66 THE UNITED STATES GEOLOGICAL SURVEY. [bull.227. 
The precision of measurement is represented by a probable error of 
from 1 100000 to 1/300000, sufficiently accurate for all primary trian- 
gulation not required in the solution of geodetic problems. 
Primary traverse. — Where it is inexpedient, because of great expense, 
to procure primary control by means of triangulation, as in a heavily 
forested and level country or in the prairies of the Central West, the 
means adopted is the running of traverse lines of a high degree of 
accuracy. The instruments used are, a good transit reading by vernier 
to 30 seconds, a 300-foot steel tape, a 100-foot steel tape, 2 standard 
thermometers, 4 hand recorders, 2 flag poles, and for azimuth observa- 
tions a good watch and electric hand lamps. In order that the effect 
of errors of angular measurement may be minimized, astronomic azi- 
muths are observed at intervals of 8 to 10 miles. The effect of errors 
in both azimuth and distance is practically eliminated by closing the 
traverse back on itself or on some other adjusted primary traverse, or, 
better still, by connecting with primary triangulation stations or astro- 
nomic positions. 
The initial and terminal points of the primary traverse, and inter- 
mediate points on the line of the traverses which may be used as tie 
points for primary or secondary control, are indicated by permanent 
marks — bronze tablets cemented in masonry, or bench-mark posts set 
in the ground. 
In localities over which the United States land surveys have been 
extended township and section corners serve as such permanent marks. 
Spirit leveling. — beginning with the season of 1896 the field parties 
engaged in the topographic mapping have conducted careful spirit 
leveling on a scale more extended and of a quality more accurate than 
had ever before been attempted in this country. This was in pur- 
suance of the act of Congress providing for the sundry civil expenses 
of the Government for the fiscal year 1896-97, a paragraph of which 
is as follows: 
For topographic surveys in various portions of the United States * * * Pro- 
vided, That hereafter in such surveys west of the ninety-fifth meridian, elevations 
above a base-level located in each area under survey shall be determined and marked 
on the ground by iron or stone posts or permanent bench marks, at least two such 
posts or bench marks to be established in each township, or equivalent area, except 
in the forest-clad and mountain areas, where at least one shall be established, and 
these shall be placed, whenever practicable, near the township corners of the public- 
land surveys; and in the area east of the ninety-fifth meridian at least one such post 
or bench mark shall be similarly established in each area equivalent to the area of a 
township of the public-land survey. 
It was realized by the framers of the above act that it would be 
impossible to establish and mark on the monuments their exact height 
above sea level. Any attempt to do this would have necessitated the 
running of thousands of miles of precise levels in order to connect 
with sea level the initial points within the various areas under topo- 
