From column 9: +0.232
From column 4: +0.231
(+0.463)/2 = + 0.232 (mean difference)
Enter the following value in column 10, opposite station 4: + 0.232.
a. The field computations and entries for one complete station have been entered. Make the entries
for all other stations in a similar manner. The surveyor should be primarily concerned with the data
contained in columns 1, 2, 3, 6, and 7.
b. The object of profile leveling is to find the elevations of points at known distances apart and thus
obtain the profile over a given line. The profile should show the elevations of the tops of all posts along
the baseline, as they would be seen in from the side.
c. When a level is properly set up, the line of sight is perpendicular to the line of gravitation and
revolves in a horizontal plane. The line of sight is the basis for determining elevations and establishing
points.
d. In all surveys, the elevations are referred to some common datum that is designated as zero
elevation. To obtain the elevation of any point not in the datum, add or subtract the point's vertical
distance above or below that plane. When an elevation has not yet been established, an elevation
(referred to as MSL, which is considered zero) simply assumes a datum and assigns an arbitrary
elevation to the starting point. All level shots taken within the area are then relative to the arbitrary
elevation assigned. At a later date, the assumed elevation of the starting point must be tied into a
benchmark of known elevation.
e. Since the lines of a scheme of triangulation are reduced to their equivalent lengths at sea level,
the length of any base must be likewise reduced to sea level before it can be used in adjusting the
triangulation to which it is connected. This requires the connection of the baseline levels to a
benchmark and the computation of the elevation above sea level of the tape supports in order to obtain a
mean elevation for the base. This reduction to sea level is peculiar to triangulation work and is not
necessary for ordinary level work.
3-10. Measure the Baseline. To accurately measure a baseline, all party members must function as a
single unit at the precise moment of marking. The following procedures are recommended to obtain a
successful measurement:
The rear stretcher man, the middleman, and the front stretcher man are positioned as
previously outlined in paragraph 3-7d and e, pages 3-6 and 3-7.
The rear contact man stands directly opposite the mark, which is on the copper strip nailed to
the top of the post. With one hand he firmly grasps the tape between the rear tape stretcher
and the mark on the copper strip. With the other hand he lightly touches the tape on the
opposite side of the
3-11
EN0593
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