PHOTOGRAPHIC SURVEYING. 
131 
pictures from any number of stations, and intermediate details between 
the points plotted bv intersection can be sketched in from inspection of 
the pictures, the accuracy of the sketching being tested from* time to time 
by intersection tests by aid of the stretched hairs from the stations. 
Contours . 
For plotting contours, advantage is taken of the fact that all points on 
the horizon line of any picture are at the same level as the camera at the 
station, so that if a number of points on the horizon line of a picture are 
plotted on the plan by the method of intersections before described, it is 
only necessary to join those points to obtain a correct contour line. In 
this way a number of contour lines corresponding to the different 
altitudes of different stations can be easily and rapidly laid down on the 
plan. Intermediate contours can be sketched in. 
Sometimes it is desirable to ascertain the altitudes of particular points 
visible in a survey picture. This can always be done when the horizontal 
distances of the points from the station are known. The altitude of any 
point bisected by the principal plane of the picture can be obtained, at 
once from the formula h = cl tan a, d being distance in feet, a the angle 
subtended at the station, and h the height in feet, a can be ascertained 
at once by measuring the distance along the tangent scale equal to the 
distance of the point on the picture above or below the horizon line. If 
the point whose altitude is required occupies any position on the picture 
not bisected either by the principal vertical or by the horizon plane its 
altitude can be determined from the same formula, only to ascertain the 
value of tan a it is necessary to substitute values in the formula tan 2 a - 
— d .— where x and y are distances measured along the horizontal and 
/ 2 + x 2 
vertical lines respectively to the bases of perpendiculars let fall from the 
point upon those lines, and / the focal distance. 
Conclusion . 
There are other methods, also, which can be used to assist in the 
preparation of the plan and for plotting in contours, but the amount of 
space available does not permit of a description here of those other 
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