A head-mounted display device includes: a camera; an inertial sensor in a fixed or adjustably fixed spatial relationship with the camera; a display; and a processor. The processor is configured to derive a first position of a feature element in an image frame in an image data sequence using an image frame, and derive a second position of the feature element using a sensor data sequence and the spatial relationship. The processor is further configured to display, with the display, a display image containing: i) a first visual element at a first location corresponding to the first position, and ii) a second visual element at a second location corresponding to the second position. The processor is further configured to perform calibration when a received signal indicates that the camera and inertial sensor are not sufficiently calibrated.