PHOTOGRAMMETRY. 595 



the same way as the platelayers deal with it. Assume a radius 

 for the auxiliary curve and solve the transition curve for that 

 radius. To obtain P find angle 



, . shift 



from this find length G P. To find G, T H = sin. X (R — R^). 

 It will be seen that the smaller the radius of the auxiliary ciirve is 

 made the shorter the length of line to be altered. 



-0-I.JI-0 — 



7.— PHOTOGRAM.VIETRY. 

 B>i a HAS. HOPE HARRIS. 



••' Pliotogrammetry may be defined as the practice of plotting 

 the positions of variovis points in a landscape to scale upon a plan, 

 by intersection of angular lines taken from two or more stations 

 previously fixed with sufticient precision to serve as base lines for 

 the plan to be prepared, or which shall be fixed during progress of 

 the work by chainage of a single line. 



The principles which form the basis of this science belong to 

 trigonometry and perspective, and their application is simple 

 to anyone accustomed to plotting with a protractor or scale 

 of chords. The angles contained between various points visible 

 in a landscape may be obtained from a photograph of them as 

 accurately as required for the pur2)0ses of mapping, provided the 

 lens used is of suitable construction, and attention is paid to the 

 general rules hereafter laid down for securing satisfactory results. 



The photographic angles just referred to are always too large to 

 suit the spot at which the view is taken. There is a point for 

 which they are all true, though its situation is by no means evident 

 to the uninitiated, as it depends principally upon the focal length 

 of the lens employed. Out of doors it occurs between the camera 

 and the objects viewed, yet in the print lies from 4in. to lOin. 

 below the middle, upon a line dividing the picture equally left and 

 right. This state of things may be expressed by the following- 

 formula : — The tangent of the photographic angle, divided by the 

 focal distance between the lens and the plate, gives the tangent of 



the true angle ; or tan. A = — W — This ratio of the tangent ex- 

 plains how it is that the parts of a building near at hand seem to 

 be represented disproportionately larger by the lens than others 

 more remote. 



• The term " ["hotosiammetry" was an outcome of a convention held on thp Continent 

 a yeai- or two ago with reference to the subject. Effectual objections were made to every 

 name suggested for the practice excepting to this one, which was tacitly Bcknowledged. 



