Data layers provide information worth even more than the sum of their parts.
Although it is good to show both details and their context, this can be difficult to achieve.
Fisheye views focus in on a particular area.
Repetitions can provide context for viewing the data.
This example uses spatial relations to show changes in Wall St. over time.
EPA's Envirofacts and My Environment
Census Bureau's data maps
NASA's Interactive Mars data maps
Guidelines for creating perceptual layers