A system for generating a three-dimensional geometric model of a structure, the system including a portable sensing device that can be transported through the structure, wherein the sensing device includes one or more sensors that generate sensor data at least partially indicative of a position of the sensing device relative to at least part of the structure and at least one electronic processing device that determines a captured map of at least part of the structure using the sensor data, determines at least part of a path traversed by the sensing device using the sensor data, identifies one or more model features using the part of the path and the captured map and generates a three-dimensional geometric model of the structure using the model features.