After development and drying, and before filing or printing, each plate should be marked with data for purposes of future identification. This is most easily done with pen and ink on the film side (in reversed lettering) either along an edge in the unexposed portion covered by the sheath or in a corner, so as to lose as little of the photograph as possible. Just what data shall be inscribed is dictated by the purpose for which the negative was made. The date, altitude, time of day, true north (from known permanent features or from shadow direction and time of day), number of the camera used, the focal length of the lens. Other records, such as the plane and squadron numbers, or even the pilot's and observer's initials, may be called for (Fig. 75). For mapping work the scale of each of a set of negatives, once found, may be marked, either in figures or by means of a line of length corresponding to a fixed distance on the ground. Rectifying data can similarly be inscribed, so that the negative can be printed in the enlarging and rectifying camera with the minimum of delay.