178 THE PHOTOGRAPHIC WORK OF GEOLOGICAL SURVEYS. 
ecnstructed from photographic survey of broken and difficult 
parts of the Alps, which were executed by Paganini in 1879- 
85, so clearly proved the efficiency and economy of the method 
that the new survey of the whole country is now being made by 
the aid of photography. 
The work done in Italy is universally admitted to be of the 
highest degree of perfection attainable. 
The difficult photo-survey of portions of the country, includ- 
ing elevations of over 15,000 ft., such as Mount Rosa, has been 
accomplished also. 
At the Vienna Exhibition of charts and maps, which was 
held in 1891, under the auspices of the nineteenth Congress of 
German Geographers, an Italian topographic map of a scale of 
1/50,000, which had been made in 1889 from photo-surveys, 
gained first place. 
These are sound proofs of the value of photo-topography. 
In Canada the results have been still more remarkable ; 
photo-surveying has been conducted over vast areas by the topo- 
eraphers of the Dominion under the direction of the Surveyor- 
General, Capt. E. Deville. 
The regions dealt with include the Rocky Mountains along 
the Canadian-Pacific Railway, and they extend for many hun- 
dreds of miles, comprising altitudes exceeding 10,000 ft. From 
these surveys a topographic chart was constructed to a scale of 
1/40,000, with contour lines at vertical intervals of 100 ft. The 
chart is in fifteen sections, and it represents a zone 20 miles 
on each side of the railway, and 1500 miles in leneth. The 
surveys were accomplished in three years by the engineer, Mr. 
M‘Arthur, with the assistance of a topographer and two labourers, 
and Capt. Deville states that the cost of the field work and the 
preparation of the map did not exceed $2.84 per square mile. 
This party can actually complete photo-survey work at the 
rate of 500 square miles per annum. 
The chart just mentioned was exhibited at the Columbian 
World’s Fair, and the instrument used will be described under 
the head of “ Instruments.” 
It is stated that the degree of accuracy attained in mapping 
from data derived from these photographic perspectives is 
equal to that of the ordinary plotting by protractor and scale, 
or of a map executed with the plane table. With regard to the 
rapidity with which these photo-surveys are accomplished, Cap- 
tain Javary, in France, made in the year 1874 in one day a 
survey extending 14 miles in length, and 1 mile on each side 
of the route. 
Lieut. Reed, in the United States, completed in ten hours all 
the field work for the accurate mapping of an area of 27 square 
miles with the necessary levels for hypsometric contouring, 
taking four or five views at each of three principal and two 
auxiliary stations. 
