Journal . 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 undulatin
Journal . 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 Fig. —The photographs of Figs. 5 and 6 are the pho-tographs obtained at our first and second camera stations respec-tively. The optical centres have been obtained in the usual wayby means of fiducial marks imposed upon the negatives at themoment of exposure. The horizon points have been foundby a point in the distant landscape too feebly marked to bevisible in the reproduced photographs, but perfectly visiblein the negatives. The rule for determining the dip angle of theoptical axis below the horizontal plane from these two points issimple, and may be taken to be known.* The instructions to thephotographer were to give photograph Fig. 5 a dip angle ($)of 5°, and photograph Fig. 6 a dip angle of 0°. The photographs,when measured up, show dip angles of = 4° : 25 and = 0respectively. The camera had a focal length of 7-196 ins., * See p. 22 of Generalised Linear Perspective. 214 RAILWAY SURVEYING BY PHOTOGRAPHY. so that the parameters of the two photographs were respectivel
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Keywords: ., bookcentury1800, bookdecade186, bookpublisherlondon, bookyear1861
