evaluate and Related Literature algorithms for cartographic generalization date The process of modifying details in spatial data back forty years at least, with early work focused has been a subject of longstanding interest in on simplistic coordinate reduction of linear fea- many disciplines, judging by publications in tures and polygon boundaries. Subsequently, the Cartography, GIScience, and Computer Science emphasis shifted to “… address the interdepen- literature. Data reduction through generaliza- dent generalization of related features (i.e., the tion can effect varying and sometimes dramatic elements of a topographic map)” (Weibel 1991, p. changes in line length, local coordinate density, 172). In the past decade, generalization work has 4 Cartography and Geographic Information Science formalized knowledge-based rules (Kilpelainen and Sester 2004; Bobzien et al. 2005), and other 1997), context-sensitive algorithms (Jones et al. interactive cartographic applications. 1995; Burghardt and Cecconi 2003), data modeling In multi-resolution work, symbolization is typi- (Buttenfield 1995; Ruas and Lagrange 1995), and cally pre-established, perhaps based on the national software agents (Galanda and Weibel 2002). It is look of a topographic series. Thus adjustment of important to note that the majority of recent work symbols is not always discussed as a generaliza- in cartographic generalization has taken place in tion operation. However, these authors do discuss Europe. Chronologic overviews can be found in it. Bédard and Bernier (2002) explicitly involve Brassel and Weibel (1988), Buttenfield and McMaster variation in visual variables in their discussion of (1991), McMaster and Shea (1992), Meng (1997), multiple representations and their proposed VUEL Weibel and Dutton (1999), and Mackaness et al. (view element) concept for managing geometric, forthcoming). semantic, and graphic multiplicities. Rayson (1999) Another thread of relevant literature has focused proposes a method of aggregating point symbols on database support for multi-scale data manage- into stacks for military map display while zooming. ment and flexible zooming through many scales Torun et al. (2000) discuss selection decisions for and for many map purposes (see, for example, web mapping at scales roughly between 1:15K and Jones et al. 2000). Many European authors work 1:80K. They develop hierarchies of feature codes and within contexts of a national mapping agency eliminate progressively larger numbers of feature producing databases containing linked multiple code groups at smaller map scales. Their classing- representations of detailed, high-resolution objects. based method sets thresholds at which groups of These databases are referred to as multi-resolu- features are removed from a map display, with class tion or multi-representation databases (MRDB) visibility becoming more exclusive in displays at (Kilpelainen 1997; Spaccapietra et al. 2000). smaller scales. They acknowledge the interaction Updates to these detailed objects autonomously between generalization constraints (e.g., graphical propagate to all resolutions, reducing the work limits of output media and reading conditions) involved in updating maps (Stoter et al. 2004; and symbol choices for feature classes. Bobzien et al. 2005). Objects are linked through Arnold and Wright (2005) focus on the inter- all resolutions to accomplish efficient updates, action between feature updates and production so the approach emphasizes object-level rather of multiple cartographic representations from a than scale-level representations. Some researchers single detailed database. They emphasize that approach object-level generalization and update mapmakers may have varied purposes (such as using representation stamps (Balley et al. 2004). “The producing tourist, road atlas, or electoral maps) as concept of multiple representations includes both they move from raw data to high-quality finished the various representations stored in the database presentations. They contrast these more custom- and also those that can be derived from existing ized mapping goals to mapping that conforms to a representations” (Kilpelainen 1997, p.14),

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