1. A method of computing a relative pose for autonomous localization for a mobile device, the method comprising: identifying matching features of a stored landmark and of an image, where the image is provided by a visual sensor coupled to the mobile device; determining 2-dimensional coordinates within the image for the matching features of the image; retrieving 3-dimensional coordinates of the matching features of the stored landmark; computing a hypothetical device pose by: projecting the 3-dimensional coordinates of the matching features of the stored landmark onto new 2-dimensional coordinates of a hypothetical image, where the hypothetical image corresponds to an image that would be observed by the visual sensor if the device were to be re-posed according to the hypothetical device pose; generating a projection error by comparing the new 2-dimensional coordinates to the 2-dimensional coordinates for the matching features of the image; and solving for the hypothetical device pose that corresponds to a low projection error; and using the hypothetical device pose as the computed relative device-pose.