1. A method of calculating a navigation phase for a carrier in a navigation system involving terrain correlation, modeled by a system of equations in discrete time in accordance with a discrete-time relation of the type: { X t + 1 = F t X t + V t Y t = h t ( X t ) + W t , where the term X t , comprises at least the kinematic state of the system, the initial state X 0 containing at least the error of the initial inertial platform modeled as a centered Gaussian random variable with covariance P 0 ; the matrix F t of the linear evolution of the state contains at least the inertial platform model used; Y=(Y t ,t≧0) the observations vector is formed at least of the measurements of the terrain sensor; V t and W t are centered independent Gaussian white noise with respective positive definite covariances Q t and R t , said noise being mutually independent and independent of the initial state X 0 ; the term h t represents the combination of the model of the terrain sensors and of the onboard maps, the navigation system comprising at least: a navigation block comprising an inertial platform and at least one terrain sensor which are accompanied by their error models, at least one onboard map, the method calculating a navigability map determining for a plurality of points of interest of the onboard map, navigability scores, the calculation method taking into account an error model of said onboard map, the navigability score at a point X of the onboard map being a function of the terrain gradient, the terrain gradient being defined by the gradient of the function h=h terrain .