UPDATED 28 Sept 2017: The altitude at which this image was taken has been revised to 23.3–26.2 m above the surface, corresponding to an image scale of about 2.5 mm/px. The uncertainty arises from the exact method of altitude calculation and the comet shape model used.
UPDATED 30 Sept 2016:
Rosetta's last image of Comet 67P/Churyumov-Gerasimenko, taken with the OSIRIS wide-angle camera shortly before impact, at an estimated altitude of about 20 m above the surface.
The initially reported 51 m was based on the predicted impact time. Now that this has been confirmed, and following additional information and timeline reconstruction, the estimated distance is now thought to be around 20 metres, and analysis is ongoing