1. A method of predicting collisions with obstacles on the ground and generating warnings, said method implemented by an instrument installed in an aircraft and comprising the following steps: providing (a) at least one predicted path of the aircraft that represents an altitude of the aircraft and (b) an extraction zone; extracting, from an obstacle database, a list of pointlike obstacles, the list of pointlike obstacles comprising, for each of the pointlike obstacles, a horizontal distance separating the pointlike obstacle from a current position of the aircraft, horizontal accuracy and a height of the pointlike obstacle; extracting, from the obstacle database, a list of linear obstacles, the list of linear obstacles comprising, for each of the linear obstacles, a list of pointlike obstacles corresponding to each end of the linear obstacle; determining, according to the altitude of the aircraft, risks associated with the extracted pointlike obstacles and generating warnings; and determining, according to the altitude of the aircraft, risks associated with the extracted linear obstacles and generating warnings, wherein said step of determining the risks associated with the extracted linear obstacles and generating the warnings includes the following steps of: processing the ends (E 1 , E 2 ) of the linear obstacle by the step of determining the risks associated with the extracted pointlike obstacles and generating the warnings; calculating (a) a point P of the predicted path of the aircraft if no warning is generated in the step of the processing, the altitude of the point P being less than that of the other points of the predicted path of the aircraft, and (b) a distance d(P) between the position of the aircraft and the point P; calculating a distance d(E 1 ) between the position of the aircraft and the point P whose coordinates are those of one of the ends (E 1 ) of the linear obstacle; calculating a distance d(E 2 ) between the position of the aircraft and the point P whose coordinates are those of another of the ends (E 2 ) of the linear obstacle; determining that the distance d(P) belongs to the range [d(E 1 ),d(E 2 )]: when the distance d(P) is included in the range [d(E 1 ),d(E 2 )], the method goes on to comparing the altitude of the point and the altitude of the linear obstacle, and then calculating, based on the comparing, a warning level; when the distance d(P) is not included in the range [d(E 1 ),d(E 2 )], the method is resumed at calculating a point Δ corresponding to the point of an intersection between a segment defined by said two ends (E 1 , E 2 ) of the linear obstacle and a straight line which passes through the position of the aircraft and is perpendicular to the segment defined by said two ends (E 1 , E 2 ) of the linear obstacle; and verifying (i) that the point Δ belongs to the segment defined by said two ends (E 1 , E 2 ) of the pointlike obstacle and (ii) that the distance d(P) belongs to the range [d(Δ);d(E 1 )], d(Δ) representing the distance between the position of the aircraft and the point.