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1.2.3. Adaptive Zooming

When zooming in the presented map of Switzerland you only get an enlarged view of the given objects. The symbolization and the amount of objects that are visualised on the screen do not change whether you look at the original view of the map or at an enlarged section of the map. Different from that map is the next example, which adapts the visualised objects depending on the zoom level.

remark

You have to watch the film several times to realise what happens to the objects. While watching the film, concentrate on one object type and look what happens. Watch the film a second time and look what happens to another object type. Examples for object types are "city names", "hydrology", "streets", etc. Watch the film as often as you want. Click on the thumbnail to reach the quicktime movie.

Example of adaptive zoomingExample of adaptive zooming (Brühlmeier 2000)

The method that is applied to the presented example is called adaptive zooming.

A zooming is called adaptive when the representation of a screen map is adapted to the zoom level. Therefore for each zoom level, the quality of the map is always high and the cartographic principles are conformed.
Hence, adaptive zooming describes the adjustment of a map, its contents and the symbolization to target scale in consequence of a zooming operation (=scale change) (Brühlmeier 2000)

In web mapping commonly the concept of term levels of detail (LoD) is applied so far. That is, a certain sequence of pre-calculated maps that cover the mapped area at different scales are used to adapt to the target scale. (Cecconi et al. 2002)

Have a look at the web site of MapQuest by clicking on the thumbnail. Type in your address and watch the responded map. Check out the zoom levels and watch how the map gets adapted.

MapQuest MapQuest (MapQuest)

The MapQuest application consists of a multiscale database. This database represents a special database where datasets from different pre-defined scales are stored. In such a database each termfeature class is stored at different scale levels (e.g. 1:25'000, 1:100'000, 1:200'000, etc.). Examples of such different scale levels are:

MapQuest zoom level 1 (Map content (c) 2007 by MapQuest, Inc and NavTeq. Used with permission)MapQuest zoom level 1 (Map content (c) 2007 by MapQuest, Inc and NavTeq. Used with permission) (MapQuest) MapQuest zoom level 4 (Map content (c) 2007 by MapQuest, Inc and NavTeq. Used with permission)MapQuest zoom level 4 (Map content (c) 2007 by MapQuest, Inc and NavTeq. Used with permission) (MapQuest) MapQuest zoom level 6 (Map content (c) 2007 by MapQuest, Inc and NavTeq. Used with permission)MapQuest zoom level 6 (Map content (c) 2007 by MapQuest, Inc and NavTeq. Used with permission) (MapQuest)

These levels are commonly called termlevels of detail (LoD). The LoD are derived either from different sources (maps) or beforehand by means of generalization of the base dataset. The selection of the scales at which LoD are stored in a multiscale database has to be a compromise between the number of LoD and the optimal adaptation to arbitrary scales. (Cecconi et al. 2002)

remark

You may wonder why we talk about features (vector data) that are stored in the database but in the MapQuest-application you get a raster map as response. That is because the webserver creates a raster image out of the requested features and sends this raster image back to the browser where it is visualised.

You might have noticed that the maps of MapQuest do not really follow cartographic principles (especially between the zoom levels 5 and 7). The reason why we presented this example is because you can choose between many zoom levels.

The next example shows a map that includes adaptive zooming AND follows the cartographic principles.

remark

In the next application you have the possibility to change between six different zoom levels and to scroll and pan in all directions, additionally, independent from these fixed steps, a user-defined map scale can be set. The value entered is equivalent to the proportions of the extent to the original viewport in %.
Check out the various zooming levels and look what happens to the map's objects. Observe the legend and look what happens to the legend when changing the zoom level of the map.

Example with implemented adaptive zoomingExample with implemented adaptive zooming (Galanda et al.)

Exercise

Search the internet for two maps that have not implemented an termadaptive zooming and for which an adaptive zooming would be recommendable. Place the links of these maps on the discussion board "Adaptive Zooming". Select three maps which your colleagues placed on the discussion board and study them carefully. Comment (on the discussion board) on why you recommend to implement the adaptive zooming for the three maps that you have selected.



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