This study investigates the readability (interpretability) of information presented on a geographical map onto which a semi-transparent multivariate selection layer has been overlaid. The investigation is based on an information visualization prototype developed for a mobile platform (tablet devices) which aimed at supporting epidemiologists and medical staff in field data collection and epidemiological interpretation tasks. Different factors are analysed under varying transparency (alpha blending) levels, including: map interpretation task (covering "seeing map" and "reading map" tasks), legend symbol and map area type. Our results complement other studies that focused on the readability characteristics of items displayed on semi-transparent foreground layers developed in the context of "toolglass" interfaces. The implications of these results to the usability of transparency variable selection layers in geographical map applications are also discussed.