Method for overlaying an image arising from a sensor on a synthetic image by automatic detection of the limit of visibility and associated viewing system

An on-board viewing system for aircraft comprising at least one image sensor of the outside landscape, a cartographic database, a graphics computer arranged so as to generate a synthetic image of the same outside landscape, a viewing screen displaying a first image arising from the image sensor overlaid on a second image representing the synthetic image, wherein the viewing system comprises:

• means for image analysis, at a temporal frequency, of a first series of first images arising from the image sensor so as to determine, inside each image, a boundary between the information useful for piloting and the information not useful for piloting, the useful information consisting of portions of terrain or of obstacles or of elements of interest that are noteworthy, the said temporal frequency being an order of magnitude lower than the video refresh frequency of the images displayed on the viewing screen, these first images being denoted analysed images;
• first means for computing a transition zone in the vicinity of each boundary of a determined width;
• the graphics computer displaying, at the video refresh frequency, a second series of first images between two successive analysed images, each first image being displayed overlaid on a second image representing a synthetic image of the same outside landscape, the first image being opaque under the transition zone, totally transparent above the transition zone and of continuously variable transparency varying between opacity and total transparency in the transition zone.