Elsevier · Taylor, Lauriault: Developments in the Theory and Practice of Cybercartography, 2nd Edition · Chapter 6: Tile-Based Mapping with Opacity

Chapter 6: Tile-Based Mapping with Opacity

Andrew Clouston¹, Michael P. Peterson²

¹Geography, Environment and Earth Sciences, University of Victoria, Wellington, New Zealand

²Department of Geography/Geology, University of Nebraska at Omaha, Omaha, USA

Abstract
Splitting maps into small contiguous tiles increases the speed of Internet map distribution, improves the panning of the map, and facilitates the presentation at multiple scales. This online method of map distribution was introduced by Google in 2005 and all the major online-mapping services including Bing, Yahoo, OpenStreetMap, Nokia, and MapQuest have since adopted the technique. User-generated tiles can be overlayed on top of these maps, obscuring the standard map or satellite views. Any digital map whether a scanned historic one or a computer-generated thematic map can be converted to tiles. A multilevel overlay can be created by tiling several different maps or orthophotos. By enabling the presentation of more targeted map information, such overlays are an increasingly important method of map presentation through the Internet.
A significant disadvantage of tiled overlays is that they totally obscure the underlying base map. By providing an interactive tool to control level of opacity on tiles, the cartographer can effectively combine two views. Benefits include, image comparison, map merging, and masking effects. The combination of tile mapping and opacity control provides a new way of enabling user interaction with online maps and provides a valuable new tool for the modern cartographer.

Keywords: Interactivity; Opacity; Overlay; Tilemapping.

Figure 6.1 New Zealand Topographic Mapping.

Figure 6.2 The method of tile indexing used by the major online map providers.

Figure 6.3 Tiles from two maps that have not been mosaiced prior to tiling.

Figure 6.4 The hierarchical Maptiler file structure for the different zoom levels.

Figure 6.5 Code for converting tile mapping service references to Google references.

Figure 6.6 Code for retrieving the tiles from the directory structure where map_name is the top level directory and the zoom levels are limited to between 9 and 14.

Figure 6.7 A portion of the New Zealand Topo250 map (1:250,000) is displayed in the left panel. The map at right is the New Zealand Topo50 map (1:50,000).

Figure 6.8 Code for creating a multilevel overlay consisting of two different topographic maps. Each map is associated with specific zoom levels in the Google Maps display.

Figure 6.9 Totally obscuring the underlying map tiles.

Figure 6.10 The Opacity Slider Bar.

Figure 6.11 Two images of the PGC building in downtown Christchurch, New Zealand, before and after collapse as the result of an earthquake.

Figure 6.12 Merging using a partially transparent topographic overlay.

Figure 6.13 Zooming through an opacity enabled, tile limited multilayer mashup.

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