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Cartography for Swiss Higher Education

Solutions for LBS maps >> Physical Constraints >>

1.1.1.

Device constraints on mapping

In the lesson Foundations of Location Based Services various limitations of mobile client devices were described. These included:

Computing and memory resource relate to the speed of the processor, the size of the memory available for computations (RAM), and the size of the memory available for data storage. These present constraints on generating maps in two ways, the effect how quickly a map can be drawn and how much map data can be stored by the device. With advances in technologies these types limitations are decreasing. In addition they can often be traded-off amongst each other. For example, maps can be pre-computed and cached in the data storage requiring little computation at runtime for their rendering. The map response section described in an earlier lesson provided examples of how this can be achieved by cutting the map up into tiles and caching these. Batteries are also becoming longer lasting though not at as fast a rate as other computing technologies. The main way this constraint can be dealt with is by limiting the amount of computation performed. Unfortunately, otherwise there is little that can be done to allievate it.

The size and resolution of displays is a factor that is likely to remain more constant with technological developments, since the small display size is one of the factors that makes a device mobile. The resolution of display screens may improve but it is ultimately tied to the longevity of the battery so it is also likely to remain a constraint for some time. These factors effect maps in several ways. The low resolution requires larger map symbols to be clear. To some extent problems cause can be improved using graphical rendering techniques such as anti-aliasing and dithering (Foley et al. 1990), or by using vector graphics whose rendering can be optimised to the resolution of the screen. The small screen limits the available space to show map information, a problem clearly exaccerbate by the low resolution. The earlier interaction clearly illustrated this. One problem that this results in an overcrowded map containing too much detail and foreground features that are overlapping and so can not be interacted with (Anonymous). We will consider solutions to these problems in the later units of this lesson.

The speed of communications networks effects access to geographic information that is stored on remote web map servers and accessed over the Internet. If the data volume that needs to be transfered is quite large it will map rendering to be slow. Using vector graphics can reduced the volume of data that needs to be transfered to the device and at the same time allow the final display of the map to be optimised to any particular resolution. However, vector data can also involve the transfer of large volumes of data, so streaming techniques are required to assist this.

In the next section we will look at a way of streaming vectors representing geographic information to a mobile client device. This is done so the map user obtains a readable map long before the data has completely downloaded.

Progressive vector transmission



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