1. A method for estimating in a mobile device, the method comprising: determining a first pose, from a first image captured at a first time, between a target and a first position of a camera of the mobile device, wherein the first image contains the target; determining a second pose, from a second image captured at a second time, between the target and a second position of the camera, wherein the second image contains the target; computing an image-based pose between the first pose and the second pose using a first estimation of a scaling factor; receiving measurements from an accelerometer of the mobile device from the first time to the second time; forming an inertia-based pose based on the measurements from the accelerometer and a first estimation for a gravity vector; computing a difference between the image-based pose and the inertia-based pose; and forming at least one of a second estimation of the gravity vector or a second estimation of the scaling factor based on the difference.