1. A vision based navigation system, the system comprising: an inertial measurement unit configured to provide inertial measurements for a vehicle; at least one image sensor configured to acquire measurements of the environment of the vehicle; one or more processing units configured to: calculate a navigation solution for the vehicle based on the inertial measurements; identify a plurality of landmarks in the acquired measurements; identify a plurality of usable landmarks from the plurality of landmarks; select a subset of useable landmarks from the plurality of useable landmarks such that the subset of landmarks has a smaller dilution of precision than other possible subsets of landmarks from the plurality of useable landmarks, wherein the dilution of precision is an amplification factor of measurement errors derived from the geometry of the subset of useable landmarks; and calculate an updated navigation solution based on the subset of landmarks.