1. A computer-implemented method comprising: using a computer processor to assign observations comprising: receiving first observations of a first sensor system; receiving second observations of a second sensor system; assigning a set of pairs of the first and second observations predicted to correspond to the same physical position, the assigning comprising: using a likelihood function that specifies a likelihood for each assigned pair; and determining the set of assigned pairs for the first and second observations based on the likelihood function, wherein the likelihood function uses a gate value determined from estimating a true volume using nearest neighbor distances determined from the first and second observations.