Geodesy and Geodynamics 9 (2018) 115e120
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Geodesy and Geodynamics
journal homepages: www.keaipublishing.com/en/journals/geog;
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Comparative study on vertical deformation based on GPS and leveling
data
Shanlan Qin a, *, Wenping Wang a, Shangwu Song b
a
b
Second Monitoring and Application Center, China Earthquake Administration, Xi'an, 710054, China
Discipline and Graduate Management Office, Institute of Disaster Prevention, Beijing, 101601, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 December 2016
Accepted 30 July 2017
Available online 16 October 2017
The development of GPS (Global Positioning System) technology has led to increasingly widely and successful applications of GPS surveys for monitoring crustal movements. However, multi-period GPS survey
solutions have not been applied in monitoring vertical crustal movements with normal backgrounds. In
this paper, we carried out a comparative study on the vertical deformation of the comprehensive profile of
the cross-fault zone in Shanyin, Shanxi province, China, based on GPS and precise leveling observation data
for multiple time periods. The vertical deformation rates observed with repeating GPS survey are obviously
different (over 20 mm/y at some sites) from those with repeating leveling survey within a relatively short
period. However, the deviations in the vertical displacement between GPS and leveling in a long-term
survey (over three years) showed good consistency at 3e4 mm/y at most sites, on GPS forced offset
surveying and fixed survey instruments in a long-term survey (over three years). Therefore, GPS vertical
displacement results can be applied to the study of vertical crustal movements.
© 2017 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier
B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords:
Vertical deformation
GPS
Precise leveling
Deviations
1. Introduction
Surveying vertical crustal movements provides an important
foundation for the study of earthquakes, subsidence, and the global
or regional maintenance of precise elevation baselines. For years,
precise leveling has been the only technique for obtaining vertical
deformation information of the Earth's surface, with the precision of
millimeters or even sub-millimeters. Precise leveling is highly accurate, but has obvious disadvantages, such as low efficiency, high
labor costs, and high transmission errors. In comparison, GPS possesses some advantages such as all day observation, high efficiency,
low labor intensity, and it also provides three-dimensional deformation information of the Earth's surface. In recent years, with the
increasing stability of the ITRF (International Terrestrial Reference
* Corresponding author.
E-mail address: shanlan_qin@163.com (S. Qin).
Peer review under responsibility of Institute of Seismology, China Earthquake
Administration.
Production and Hosting by Elsevier on behalf of KeAi
Frame), improvement of the antenna phase center model, development of the Earth load correction model, and accumulation of
survey data over longer time periods [1e4], GPS has been increasingly applied in surveys of vertical crustal deformation.
Scholars at home and abroad have conducted many studies on
this subject. Aoki et al. plotted a velocity field of vertical deformation
in the Japanese archipelago by calculating the continuous GPS data
collected from 1996 to 1999 [5]. They think that the vertical components of the GPS velocities have rarely been used, because of their
higher noise level, information on the vertical deformation field will
enable us to separate rigid plate motions from deformation due to
interplate coupling. Ching et al. plotted a velocity field of vertical
deformation in Taiwan by calculating the regional continuous GPS
data collected from 2000 to 2008 [6]. They model the present-day
and geologic vertical velocities and published GPS horizontal velocity data across southern Taiwan using a 2-D lithospheric model.
Wang Min plotted a long-term velocity field of vertical deformation
in mainland China from 1999 to 2007 based on years of survey data
collected from network engineering baseline sites and basic sites [7].
In the last few years, GPS has showed good application prospects in
high-precision surveys of slow vertical crustal movements. However, the consistency of GPS-derived vertical displacement results
with the results derived from leveling surveys remains a cause for
https://doi.org/10.1016/j.geog.2017.07.005
1674-9847/© 2017 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an
open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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S. Qin et al. / Geodesy and Geodynamics 9 (2018) 115e120
concern. Based on previous studies, in this paper the GPS and precise
leveling observation data of the comprehensive profile of the crossfault zone in Shanyin, Shanxi province, China, collected in multiple
periods from 2006 to 2016, were used to determine whether the GPS
vertical displacement results and leveling vertical displacement results were in good agreement, observation and reveal the reliability
of GPS technology in studying the vertical deformation of the Earth's
surface in the study region.
2. Data sources and processing methods
2.1. Data sources
The comprehensive profile of the cross-fault zone in Shanyin of
the Shanxi province was built as part of the digital earthquake
network engineering project in China's Tenth Five-Year Plan. It
covers an area from the edge of the Ordos block near the Kouquan
fault in the north to the uplifted region of Mount Heng in the south.
The first survey was carried out in 2006. From 2009 to 2014, this
profile was surveyed at the same site by GPS and precise leveling.
Table 1 lists the time of GPS surveys and precise leveling surveys.
The GPS data are observed over 6 time periods with 4 days for each
period and the observation sampling rate is 30s. The GPS stations
have reinforced concrete monuments with forced-centering
apparatus for antennas were applied under excellent surveying
conditions with the survey instruments relatively fixed in GPS
surveys (Fig. 1). TOPCON receivers and choke coil antennas were
applied in all the surveys, except in 2006, when LEICA and ASHTECH receivers were used. Fig. 2 illustrates the site distributions of
the comprehensive profile of the cross-fault zone in Shanyin.
2.2. Processing methods
We used the GAMIT/GLOBK10.40 software to calculate the GPS
data collected in each time period and the four IGS sites data (bjfs,
Table 1
The observation time of GPS and precise leveling.
Precise leveling observation time
GPS observation time
Sep. 14, 2006eSep. 25, 2006
Jul. 28, 2009eAug. 07, 2009
Jan. 29, 2010eFeb. 06, 2010
Apr. 25, 2011eMay. 05, 2011
May. 4, 2012eMay. 12, 2012
Jul. 13, 2013eJul. 18, 2013
May. 5, 2014eMay. 09, 2014
Nov. 23, 2006eNov. 26, 2006
Sep. 30, 2009eOct. 3, 2009
Feb. 04, 2010eFeb. 07, 2010
Aug. 10, 2011eAug. 13, 2011
Oct. 05, 2012eOct. 08, 2012
Aug. 29, 2013eSep. 01, 2013
Sep. 12, 2014eSep. 15, 2014
shao, wuhn, urum) in China [8]. We used the ELEV model to
correct the antenna phase center offset during data processing.
The GPT model and Markov random procedure were used to estimate, correct, and eliminate the impact of the tropospheric
zenith delay during GPS signal transmission. The tropospheric
zenith delay parameters were estimated every hour at each GPS
site, for which linear interpolations on time dimension were also
estimated [9]. The Vienna Mapping Function 1 (VMF1) with the
highest precision in the elevation direction was used to improve
the precision for which linear interpolations on time dimension
on time dimension were also estimated calculations [7]. We obtained the relaxation solution of GPS sites every day, and then
bound it with those of IGS sites. The GLOBK software was used to
calculate the coordinates of the GPS sites under the ITRF2008
frame [10]. The results showed that the mean squares error in the
elevation direction at almost every GPS site was less than 4.8 mm.
Fig. 3 illustrates the coordinate time series for each GPS site.
During GPS surveys at a site, receiver antennas varied in multiple
time periods. Based on the GPS ultra-short baseline network (the
lengths of baselines are 3e5 m) which was constructed by the
Second Monitoring and Application Center, China Earthquake
Administration, we used the absolute antenna phase center model
to correct the antenna phase center variations. The results indicated that LEIAT504, ASH701945C_M, and ASH700936E_C/
ASH701945B_M had vertical deviations of 8.4 mm, 5.5 mm, 3
mm and 2 mm, respectively relative to the TPSCR.G3 antenna
[11]. The vertical deviations in the phase centers from the same
antenna model generally agreed with each other within millimeters. Considering the impacts of the vertical deviations in the
antenna phase centers of different models on vertical deformation, we eventually obtained the vertical deformation rate in
different periods for each GPS site.
However, the vertical deformation rate of the precise leveling
within the same time intervals was obtained by fitting elevation
difference. Elevation differences for each measured segment were
obtained from roundtrip measurement. The mean squared error of
height for the 1 km roundtrip measurement was ±0.353 mm. Fig. 4
illustrates the vertical deformation of multi-period leveling data for
leveling points.
3. Results and discussion
In GPS and leveling surveys, elevations were obtained using
different elevation systems. However, when studying the vertical deformation, we focused on the variation of elevations
rather than the elevations themselves. Huang Liren et al. carried
Fig. 1. The GPS stations have reinforced concrete monuments with forced-centering apparatus for antennas.
S. Qin et al. / Geodesy and Geodynamics 9 (2018) 115e120
Fig. 2. Site distributions of the comprehensive profile of the cross-fault zone in Shanyin, Shanxi (The gray rhombuses are sites for leveling, while the red solid dots are
sites for both GPS and leveling. F1: Kouquan fault, observation F2: Hengshan North
fault, F3: Hengshan South fault.).
out a quantitative comparison between the U component variation in the coordinate system of the site center and the
elevation variation derived from precise leveling [12]. They
found that the U component variation in local Cartesian coordinate system can be used represent the elevation variation
measured by GPS, provided that the studied area was less than
117
110 km. Therefore, when the studied area was small, the
elevation variations obtained by GPS and leveling could be
directly compared. To better compare the survey results of GPS
and leveling, we selected the northernmost point (Shanyin 01)
as the baseline site to obtain the vertical displacements in
relation to Shanyin 01 in different time periods. The comparison
results are shown in Fig. 5.
As shown in Fig. 5, the comparison results of GPS and leveling
in the comprehensive profile of the Shanyin cross-fault zone are as
follows: (1) there were great deviations in the vertical displacement in one year, for example, over 20 mm per year at some sites;
(2) although there were deviations in the vertical displacement
represent in two years, the consistency between them was greatly
improved; (3) the deviations in the vertical displacement between
GPS and leveling in three years were small at most sites at 3 mm
per year. Blewitt et al. showed that GPS vertical displacement
results were highly subject to the impacts of the reference frame,
atmosphere, and water load when compared with horizontal
movement results [13]. If the GPS survey time period was shorter
than 2.5 years, the site velocity was greatly affected by seasonal
signals. If the survey time period was longer than 3 years,
particularly 4.5 years, the velocity was less influenced by seasonal
signals. This is consistent with the comparison results in this
paper.
The great disparity obtained in short periods may be attributed
to three reasons. First, most GPS surveys were conducted from
August to November every year, except for the emergency survey
in February 2010. The leveling survey times were greatly different
from the GPS survey time except in 2009 and 2013. Due to the
seasonal movement of surface substances (such as atmospheric,
terrestrial water), the results of GPS have significant seasonal
variations. In order to get the annual/semi-annual variation of the
vertical deformation of GPS measurement, we obtained the coordinate time series of three continuous GPS stations (SXLQ, SXGX,
and SXLF) in the area (Fig. 6). The SXLQ is the nearest GPS station
Fig. 3. The coordinate time series for GPS sites (relative to sy01).
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S. Qin et al. / Geodesy and Geodynamics 9 (2018) 115e120
Fig. 4. The vertical deformation of multi-period leveling data.
Fig. 5. Comparisons between the GPS and leveling vertical deformation rates in the comprehensive profile of the Shanyin cross-fault zone (The hollow circle represents the vertical
displacements obtained by leveling, while the triangle represents the vertical displacements obtained by GPS.).
(about 100 km) from the comprehensive profile of the cross-fault
zone in Shanyin. The Fig. 6 shows the obvious periodicity and the
largest change of vertical displacement in July and August and the
smallest in January and December of each GPS station. The seasonal effect on vertical deformation of GPS is to be considered,
therefore, we should choose the same observation time as
possible.
Second, antenna phase centers were offset. Theoretically, the
phase center of an antenna should be consistent with its geometric
center. However, due to the characteristics and mechanical
manufacturing of an antenna, the phase center changes with the
signal input direction and signal intensity [14]. Therefore, the
momentary position of the antenna phase center does not coincide
with the geometric phase center position. During the GPS calculation process, although the ELEV model was used to correct the
phase center offset, the impact of such deviation on the vertical
deformation still existed. Third, the GPT model and Markov random
procedure were selected to estimate, correct, and eliminate the
impact of the tropospheric zenith delay on GPS signal propagation.
However some researchers [15,16] think that GPT2 could better
S. Qin et al. / Geodesy and Geodynamics 9 (2018) 115e120
119
Fig. 6. The coordinate time series of continuous GPS stations near the Shanyin.
improve the vertical precision of GPS. We will study the impact of
different models on the precision of vertical deformation in the
future.
4. Conclusion
In this paper, we processed the GPS and precise leveling observations in the comprehensive profile of the cross-fault zone in
Shanyin, Shanxi, over six time periods. The comparison results
showed that there was a great deviation in the vertical deformation
rate between them if GPS and leveling observations in short periods
(several months or one year).While in long survey periods (three
years or longer), GPS surveying with forced-centering apparatus for
antennas and fixed survey instruments, the vertical deformation
rates derived from GPS and leveling were highly consistent, with
good external compatibility. Those results indicated that GPS observations could be applied to the study of vertical crustal
movements.
Acknowledgments
This work was supported by the China National Special Fund for
Earthquake Scientific Research in Public Interest (201508009).
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Qin Shanlan, Engineer at Second Monitoring and Application Center, China Earthquake Administration. Her
research interests mainly focus on GPS data processing,
applications of GPS in monitoring crustal movement and
earthquake prediction.