Journal . urse, itprevented the further circulation of plates in the trifling accident, therefore, brought our photographicoperations to an end. In future we shall use dark slides andnot a magazine for exposing our plates. The second point to be noticed is the enormous scale of aphotograph taken near the ground, and the necessity of takingthat into account in making a plan from it. The scale of thephotograph is proportioned simply to the altitude of the camerastation, for the variation of scale due to perspective is allowedfor in the plotting operation. Now a plan to a scale of 1
Journal . urse, itprevented the further circulation of plates in the trifling accident, therefore, brought our photographicoperations to an end. In future we shall use dark slides andnot a magazine for exposing our plates. The second point to be noticed is the enormous scale of aphotograph taken near the ground, and the necessity of takingthat into account in making a plan from it. The scale of thephotograph is proportioned simply to the altitude of the camerastation, for the variation of scale due to perspective is allowedfor in the plotting operation. Now a plan to a scale of 1/2500(26 ins. to the mile) is equal to that of a photograph taken2500 F. above the photographed area. The camera which wewere using has a focal length (F.) of 0-6 ft. To produce a photo-graph by its means on the scale of the large scale ordnance mapthe camera would have to be raised 1,500 ft. above the level ofthe photographed ground. If we reduce that height to 50 ft. 212 RAILWAY SURVEYING BY RAILWAY SURVEYING BY PHOTOGRAPHY. 213 we get a map to a scale of 75 ins. or upwards of 6 ft. to the mile,a scale suitable only for dealing with very small areas. Forrailway surveying something very different is required. Themodification of the scale presents no particular difficultv, butif the scale of the photograph is large the area representedin any one photograph will be reciprocally small. The number ofphotographs to be produced and dealt with is proportionatelygreat. This is the only inherent difficulty in land surveving byphotography. It disappears in hilly or deeply undulatingcountry. It has to be surmounted on level country by the useof a portable tower or equivalent device.
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Keywords: ., bookcentury1800, bookdecade186, bookpublisherlondon, bookyear1861
