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H. Huang, G. Gartner:
"A Technical Survey on Decluttering of Icons in Online Map-based Mashups";
in: "Online Maps with APIs and WebServices", Springer Verlag, 2012, 157 - 175.

Recent years have witnessed rapid advances in online map-based mashups with Application Programming Interfaces (APIs) and web services. Map-based mashups often display different kinds of information (e.g., POIs, represented as icons) on base maps, such as Google Maps and Bing Maps. The visualization of large number of icons in a map on web browsers or mobile devices often results in the icon cluttering problem with icons touching and overlapping each other. This problem decreases map legibility, and thus prevents users from effectively processing the information presented in the map. It also leads to a dramatic degradation of performance, and a high transmission load. All these problems will greatly decrease the usability of a mashup application.
This paper surveys and assesses approaches from different disciplines (i.e., computer science and cartography) for avoiding icon cluttering in online map-based mashups. We focus on two issues: filtering of irrelevant POIs, and icon placement and aggregation. Different techniques from information filtering research are analyzed and compared for reducing the number of icons to be displayed in a map. For the latter issue, approaches of aggregating and placing icons from map generalization research are discussed and assessed for their applicability in online mashups. Some related APIs and typical mashup examples are also discussed and compared. This paper concludes that in order to provide more cartographically pleasing maps in mashups, techniques from computer science and cartography should be seamlessly integrated.

http://dx.doi.org/10.1007/978-3-642-27485-5_11