FORUM• VOL. VII being affected by aspect, and by distance in so far as detail is concerned, it is also dependent upon the season, the weather, and the time of day. A coastline in winter, covered with snow, on a dull overcast day or with long shadows cast by a low sun, will be very different in everything save silhouette when seen on a bright summer's day. Such drawbacks have not, however, deterred publication of such views in the Pilots, even when there is little choice of quality.As to the position from which each radar photograph is taken, this will almost settle itself. For pictures intended as an aid to landfall, the range should perhaps be that at which just sufficient land will paint on a normal set under normal conditions for identification to be positive. If super-refraction conditions exist when the landfall is being made, so much the better for identification; while sub-refraction of any material consequence is unlikely. The bearing of the position may be stated as well as the range-thus conforming to the practice established on so many of the existing views.The writer has drawn the accuracy contour maps of a ship's position fixed by cross bearing, horizontal sextant angle method and Loran aid. The accuracy of the fixes has been evaluated in terms of the probability density.In Fig. i, Pis the ship's position fixed from cross bearings of the objects A and B, and d is the angle between two bearings, d i and d 2 are the distances of the objects. The displacement probable error in each of the lines of bearing at P is r = d sin AZ (i) where AZ is the probable error in observed bearing.
t-\,o)(4,0) ---X 0 Fig. 1.The probability density at P is given by p = (I/27TCT 1 O-2 ) sin 6 = (0-0724/1-^2) sin 9( 2) where c lt a 2 are the displacement standard errors in each line of bearing at P. From equations (1) and (2) where k = o-oj24.jsin 2 AZ.(3)
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