## 2 Technology and Design
### Filtering and Relevance Approaches
Filtering information based on the usage context and the resulting relevance of the information also has the advantage that the selected information is likely to fit the cognitive abilities of the users and can more easily be processed and connected to existing knowledge. Too much information, as well as irrelevant and hence useless information can actually bind cognitive resources for making sense of this information overload and limit higher-level cognitive processes, such as decision-making or planning [57]. Filtering, as discussed in the previous section, is one of the methods applied in the mapping process, specifically in the generalization process. Generalization is an abstraction process necessary to make maps of reduced scale still legible and understandable. In generalization filtering, often called _selection_ or _omission_, is applied to reduce the amount of features that will be represented on the final map. The number of map features is one factor influencing the map's complexity. Other factors are the complexity of the phenomena to be represented and their relations [66, 67]. Generalization also aims at reducing the complexity of the map in other ways, such as simplifying linear and areal features, aggregating areas, as well as reducing semantic complexity by aggregating categories to higher-level semantic units. In that way generalization also reduces the size of the data, e.g., less points, less attributes, less categories, less different colors, etc. As discussed previously, a major problem of geographic services is information overload. Filtering is a powerful instrument to reduce the amount of data and potentially prevent information overload. However, for geographic information, and mainly for map representations of geographic information, the question remains how much information can be filtered and which information should not be represented. If we filter too much geographic information or if we represent too few spatial reference information the geographic context might get lost or it becomes impossible for the user to construct a consistent information structure without gaps from the representation. As in the case of information overload the result might be in the worst case a useless and unusable map representation. Too little contextual information in a map can cause no or wrong references, errors in distance and direction estimates, invalid or wrong inferences, misinterpretations due the missing corrective function of context, problems in relating the geographic information on the map to the cognitive map, and incongruities between the perceived environment and the internal, mental representation of it. Whereas the problem of too much filtering is evident, it is very difficult to know and even to measure a good map design and an appropriate degree of filtering. As theoretical guidelines in deciding which geographic information to represent on a map may serve research on spatial cognition, e.g., the elements that make up a city, such as paths, edges, districts, nodes, and landmarks [68] or the fundamental geographic concepts, such as identity, location, distance, direction, connectivity, borders, form, network, and hierarchies [69]. Nevertheless, there are some guidelines in designing geographic services that can handle the bandwidth limitations. Since different visual representation forms of geographic information--commonly maps, city models, digital shaded reliefs _etc._--require different bandwidth capacities, services targeted at clients that have limited bandwidths could be restricted to light visual representation forms. Conversely, the different bandwidth capacities available for the transmission of visual representations of geographic information have a strong influence on the way such representations should be designed. Fixed line connections common for desktop applications usually offer more bandwidth than mobile network data connections. As a consequence different types of services have to be designed to meet the different capabilities.