is the product of 01´ times the square root of
3, or about 1.73´. The prescribed maximum
angular error of closure has therefore been
exceeded.
Meeting Precision Specifications
The following specifications are intended to
give you only a general idea of the typical
precision requirements for various types of transit-
tape surveys. When linear and angular errors of
closure are specified, it is understood that a
closed traverse is involved.
For many types of preliminary surveys and for
land surveys, typical precision specifications may
read as follows:
l Transit angles to nearest minute, measured
once. Sights on range poles plumbed by eye. Tape
leveled by eye, and standard tension estimated.
No temperature or sag corrections. Slopes under
3 percent disregarded. Slopes over 3 percent
measured by breaking chain or by chaining slope
distance and applying calculated correction.
Maximum angular error of closure in minutes is
1.5 ~n. Maximum ratio linear error of closure,
1/1000. Pins or stakes set to nearest 0.1 ft.
For most land surveys and highway location
surveys, typical precision specifications may read
as follows:
l Transit angles to nearest minute, measured
once. Sights on range poles, plumbed carefully.
Tape leveled by hand level, with standard tension
by tensionometer or sag correction applied.
Temperature correction applied if air temperature
more than 15° different from standard (68°F).
Slopes under 2 percent disregarded. Slopes over
2 percent measured by breaking chain or by
applying approximate slope correction to slope
distance. Pins or stakes set to nearest 0.05 ft.
Maximum angular error of closure in minutes
is l~n. Maximum ratio linear error of closure,
1/3,000.
For important boundary surveys and extensive
topographical surveys, typical precision specifica-
tions may read as follows:
. Transit angles by 1-rein transit, repeated
four times. Sights taken on plumb lines or on
range poles carefully plumbed. Temperature and
slope corrections applied; tape leveled by level.
Pins set to nearest 0.05 ft. Maximum angular
error of closure in minutes is 0.5~n. Maximum
ratio linear error of closure is 1/5,000.
Note that in the first two specifications,
one-time angular measurement is considered
sufficiently precise. Many surveyors, however, use
two-line angular measurement customarily to
maintain a constant check on mistakes.
Measuring Angles vs. Measuring Distances
It is usually the case on a transit-tape survey
that the equipment for measuring angles is
considerably more precise than the equipment for
measuring linear distances. This fact leads many
surveyors into a tendency to measure angles with
great precision, while overlooking important
errors in linear distance measurements.
Making the precision of angular measurement
greater than that of linear measurement is useless
because your angles are only as good as your
linear distances. Suppose that you are running
traverse line BC at a right deflection angle of
63°45´ from AB, 180.00 ft to station C. You set
up at B, orient the telescope to AB extended, and
turn exactly 63°45´00´´ to the right. But instead
of measuring off 180.00 ft, you measure off
179.96 ft. Regardless of how precisely you turn
all of the other angles in the traverse, every
station will be dislocated because of the error in
the linear measurement of BC.
Remember that angles and linear distances
must be measured with the same precision.
IDENTIFYING ERRORS AND
MISTAKES IN TRANSIT WORK
In transit work, errors are grouped into three
general categories; namely, INSTRUMENTAL,
NATURAL, and PERSONAL errors. First, we
will discuss these errors, and then, later, we will
explain the common mistakes in transit work.
Identifying Instrumental Errors
A transit will not measure angles accurately
unless the instrument is in the following condition:
1. The vertical cross hair must be perpen-
dicular to the horizontal axis. If the vertical cross
hair is not perpendicular, the measurement of
horizontal angles will be inaccurate.
2. The axis of each of the plate levels must
be perpendicular to the vertical axis. If they are
not, the instrument cannot be accurately leveled.
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