1. A method for geolocating an image sensor having a line of sight and installed on board an aircraft, characterized in that the geographical position of the sensor and the orientation of its a line of sight being approximate, it comprises: a step of creating at least one opportune landmark comprising the following substeps: an operator locating, on a screen for displaying acquired images, an observable stationary element, on the ground the axis of a telemeter being indicated in these images by means of a “reticle” type overlay the direction of which in the scene space represents the line of sight of the image sensor; the operator moving the line of sight in order to place the reticle marking the axis of the telemeter on this stationary observable element; tracking of this stationary element by tracking means; estimating the geographical position of this stationary element from image capture conditions delivered by a device able to determine them, this position also being approximate; on the basis of the approximate position of this tracked stationary element, searching in a terrain database for the location corresponding to a zone centered on the approximately positioned stationary element, this zone being of preset size; displaying an image of the terrain of this location, said image being constructed from the terrain database, the operator locating the observable element in this image; and the operator pointing to this observable element in the displayed terrain image, the geographical coordinates pointed to being retrieved from the terrain database, this observable stationary element that the line of sight is tracking thus becoming an opportune landmark; and the sensor moving relative to the opportune landmark, a step of determining the accurate location of the sensor, from the geographical coordinates of this tracked opportune landmark, using a Kalman filter supplied with a plurality of measurements of the distance between the sensor and the opportune landmark and with a plurality of measurements of the orientation of the line of sight of the sensor toward the opportune landmark, there being one orientation measurement for each telemetry measurement, simultaneously allowing the orientation of the line of sight to be accurately estimated.