554 
Smith—Stadia Measurements. 
triangulation or steel tape checks on the stadia measurement, 
each check may be used as a determination of interval, and the 
average of a number of such determinations would give the very 
best value for the interval, without the expenditure of an extra 
dollar or an extra hour of time. 
Madison. Wis. 
APPENDIX. 
THEORY OF STADIA MEASUREMENTS. 
An explanation 1 of the theory of Stadia measurements is here given, 
that the general reader may better understand this paper. 
The stadia 2 is an instrument for determining the distance of a point 
from the observer by the visual angle subtended by an object of known 
size, placed at the point. Ordinarily, not only the distance but also the 
horizontal and vertical angles are observed; these three being sufficient to 
determine the direction, distance, and elevation of the point upon which 
the rod is placed. 
A graduated rod and a mounted telescope are used, the latter containing 
two extra horizontal cross wires. When the objective of the telescope is f o- 
Fig. 21. 
cused on the rod, these wires seem projected on it. Thus in Fig. 21, a and h 
represent the stadia wires, i the distance between them, s the distance p—q 
on the rod intercepted between the wires, / the principal focal distance of 
the telescope objective, e a point at a distance f in front of the optical cen¬ 
ter of the objective (e is the principal focus of the objective;, c the 
distance from the plumb-line of the instrument to the optical center of the 
1 This explanation is taken mainly from Prof. Ira O. Baker’s “Engineer’s Surveying 
Instruments.” 
2 The word stadia was first applied to the rod, but as now generally used means both 
alidade and rod as a whole. In Great Britain the instrument by which the observation is 
made is called a tacheometer, and the rod is called a stadia. On the U. S. C. & G. S. the 
term telemeter is used instead of stadia. 
