Go to previous pageGo to next page

1.3.3. Data Amount

In the last unit the difference between raster and vector graphics were shown. The file size of a raster image is only dependent of the image size (amount of pixels). The file size of the vector image however depends only on the amount of information, thus on the amount of different objects, shown in the image. In most LBS scenarios the maps and other graphics are delivered from the service provider to the mobile device via a wireless network. Thus the file size of those graphics influences the transfer time and thus the map response.

raster map example (27KB)raster map example (27KB) vector map example (SVGZ 81KB, SVG 335KB)

The examples above show 829 map features (443 roads, 380 buildings, 6 surfaces). So at this display size the raster map (27KB) is much smaller than the vector map (81KB compressed, 335KB uncompressed). Thus the choice of the data format is also depending on the amount of information (map features) to display in relation to the size the map will displayed at.

Vector graphics have no specific display size and can be displayed at every desired scale. This resizing can also be seen as zooming. All the information which is present in the vector graphic can be presented enlarged without having to download the graphic again. Click at the vector map example above and choose Zoom In. You can see how the vector map is enlarged and the single items are better to recognize. But there is no new more detailed information shown. Thus if new information, e.g. a map with more details, has to be displayed always new has to be downloaded (see table map navigation and data transfers below).

operation raster vector
panning download new data download new data
zooming (same map) download new data use old data
zooming (other map) download new data download new data
map navigation and data transfers

Rendering of Vector Graphics

Computer displays are always showing rastered graphics. Thus a vector map has to be rendered (converted to raster, see lesson Computer Graphics) on the device, before displaying it. The processing speed as well as the memory is much smaller on a mobile device compared to a normal desktop PC. This lack of performance influences the rendering speed of vector graphics, e.g. in the SVG format:

"What considerations should be taken into account when generating SVG content for mobile devices? As a very general rule, the fewer the number of graphics objects the faster the rendering speed." (Robinson 2002)

So again vector graphics are more suitable for graphics with a limited amount of graphic objects (map features). Exact numbers when vector and when raster is better are impossible to say because they depends on the purpose of the graphic. The performance of mobile devices is still raising with time so that soon also the rendering speed on mobile devices will increase.

Background and Foreground Layers

A common way of overcoming these rendering problems are for example the use of background and foreground layers. The background layers contain the map details that are not dynamic. An example would be a LBS map showing some termPOIs. While the POIs are created dynamically for the specific request, the map with the topographic information such as roads and houses are static. Thus theis background information can be term cached on the mobile device instead of having it to reload with every new request.

background raster map and vector POI overlaybackground raster map and vector POI overlay

The examples show how a raster background map can be combined with a vector foreground showing POIs. This combination of the different formats results in a good performance, as only the few POIs have to be downloaded and rendered dynamically, wile the raster background is only downloaded once and then cached.

combined vector-raster map

This combination of different layers can also be done with multiple vector or multiple (partially transparent) raster images. Also a combination with tiling (see next section) in order to enhance the response is common.



Go to previous page
Go to next page