6.2 Open Issues

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6.2 Open Issues

A basic problem in the use of tiling in web mapping applications is the WMS server's ability to automatically place a scale bar, pre-defined logo, watermark or signature on every requested map. Figure 6.2 illustrates the potentially unaesthetic tile effects in OpenLayers using the examples of two WMS services. Maps that are supplemented with externally-generated and repetitive details divert the user's attention from the map content and therefore have a negative effect on Smart Map Browsing.
TileCache offers a technical approach to dealing with display issues such as these: with the Metatile option the WMS server loads a large tile, which is subsequently provided to the web mapping application in the form of numerous smaller, subdivided tiles. By default a Metatile is divided into 5x5 tiles. Taking into account the recommended maximum tile size of 2048px by WMS servers, the complete display of large maps with only one meta tile would generally not be possible. Therefore this solution could still result in a recurrence of this undesired element.

A second tiling problem is the positioning of labels which extend beyond tile borders. In Figure 6.2 (left picture) these labels are simply cut off, which clearly impairs the readability of the map. A discussion of potential problem solving approaches would go beyond the confines of this thesis, therefore this issue is merely pointed out.

   

Tiling Problems with Scale Bar, Logos and Labels in WMS services [GRASS user-map] and [GDI Bayern]

Furthermore two open issues are observed in the developed OpenLayers extension animated zooming:

1. Map scaling can at times result in a significant loss of quality. As the zoom level increases the tiles display an increasing coarsening of the pixel structure, which can result in a near illegibility of the map content. This is particularly evident during zoom-out from a high zoom level, where the ZoomOut tile intially provides barely usable information. The only zoom-interaction confirmation the user receives is the pixel movement itself.
   A possible solution would be to use several ZoomOut tiles (based on different zoom levels). As soon as the map exceeds a certain zoom level, an additional ZoomOut tile could be loaded. However, this alternative requires additional load and processing time which could negatively impact performance.
2. Because of the performance problems touched upon earlier, overlay scaling was abandoned - an aspect that has negative effects on Smart Map Browsing. Moreover, compared to animated panning the automatic zoom animation process is still very sluggish. For purposes of usability, a seamless and quick zoom animation is highly desirable. The mentioned performance problems show clearly that because of the large processing requirements, scaling processes in browsers are still very limited by current technology. Ideal animated zooming without noticable delays does not seem to be feasible with current client-hardware. However, given future technological developments this problem is likely to be solved in the next few years.

6.2 Open Issues

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