A device for compressing image data, the device comprising a controller configured to: define a region of interest in a reference coordinate system, wherein an origin of the reference coordinate system is located in the optical centre of a camera, so that the reference coordinate system is concentric with the optical centre of the camera and is fixed with respect to geographic compass points and with respect to the geographic horizontal plane; receive image data comprising images recorded by the camera; receive attitude data indicating the attitude of the camera when each of the images was recorded, wherein the attitude data indicates, for each image, one or more of the pitch, roll and yaw of the camera with respect to the compass points and to the geographic horizontal plane; in each of the images, define a section of the image within the region of interest and a section of the image outside the region of interest by transforming the boundary of the region of interest defined in the reference coordinate system into a boundary of the region of interest within the respective image based on the attitude data for the image, wherein the boundary of the region of interest is defined in the reference coordinate system on a two dimensional surface of a sphere whose centre is located at the origin of the reference coordinate system, and the boundary of the region of interest is defined using a series of points located on the surface of the sphere; and compress each of the images, wherein the compression ratio applied to the section of the image outside the region of interest is higher than that applied to the section of the image within the region of interest. define a region of interest in a reference coordinate system, wherein an origin of the reference coordinate system is located in the optical centre of a camera, so that the reference coordinate system is concentric with the optical centre of the camera and is fixed with respect to geographic compass points and with respect to the geographic horizontal plane; receive image data comprising images recorded by the camera; receive attitude data indicating the attitude of the camera when each of the images was recorded, wherein the attitude data indicates, for each image, one or more of the pitch, roll and yaw of the camera with respect to the compass points and to the geographic horizontal plane; in each of the images, define a section of the image within the region of interest and a section of the image outside the region of interest by transforming the boundary of the region of interest defined in the reference coordinate system into a boundary of the region of interest within the respective image based on the attitude data for the image, wherein the boundary of the region of interest is defined in the reference coordinate system on a two dimensional surface of a sphere whose centre is located at the origin of the reference coordinate system, and the boundary of the region of interest is defined using a series of points located on the surface of the sphere; and compress each of the images, wherein the compression ratio applied to the section of the image outside the region of interest is higher than that applied to the section of the image within the region of interest.