A method for determining bias in an inertial measurement unit of an image acquisition device comprising: mapping at least one reference point within a reference image frame into a 3D spherical space based on a lens projection model for the image acquisition device to provide a respective anchor point in 3D space for each reference point; acquiring successive image frames; for at least one of said at least one reference points within a given acquired image frame: obtaining an estimate of frame-to-frame motion at said reference point between said given acquired frame and a previously acquired frame; obtaining from said inertial measurement unit a measure of device orientation for an acquisition time of said reference point in said given acquired frame and said previously acquired frame, said measure including a bias component; projecting a corresponding anchor point in 3D space according to a difference in said measure of device orientation in said given acquired frame and said previously acquired frame to provide a 3D vector V m; projecting a result of said estimated frame-to-frame motion for said point from said given acquired frame into said previously acquired frame into 3D space to provide a 3D vector V e; providing a cross product V c of said 3D vectors V m and V e, and obtaining an updated bias component value as a function of said cross product V c for a plurality of reference points in one or more acquired image frames. mapping at least one reference point within a reference image frame into a 3D spherical space based on a lens projection model for the image acquisition device to provide a respective anchor point in 3D space for each reference point; acquiring successive image frames; for at least one of said at least one reference points within a given acquired image frame: obtaining an estimate of frame-to-frame motion at said reference point between said given acquired frame and a previously acquired frame; obtaining from said inertial measurement unit a measure of device orientation for an acquisition time of said reference point in said given acquired frame and said previously acquired frame, said measure including a bias component; projecting a corresponding anchor point in 3D space according to a difference in said measure of device orientation in said given acquired frame and said previously acquired frame to provide a 3D vector V m; projecting a result of said estimated frame-to-frame motion for said point from said given acquired frame into said previously acquired frame into 3D space to provide a 3D vector V e; providing a cross product V c of said 3D vectors V m and V e, and obtaining an estimate of frame-to-frame motion at said reference point between said given acquired frame and a previously acquired frame; obtaining from said inertial measurement unit a measure of device orientation for an acquisition time of said reference point in said given acquired frame and said previously acquired frame, said measure including a bias component; projecting a corresponding anchor point in 3D space according to a difference in said measure of device orientation in said given acquired frame and said previously acquired frame to provide a 3D vector V m; projecting a result of said estimated frame-to-frame motion for said point from said given acquired frame into said previously acquired frame into 3D space to provide a 3D vector V e; providing a cross product V c of said 3D vectors V m and V e, and obtaining an updated bias component value as a function of said cross product V c for a plurality of reference points in one or more acquired image frames.