1. A method of evaluating the horizontal translation speed of a drone, in particular rotary wing drone capable of hovering flight under autopilot, the drone comprising: an altimeter suitable for measuring the altitude of the drone relative to the terrain over which the drone is flying; and a downwardly-looking vertically-oriented video camera suitable for picking up successive digital images of a scene of said terrain; the method comprising estimating the movement of the scene picked up by the camera from one image to the next, and applying to said estimated movement a scale factor that is a function of the measured altitude, estimation of the movement comprising: periodically and continuously updating a multiresolution representation of the pyramid of images type that models a given picked-up image of the scene at different, successively-decreasing resolutions; and for each new picked-up image, applying an optical flow type iterative algorithm to said multiresolution representation for estimating the differential movement of the scene from one image to the next; the method further comprising: obtaining, from the data produced by the optical-flow algorithm at least one texturing parameter representative of the level of microcontrasts in the picked-up scene and derived from the matrix: [ ∑ E ⁢ I x 2 ∑ E ⁢ I x ⁢ I y ∑ E ⁢ I x ⁢ I y ∑ E ⁢ I y 2 ] of the gradient components of the image, where I x and I y are the gradient components of the image and E is the set of points to which the gradient presents a norm greater than a predetermined first useful threshold; obtaining an approximation of the horizontal translation speed of the drone; applying a first battery of predetermined criteria to said texturing parameter(s) and to said speed approximation; and if said first battery of predetermined criteria is satisfied, switching from the optical-flow algorithm to an algorithm of the corner detector type in order to estimate the differential movement of the scene from one image to the next.