1. A method in a visual simultaneous localization and mapping (VSLAM) system of fusing sensor measurements for localization of a mobile device, the method comprising: using a probability density function p(x) represented by a plurality of particles to estimate a value for a pose of the mobile device, where the pose includes both location and orientation; observing visual features that are associated with a landmark pose; receiving an indication of whether dead reckoning data corresponding to a dead reckoning measurement is trustworthy, where the dead reckoning measurement corresponds to an incremental change in pose from a prior pose; receiving an indication of whether visual data for a visual measurement is trustworthy, where the visual measurement is associated with the landmark pose; if the dead reckoning data and the visual data are trustworthy, then performing: apportioning a particle in the plurality of particles to one of at least a first subset and a second subset of the plurality of particles, where the first subset and the second subset are mutually exclusive; performing for a particle associated with the first subset: computing a predicted robot pose by combining the dead reckoning measurement and the prior pose for the particle; perturbing the predicted robot pose to generate a perturbed predicted pose; and using the perturbed predicted robot pose as the estimate for the pose associated with the particle; performing for a particle associated with the second subset: retrieving the associated landmark pose; using the associated landmark pose and the visual measurement to generate an estimated visual pose; perturbing the estimated visual pose to generated a perturbed estimated visual pose; and using the perturbed estimated visual pose as the estimate for the value of the pose of the mobile device for the particle; and, if one of the dead reckoning measurement and the visual measurement is not trustworthy, then performing: estimating a first pose by using a selected one of the dead reckoning measurement and the visual measurement, where the selected one is trustworthy; perturbing the first pose to generate a perturbed pose; and using the perturbed pose as the estimate for the value of the pose of the mobile device for the particle.