120 
KIKTLEY F. MATHER 
enough into the paper to make a permanent indentation, being 
careful to hold the needle vertically. 
Accuracy of the stadia method. The telescopic alidade and 
stadia rod are to be looked upon as instruments of precision; 
distances are not estimated, but accurately determined in well- 
conducted stadia work. Although essentially intended to secure 
rapidity rather than accuracy, the stadia method employed with 
due care to eliminate the chief sources of error is capable of 
attaining a high degree of accuracy. 
Perhaps some of the most interesting results obtained with stadia? 
as showing its precision, were those obtained by Mr. J. L. Van Ornum 
in taking topography on the international survey of the Mexican 
Boundary. The whole of the boundary line was .measured with the 
stadia, and a large portion of it by the chain, and always tied in by a 
system of accurate primary triangulation. Corresponding distances 
were found by stadia and chain and compared with the known dis- 
tances as obtained by triangulation, with the following results: 
Of five different stretches measured by the three methods, the total 
distance shown by triangulation was 99,110 meters, by stadia 99,025 
meters, by corrected chain 99,041 meters Other sections of 
the line were measured by stadia and triangulation, but not by chain. 
In all there were measured 182.5 miles by stadia w;hich were triangu- 
lated and in which the total difference in length was plus 50 meters, or 
1 in 5837. It tnay be noted that the chained distance was marked 
corrected chain, because in six measurements of the chained distance, 
dropping or omission of chain-lengths occurred which were detected 
in every instance by the stadia. 
To determine differences in elevation 
Methods of determining differences in elevation by means of 
the telescopic alidade are even more numerous than those in 
vogue for measuring distances. The good instrument man will 
know several different methods and select the one best suited to 
the particular sight, depending upon the accuracy required, the 
inclination of the sight, the equipment of the instrument, and 
H. M. Wilson, Topographic Surveying. John Wiley and Sons, New York, 
1910, pp. 241-2. 
