A method for determining a corrected axial displacement parameter of a conduit inspection tool having an imaging device comprising one or more sideview cameras or a downview camera, said displacement parameter being a velocity or a displacement distance; the method comprising: obtaining, using the one or more sideview cameras or the downview camera, a video stream comprising successive axially overlapping images (201, 202) of an internal wall of a conduit during transit of the tool axially along the conduit and captured at intervals determined by a frame rate of the video stream; determining, from the images (201, 202), an observed axial displacement parameter of the tool as a function of transit time by determining an axial shift in pixels between the images; identifying, in the images, a plurality of reference features (24, 28) of fixed position in the conduit; identifying a corresponding plurality of reference transit times at which said reference features (24, 28) appear; determining an estimated axial displacement distance of the tool over each of a plurality of transit time intervals between successive reference transit times; and computing the corrected axial displacement parameter of the tool for each of the plurality of transit time intervals by applying a correction factor to the observed axial displacement parameter; wherein the correction factors are computed by applying the condition that, within each transit time interval, a total axial displacement distance of the tool determined from the corrected axial displacement parameter is equal to the estimated axial displacement distance; and wherein, when the displacement parameter is a velocity, the total axial displacement distance of the tool within each transit time interval is determined by integrating the corrected axial velocity with respect to time over the transit time interval; and when the displacement parameter is a displacement distance, the total axial displacement distance of the tool within each transit time interval is determined as the difference in corrected axial displacement distance over the respective transit time interval. obtaining, using the one or more sideview cameras or the downview camera, a video stream comprising successive axially overlapping images (201, 202) of an internal wall of a conduit during transit of the tool axially along the conduit and captured at intervals determined by a frame rate of the video stream; determining, from the images (201, 202), an observed axial displacement parameter of the tool as a function of transit time by determining an axial shift in pixels between the images; identifying, in the images, a plurality of reference features (24, 28) of fixed position in the conduit; identifying a corresponding plurality of reference transit times at which said reference features (24, 28) appear; determining an estimated axial displacement distance of the tool over each of a plurality of transit time intervals between successive reference transit times; and computing the corrected axial displacement parameter of the tool for each of the plurality of transit time intervals by applying a correction factor to the observed axial displacement parameter; wherein the correction factors are computed by applying the condition that, within each transit time interval, a total axial displacement distance of the tool determined from the corrected axial displacement parameter is equal to the estimated axial displacement distance; and wherein, when the displacement parameter is a velocity, the total axial displacement distance of the tool within each transit time interval is determined by integrating the corrected axial velocity with respect to time over the transit time interval; and when the displacement parameter is a displacement distance, the total axial displacement distance of the tool within each transit time interval is determined as the difference in corrected axial displacement distance over the respective transit time interval.