Civil Engineering Reference
In-Depth Information
paper. All field measurement techniques involve some amount of error. If a
total station is used to measure traverse lines, there will be errors in sighting the
prisms—no matter how careful the instrument person is. A tripod may not be set
up exactly over a point, or the tripod may be set up on ground which is soft or
thawing, causing the tripod to move slightly between measurements. This would
cause errors in both angles and distances. These errors can accumulate, or they
can cancel each other out. Usually it is some combination of both. The purpose of
adjusting the traverse is to remove these errors.
The reader is already familiar with the concept of a closure from our previous
discussion of the property line closure. The same closure theory which we applied
to property lines applies to traverses. The surveyor must know how well the trav-
erse closes in order to determine the precision of the measurements that were
made in the field. After the traverse is adjusted, most of the work will involve rec-
onciling the record evidence with the physical evidence. In many cases this will be
the most difficult and time consuming part of the calculation process.
A detailed discussion of the mathematics used to adjust a traverse is outside
the scope of this chapter but examples are provided in the Appendix. A few points
should be mentioned. The first point is that the traverse must close within certain
limits. These limits may be imposed by law or they may be based on the standards
of practice commonly used by surveyors in the area for the type of survey being
conducted.
Typically the surveyor will enter the raw traverse data, which consists of
angles and distances measured in the field by the instrument, into a computer
program. The program will then calculate the error of closure. In actual practice,
most surveyors use data collectors on their instruments. These data collectors
are small computers that often have the ability to inform the user, at least on an
elementary level, approximately how well the traverse will close. This informa-
tion is usually available to the surveyor while in the field as soon as the traverse
returns to the beginning traverse point. Sometimes the data collector is capable
of adjusting the traverse. However, even if a data collector is capable of adjusting
the traverse, it is often more convenient to use a more powerful office computer
that can take advantage of more sophisticated mathematical routines to adjust the
traverse.
Once the surveyor has determined that the error of closure is within allowable
limits, the traverse is adjusted. There are a number of mathematical methods of
adjusting a traverse but suffice it to say that the adjustment takes the closure error
and distributes that error at certain places in the traverse so that the adjusted trav-
erse closes perfectly. Some of the more sophisticated closure routines are able to
determine the most likely places where the errors occurred and place the neces-
sary adjustments there. Other routines merely distribute the error proportionally
over all of the traverse lines. With modern surveying instruments and techniques
the errors are usually very small and the closures usually exceed minimum closure
requirements by a substantial margin.
After the traverse is adjusted, the locations of all monuments and other
evidence found in the field are calculated, and the coordinates of the points plotted
