Cellular networks are more and more complemented by wireless LAN hotspots. However, when users are mobile, the small coverage area of hotspots is a challenge for the access network discovery and selection. Network selection mechanisms can be improved if, in addition to terminal-based measurements, hotspot information can be retrieved from an information service. In this context, the coverage area is one of the most important characteristics of hotspots. This paper studies algorithms for the precise estimation of hotspot coverage areas based on field strength measurements performed by individual terminals. Different approaches for interpolation, vectorization and complexity reduction of measurement data are presented and evaluated in different scenarios by means of simulation. As a result, guidelines for the optimal parametrization of these algorithms are presented.