5. A NOTE ON TIME SAVINGS
In this study, forward and return runs for the geometric levelling were
completed in 1.5 hours. The height differences between the profile points were
surveyed for about one hour using trigonometric levelling. Reciprocal zenith angle
observations were conducted during the survey. As seen the results from Table 4,
the short baseline static GNSS results obtained from 30 min sessions are as good as
the ones obtained from the terrestrial surveying techniques. This study also shows
that GNSS is as efficient as the terrestrial surveying methods in determining height
differences over steep slopes from the point of view of time and economy.
6. CONCLUSIONS
This paper discusses two kinds of heighting techniques that are used to survey
slopes: GNSS and terrestrial levelling techniques. The basic idea for an effective
heighting over short ranges is to determine the elevation difference by using
geometric or trigonometric levelling. However, surveying with these techniques
over steep slopes might result in degraded accuracy due to some error sources, and
also especially geometric levelling is time consuming. Moreover, some gross errors
might occur. Therefore resurveying of the site is unavoidable.
The GNSS technology is widely used for many kinds of geodetic and
engineering surveys. In this study, the use of GNSS heighting instead of terrestrial
techniques over slopes was shown. Both (space and terrestrial) techniques were
performed over the local test area, and GNSS derived height differences were
determined. The results indicate that the differences between GNSS and terrestrial
methods are obtained at about sub-centimeter for the experiments. This leads to a
good heighting by the GNSS, and is less time consuming over steep slope. For the
GNSS technique, there exists a specific disturbing effect, i.e. multipath, and it can
corrupt the results of heights. Therefore the location of the local network is
important, and one must avoid establishing the survey marks near the multipath
areas. This method explained here could only be used over short distances, i.e. in
our case it was only a 100 m. For longer distances, one has to include the effect of
geoid-ellipsoid separation. For the future work, we are going to study the issue of
the real-time kinematic GPS navigation in this scope.
ACKNOWLEDGEMENTS
We obtained CODE final orbits and the coordinates of IGS permanent stations
from SOPAC archives. Also Google Earth picture was used as Fig 3. Finally, the
authors appreciate valuable suggestions and fruitful comments by anonymous
reviewers to improve the quality of the paper.