Adaptive Zooming in Web Cartography

Alessandro CECCONI and Martin GALANDA
Department of Geography
GIS Division, University of Zurich
Winterthurerstrasse 190
CH-8057 Zurich, Switzerland

Phone: +41 1 635 51 31, Fax: +41 1 635 68 48

{acecconi,mgalanda}@geo.unizh.ch

Beyond any doubt cartographic quality lacks in most of the web mapping and web GIS applications offering zooming facilities that can be found on the web nowadays. The reasons aren't only the technical limitations related to the internet, e.g. screen resolution, data rate etc., but rather the neglection of one of the main cartographic principles of (web) mapping, namely adaptive zooming.

Adaptive zooming terms the adjustment of a map, its contents and the symbolization used to the target scale consequently to a zooming operation (= scale change). In webmapping commonly the concept of levels of detail (LOD) is applied. That is, a certain number of precalculated maps that cover the mapped area at different scales is used to adapt to the target scale (see for instance the applications http://www.mapquest.com or http://www.mappy.com). Such an approach implicates three main drawbacks:

On contrary the ideal solution would be to use a single master database and to calculate every arbitrary desired scale on demand by means of automated generaliztion (on-the-fly generalization). This approach fails too because of the limited time available to react on a zoom request (a limit of 10-15 seconds is assumed) and the complexity of the generalization task. Therefore this paper proposes a combination of these two methods to implement adaptive zooming (see figure 1) in order to compromise between flexibility in scale and loading time.

Figure 1: Principle of map generation based on LODs and on-the-fly generalization.

LODs should be used for those object classes, that require high computational costs to be generalized automatically, like road networks or buildings. On-the-fly generalization produces on the basis of the database temporary views on run time by applying generalization operators like selection, simplification or label placement. Each zooming request triggers an independent re-generalization process.

Realizing such an interactive and dymnamic concept for web mapping requires the use of vector based visulization tools, the interplay with other web components as HTML or CSS and well defined interfaces to map authoring software (e.g. Adobe Illustrator, Corel Draw, ArcGIS). The only data format meeting the criteria is the W3C standard Scalable Vector Graphics (SVG). Thus, it was used to implement the presented ideas in an application for topographic web mapping based on the landscape model VECTOR25 of the Suisse Federal Office of Topography. It covers a scale range from 1:25,000 to 1:200,000. The object classes roads and buildings are mapped by LODs at scales between 1:25,000 and 1:200,000 while map objects like rives, lakes, labels, symbols are generated on-the-fly. A first protoype showing a test area representing the current state of our implementation is located at http://www.geo.unizh.ch/gis/research/webmap/gendem/.

last update: 15.01.2002