April 14, 1898 J 



NA TURE 



56: 



PHOTOGRAPHIC SURVEYING. 



"piFTY years ago or more, M. Beautemps Beaupre 

 J- successfully employed a process which greatly 

 facilitated the work of surveying, and which in its modern 

 developments is likely to supersede the tedious work of 

 measurement in the field. Where the greatest accuracy was 

 not required, the method recommended itself on account 

 of its great practical utility, enabling contoured maps to 

 be produced without the labour of heavy calculations. 

 M. Beaupre availed himself of the principle of the camera 

 lucida, and by its aid sketched the panorama about him 

 from two ends of a measured base line. In a paper 

 recently communicated by Prof. E. J. Mills to the Institu- 

 tion of Engineers and Shipbuilders in Scotland, it is 

 contended that the best work is still done when this 

 principle is utilised. From two views taken at a known 

 distance from each other, however procured, one is able 

 to plot a map with a fair amount of accuracy, and knowing 

 the correct relative distances of the objects their vertical 

 height can be deduced. No doubt there were diffi- 

 culties in the application of the method. Any one who 

 has attempted to draw a picture of a distant object by 

 means of Wollaston's invention knows that the result is 

 apt to be a little disappointing, though successive improve- 

 ments in the mechanism have removed many of the effects 

 arising from parallax, which interfered with correct deline- 

 ation in the early days when the instrument was first used. 

 But the process still remains long and tedious, and it was 

 inevitable that the introduction of photography with its 

 rapid and accurate results, should be welcomed as likely 

 to banish the slow methods of hand drawing in the initial 

 stages of the work. 



To obtain a photograph which should be free from 

 optical distortion, and to which the laws of geometrical 

 perspective could be applied without any correction, has 

 not been an easy task. But now it may be said that we 

 do possess lenses which will cover an angular field of 

 about bd" without measurable distortion, and give uniform 

 definition all over the plate. Enlargements and printing 

 from the original negatives doubtless still present some 

 difficulties. Prof Mills, we notice, recomm.ends that the 

 prints should be made on bromide paper, and developed 

 with amidol. Shrinkage and distortion of the paper will, 

 it is asserted, be prevented, when soaked in a two per cent, 

 solution of formalin, and dried at a gentle heat. Other 

 authorities, however, distrust paper altogether, and prefer 

 to use a bromide emulsion on opaque or translucent plates 

 of flat glass as likely to give less error. But the posses- 

 sion of accurate optical arrangements, combined with 

 exact manuipulation of the photographic result, suggests 

 many new applications to which the camera can be 

 profitably applied, and the surveyor now finds himself 

 equipped with an instrument of scientific precision, in 

 which are combined the main features of a theodolite 

 and a level, and which replaces the plane table and its 

 accessories. 



In the earliest form of surveying camera orphotogram- 

 meter, to give it a polysyllabic title, the instrument con- 

 sisted of little more than an ordinary bellows camera, set 

 on a horizontal circle, and moving about a vertical axis. 

 The distance between the plate and the lens was fixed, 

 and the camera could be levelled by means of screws in 

 the head of a tripod in much the same way that a theo- 

 dolite is adjusted. In the subsequent development and 

 increased effectiveness that have been added to this 

 surveying instrument, two names stand out prominently, 

 that of Colonel Laussedat, the present Directeur of the 

 Conservatoire des Arts et Metiers, Paris ; and in this 

 country that of Mr. Bridges Lee. In the French form, 

 the camera is placed on the top of a stand carrying a 

 transit theodolite, and the disposition of the several parts 

 is arranged to ensure stability and symmetry about a 

 vertical axis, while each part of the instrument can be 



NO. 1485, VOL. 57] 



used separately. In the English form, Mr. Bridges Lee 

 has wisely determined that while each photograph should 

 offer a correct perspective view of the country it repre- 

 sents, it should also carry on its face the information 

 necessary for the correct interpretation of the picture, 

 and the subsequent construction of a map. When 

 passing through countries where roads are scarce and 

 travelling difficult, notebooks are apt to be lost ; but 

 without more words, every one must appreciate the 

 arrangement by which the "constants of reduction" 

 are made as permanent as the picture to which they 

 refer. 



The general principles on which the Bridges Lee 

 camera has been constructed, and the objects sought to 

 be attained, have already been briefly explained in 

 these columns (vol. li. p. 191). Its present form has 

 been adopted because experiment has proved the 

 necessity of great nicety of mechanical adjustment to 

 obtain the best results. To ensure the necessary 

 lightness and rigidity, the camera box is made of 

 aluminium. The direction in which the instrument is 

 pointed can be read off an azimuthal circle, graduated to 

 minutes, on which the camera turns. A level on the top 

 of the box ensures the horizontality of the instrument, 

 and when this adjustment has been made, a telescope, 

 also securely attached to the top of the camera, will move 

 in a vertical plane through a sufficient range of angle for 

 all ordinary terrestrial work. The angle through which 

 the telescope can be moved in altitude is also read to 

 minutes. This supplies the observer with a theodolite, 

 and its position is so arranged with reference to the other 

 parts of the instrument, that the line of collimation and 

 the vertical wire of the theodolite are in the same plane 

 with that which bisects the photographic lens. In this 

 same vertical plane, a " wire" is fixed to the frame of the 

 camera, cutting the optic axis of the lens at right angles, 

 and consequently marks on the photograph the median 

 vertical plane of the instrument. Another wire, also 

 through the optic axis at right angles to this, will mark the 

 horizon of the instrument on the picture, and the inter- 

 section of the two wires will give the " principal point " of 

 the perspective. Inside the camera is placed a magnetic 

 compass, and the scale being transparent, it is printed on 

 the picture. The axis of rotation of this compass is in the 

 same plane as the axis of collimation of the theodolite, 

 and of the vertical wire. The distance between the 

 scale of the compass and the vertical wire is kept 

 constant by a device which works automatically when the 

 camera is in use, and since this distance very slightly 

 exceeds the radius of the compass, the wire can be used 

 as a pointer to read the scale. One very important 

 addition to the usefulness of the instrument is the 

 insertion of a scale of angular distance, photographically 

 prepared by the same lens as that fitted in the 

 instrument when complete for surveying purposes. The 

 scale is so attached to the frame, that it is photographed 

 on every picture taken, and by its aid one can easily read 

 off the angular distances of any point in the picture 

 right or left of the median vertical plane. 



A method of surveying in which the necessary observ- 

 ations are easily and rapidly collected, or are implicitly 

 contained in a series of photographic views, has the 

 promise of a large future. In travelling over unsettled 

 districts, where it is impossible to remain for any length 

 of time on a particular spot, the photographic method 

 seems likely to supersede all others. Indeed no other 

 method seems possible. Moreover, a photograph contains 

 an amount of detailed information concerning the country 

 photographed, which it is quite impossible to gather 

 from notes of observations and sketches, although much 

 time may be spent in making these additions as carefully 

 as possible. In this connection, one might call attention 

 to the beautiful mountain maps which have been 

 prepared for the German and Austrian Alpine Club 



