1. A method for determining a linear terrain profile along a lateral approach trajectory of an airport, comprising: a first set of steps, comprising measuring, on at least one aircraft, automatically and repetitively, during at least one flight of the aircraft along the lateral approach trajectory, during an approach to a landing runway of the airport: using at least one onboard radio altimeter, a height of the aircraft relative to the ground; using at least one onboard barometric altimeter, a barometric altitude; and using onboard sensors, a total air temperature and a Mach number of the aircraft; and storing the measured values, automatically, in at least one memory; and a second set of steps, implemented after the first set of steps and comprising, automatically, after the flight of the aircraft: repetitively, for each of a plurality of different distances relative to a threshold of the landing runway along the lateral approach trajectory: a) estimating, using the total air temperature, the Mach number and the barometric altitude, measured and stored in the memory, a variation of the static temperature dependent on the altitude and a temperature on the ground; b) computing a geometric altitude, using the barometric altitude measured and stored in the memory, as well as the variation of the static temperature and of the temperature on the ground, estimated in step a); c) computing a terrain height, by subtracting, from the geometric altitude computed in step b), the height measured by the radio altimeter and stored in the memory; and for the set of the plurality of different distances relative to the threshold of the landing runway along the lateral approach trajectory: d) determining at least one auxiliary terrain profile from the set of terrain heights computed for the set of different distances; e) determining a terrain profile using at least the auxiliary terrain profile, the terrain profile representing the trend of the terrain height as a function of the distance relative to the threshold of the landing runway, the terrain height being defined relative to a reference altitude corresponding to that of the threshold of the landing runway; and f) storing, in a database, the terrain profile determined in step e).