Activity Change Index (ACI) – new aspects of indoor mobility identified from GPS and lifelogging data Sila-NowickaKatarzyna ThakuriahPiyashimita 2019 Global Positioning System (GPS) technology has changed the world. We now rely on it to navigate our vehicles, source real-time information about chosen locations and to track our movements. GPS is, however, principally limited to receiving signals outdoors and therefore not able to provide reliable information about indoor movements. This paper examines patterns of indoor physical activity and sedentary behaviour using a combination of lifelogging and GPS movement data to develop an index for capturing activity changes. In our preliminary analysis, we explore possible associations between people’s indoor physical activity levels and their socio-demographic characteristics.