1.1.1. What is Portrayal?

The International Organization for Standardization (ISO) defines portrayal as "presentation of information for humans" (ISO-19117 2002). The vagueness of this definition is representative of the abstractness of the concept in general. In the ISO standard the main concern is to define methods for styling geographic features. However it notes that, ``Portrayal shall not be limited to visual rendering, but may include audio, tactile and other media'' (p.9), In this sense, portrayal of geographic information can also be seen as being concerned with a number of other techniques for presenting information that exploit one or more properties of a map to enhance its abilities for communication in particular circumstances. These techniques include:

1. Changing the level of detail used to show map features at different scales
2. Distorting the geometry of the map, for example using map projection or a variable scale
3. Presenting the the features of the map topologically by relaxing other geometric properties
4. Using different forms media to portray the information (e.g. visual, audio, and tactile)

In the next units each of how each of these aspects can be exploited to alleviate the constraints imposed by mobile device clients will be described and techniques to perform the portrayal operations dynamically explained.