August 9, 1895.] 



SCIENCE. 



153 



The true dimensions of objects, repre- 

 sented in perspective view upon a plane 

 surface, are not obtainable by direct meas- 

 urements with any one scale, but thej^ can 

 be determined, geographically, if the dis- 

 tance line (focal leng-th), the horizon and the 

 principal point are knoton. 



Iconometry, photogrammetry or metro- 

 photography are terms applied to that art 

 which ascertains, graphically, the true di- 

 mensions of objects from their perspectively 

 correct (photographic) representations (pro- 

 duced by means of a so-called photogram- 

 meter) . 



By applying the inverse rules of perspec- 

 tive drawing and aided by a knowledge of 

 descriptive geometry, the horizontal projec- 

 tions of the terrene, obtained by means of 

 a photogrammeter, can be plotted in pre- 

 cisely the same manner as if the measure- 

 ments were made in the field, instead of 

 being obtained from such photographs. 



The fundamental principles underlying 

 this particular branch of photogrammetry — 

 'phototopography ' — are, of cou^rse, the same 

 as in all other methods of topographical 

 surveying, inasmuch as all require the de- 

 termination of lengths of lines and angular 

 measurements of the deflections of such 

 lines from a given direction. 



Generally speaking, phototopography fol- 

 lows the same lines of procedure as the plane 

 table methods, except that with the latter the 

 control underlying the map is plotted and 

 the chart is drawn in the field, the terrene 

 of the mapped area being before the eyes of 

 the plane-tabler, whereas the iconometrical 

 draughtsman has only the perspective views 

 of the terrene, as seen from known stations, 

 from which he must while in his ofiice 

 gather all the data necessary to construct 

 the same map. Still, the latter has the ad- 

 vantage that he can, at any time, refer back 

 to the terrene (pictorially at his command) 

 surrounding any station, while the plane- 

 tabler rarely occupies the same station twice. 



notwithstanding such references are very 

 essential aids towards giving the topog- 

 raphy a correct interpretation with respect 

 to forms. 



In both cases the chart is constructed by 

 means of visual rays or lines of direction, 

 drawn from different stations towards the 

 same point, the cartographic position of 

 which is found by locating the point of in- 

 tersection of such lines, observed from dif- 

 ferent stations. 



With the plane-table such intersections 

 are made (or plotted) by bisecting the ob- 

 jects or signals with the (telescopic) alidade, 

 and actualljr drawing the lines of direction 

 observed from different stations to the same 

 point, upon the plane-table sheet, thus plot- 

 ting the horizontal angles (graphically) 

 without knowing their values in arc. 



In phototopograj)hy the lines of direc- 

 tion (in both the vertical and horizontal 

 sense) to the various points, identified on 

 different panorama views and selected for 

 plotting, are found by transposing linear 

 measurements taken (as rectangular co- 

 ordinates) from the negatives or prints, 

 which, together with the camera constants, 

 will enable the iconometrical draughtsman 

 to graphically locate the lines of direction 

 and plot the points (selected on the pic- 

 tures) bj^ intersections of the same. 



Such being the case, it is evident that the 

 smallest length measurable with eye and 

 scale will represent the limit within which 

 the points of the map can be correctly laid 

 down, and it becomes advisable to use a 

 large scale for good photogranxmetrical 

 plotting. 



The work of drawing in the horizontal 

 contours, after a sufiicient number of points 

 have been located on the chart, hypsomet- 

 rically and geographically, is done in the 

 same manner (by graphical interpolation) 

 as in the field, when using the plane-table, 

 except that in iconometrical plotting fre- 

 quent reference is taken to the pictures of 



