Surveying - Carpenter's and Builder's Math, Plans, and Specifications - page 211

be necessary at the level. If the ground is still unsafe, drive
stakes or piles to support the instrument.
Elevations taken at road crossings —Take elevations both ways
for some distance.
Elevations taken at river crossings —Take elevations of high-
water marks and flood marks, with the dates of it. Question
the local residents for these dates. Also, ask for dates and data
of extreme low water.
Proper length of sights —This will depend on the distance at
which the rod appears distinct and on the precision required.
Under ordinary conditions, sights should not exceed 300 feet
where elevations are required to the nearest .01 foot, and even
at a much shorter distance, the boiling of the air may prevent
a precision reading of this degree.
Correction for refraction and earth curvature —A level line
is a curved line at which every point is perpendicular to the
direction of gravity, and the line of sight of a leveling instru-
ment is tangent to this curve. This makes it necessary to con-
sider this curve in some leveling operations. If reasonable care
is used to make the lengths of backsights and foresights ap-
proximately equal, this aberration is self-correcting. However,
in extremely long lines, it is approximately 2 inches in one-
half mile, or about 2 / 3 d 2 ,infeet, where d is equal to the dis-
tance, in miles. This correction is usually combined with that
for refraction. The combined correction is 547 d 2 , and it is
negative .
Trigonometric Leveling
Finding the difference in elevation of two points by means of the
horizontal distance between them and the vertical angle is called
trigonometric leveling .Itisused chiefly in determining the eleva-
tion of triangulation stations and in obtaining the elevation of a
plane-table station from any visible triangulation point of known
elevation.
In triangulation work, the vertical angles are usually measured at
the same time the horizontal angles are measured to obtain the ele-
vations of triangulation points as well as their horizontal positions.
The vertical angle is measured to some definite point on the signal
whose height above the center mark of the station was determined
when the signal was erected. The height of the instrument above its
station should be measured and recorded.