Revision of Survey Results of control points
Geospatial Information Authority of Japan (GSI) is the governing institute on geodetic survey and mapping. It manages about 130,000 geodetic reference points in Japan and their survey results are consistent with the International Terrestrial Reference Frame. The recent development of GNSS technology led to more than 1,200 GNSS-based control stations with operation system called GEONET as new geodetic reference points.
The 2011 earthquake that took place off the Pacific coast of Tohoku (hereafter ‘Tohoku Earthquake’) was such a major seismic event (Mw=9.0) that occurred at 05:46:23UT and triggered a huge tsunami, causing severe damage along the coast of Tohoku or Kanto district. With this, large- scale crustal deformation was detected by GEONET. The station ‘Oshika’, located close to the epicenter, was moved about 5.3m to the east-southeast direction and subsided about 1.2m for instance (Figure 1). After the main shock, post-seismic deformation has been continued widely along the Pacific coast area of eastern Japan and local scale crustal deformations have been observed due to some aftershocks (Suito et al., [2011a, 2011b]).
To carry out the public surveys, the precision of the relative position between nearby control points is crucial in terms of maintaining consistency. GSI stopped the publication of the Survey Results of the GEONET stations, triangulation stations and bench marks in the affected area on March 14, 2011, and started to discuss on their revision.
In this paper, GSI’s activity on revision of Survey Results of control points after the Tohoku Earthquake is presented.
Calculation of Survey Results of GEONET Stations
Conventionally, new Survey Results were calculated based on existing results. However, since crustal deformation occurred across a large area due to the Tohoku Earthquake, we decided to calculate the coordinates of GEONET stations that correspond to International Terrestrial Reference Frame 2008 (ITRF2008), based on the observation results at the VLBI (Very Long Baseline Interferometry) station (TSUKUB32) located in GSI, Tsukuba, Japan. These calculated coordinates were then adopted as the new Survey Results for the area where update of Survey Results were required.
Required accuracy and revision date for the Survey Results of GEONET stations
In accordance with the tolerances indicated in the General Standard of Operation Specifications for Public Surveys in Japan, the accuracy required for calculating Survey Results of GEONET stations should be to a pointto- point relative accuracy of 2ppm.
Since positional data were necessary for the restoration/reconstruction of the disaster-stricken area, and also be utilized in the resurvey of triangulation stationsand public control points, urgent revision of Survey Results of GEONET stations was required. Meanwhile, to decrease the effect of post-seismic deformation, future deformation was estimated based on the observation data of GEONET stations using the approximation formula with a logarithm function by Marone et al.,  as follows:
where c and a are the constants, τlog is the time constant and t is the time elapsed from the earthquake.
The appropriate time of re-publication of revised Survey Results was then discussed based on the predicted deformation (Fig. 2). As a result, we decided that the new Survey Results of GEONET stations should be published by the end of May 2011.
Figure 2: (Top diagram) Progression of post-seismic deformation observed at the GEONET Station ‘Yamada (950167) and (Bottom diagram) predicted deformation
Calculation of Survey Results based on VLBI and GEONET
Coordinate of TSUKUB32
The coordinate of TSUKUB32 on ITRF2008 was estimated on the analysis of VLBI observation data of international VLBI session ‘IVS-R1482’, which was conducted on May 10, 2011, to adopt the latest coordinates for the revision of the Survey Results. We constrained the ITRF 2008 coordinates of five overseas stations, which were not affected by the earthquake, and the coordinates of TSUKUB32 were calculated.
Survey Results calculation method
The calculation method for Survey Results with observation results of the VLBI and GEONET is as follows:
(1) First, the post-seismic deformation was calculated from the difference in averages of prompt GEONET solutions using IGS’s Rapid Orbit (R3 solutions) at Station No. 92110 (located in GSI, Tsukuba) between May 8-12 and May 22-26, 2011. Next, we calculated the coordinate of TSUKUB32 at 12:00 UTC on May 24 by adding the post-seismic deformation to the coordinate calculated from VLBI observation at 5:00 UTC, May 10.
(3) Lastly, we combined (1) and (2) with the collocation result in 2007 (Miura et al. ) and shifted the R3 solutions so that they were consistent with the coordinate of IGS station ‘TSKB’ (located in GSI, Tsukuba) based on VLBI. The shifted R3 solutions were used as new Survey Results of GEONET stations that are located in the area for which the publication of Survey Results had been stopped.
Adjustment calculations on boundary area
Discrepancies across the boundaries of areas with updated Survey Results and those with no updates had been caused by accumulated crustal strain, since the last update of survey results. Thus, we calculated correction values for the revised Survey Results so that the amount of discrepancy across the boundaries became 2ppm or smaller.
Re-publication of Survey Results of GEONET stations
On May 31, the updated Survey Results of 438 GEONET stations were released, and it became available to carry out public surveys using GEONET stations in the affected area and its vicinity.
Amendment of coordinate/elevation of origins of Japanese horizontal/vertical control network
Due to the Tohoku Earthquake, the positions of the Origins of the Japanese Horizontal and Vertical Control Network in Tokyo had also shifted. This resulteding in the legally prescribed coordinate and elevation being far off the actual positions after the earthquake. Tthe coordinate and elevation of those origins were amended to ensure the accuracy of surveys.
Amendment of coordinate of the origin of the Japanese horizontal control network
To again calculate the coordinate of the Origin of the Japanese Horizontal Control Network, GNSS observations were conducted between June 21 and 25, 2011 at the VLBI marker for TSUKUB32 and at the Origin of the Japanese Horizontal Control Network.The coordinates of the Origin of the Japanese Horizontal Control Network calculated again through these observations and the azimuth from the Origin of the Japanese Horizontal Control Network to the VLBI marker for TSUKUB32 are shown in Tables 1 and 2 respectively.
Amendment of Elevation of the Origin of the Japanese Vertical Control Network
GSI has kept regular leveling once per year from Aburatsubo tide station to the Origin of the Japanese Vertical Control Network to monitor vertical changes at the origin. Sea levels recorded at the Aburatsubo tide station before and after the Tohoku Earthquake showed that no significant tidal change associated with the earthquake was observed. Additionally, no significant change in ellipsoidal height was observed between pre-earthquake (January 2011) GPS observation results and post-earthquake (July 2011) results at Aburatsubo in terms of uncertainty of GPS observations. Therefore, new elevation of the Origin of the Japanese Vertical Control Network was calculated through the following procedures (1) and (2) and then verified by procedure (3).
(1) When the elevation of the Origin of the Japanese Vertical Control Network was fixed based on leveling results from the region between the Aburatsubo tide station and the Origin of the Japanese Vertical Control Network in January 2011 prior to the earthquake, the height of the reference point at the Aburatsubo tide station was determined to be 2.4173m.
(2) By fixing the elevation of the reference point at the Abratsubo tide station obtained in (1), the new elevation of the Origin of the Japanese Vertical Control Network calculated based on leveling results of July 2011 was determined to be 24.3904m.
With regard to standard deviation of the network adjustment of the leveling, since the estimated error of +/-0.0026m may be contained in the results shown in (1) and (2), the newly amended elevation of the Origin of the Japanese Vertical Control Network should be 24.3900m after rounding off the ten-thousandth place.
Revision of Survey Results of triangulation stations
To ensure accuracy of the revised Survey Results of triangulation station, it is desired that surveys are conducted onsite at triangulation stations. However, in terms of time and cost it is not always practical to conduct surveys at each and every triangulation station due to the fact that publication of Survey Results for more than 40,000 triangulation stations was stopped. Since the observed crustal deformation due to the Tohoku Earthquake was relatively similar in a wide area, precise geodetic surveys were to be conducted only at a selected number of triangulation stations – similar to practices followed after the Tokachi-oki Earthquake in 2003 (Doi et al., ) – and triangulation stations not targeted for resurvey were to have their Survey Results recalculated using correction parameters.
Figure 3: Correction parameters developed for coordinates
Triangulation station resurveys
Within the area for which publication of Survey Results was stopped, precisegeodetic surveys were conducted at 595 selected points and 1,272 points in the area where aftershocks and induced earthquakes occurred.
Recalculation of Survey Results of Triangulation Stations
For the remaining triangulation stations, a new set of correction parameters were developed and Survey Results of the triangulation stations were recalculated using PatchJGD, the software for correcting geodetic coordinates (Tobita, 2009). The number of the stations recalculated by the software was 41,392.
To recalculate the Survey Results of triangulation stations, correction parameters for coordinates (horizontal) and elevations were developed using both previous and revised Survey Results of GEONET stations and the triangulation stations for precise geodetic surveys. The correction parameters were constructed by estimating variations at each grid with a resolution of 45 seconds in longitude and 30 seconds in latitude from displacements of GEONET stations and triangulation stations using the Kriging method. It should be noted that there were some excluded area where relatively large aftershocks or induced earthquakes made complicated deformations as observed by GEONET and Synthetic Aperture Radar (SAR) interferometry. Additionally, the area around the Fukushima No.1 nuclear power plant was also excluded. The revision of Survey Results for triangulation stations not slated for resurvey was carried out based on these correction parameters (Figure 3 and Figure 4).
Figure 4: Correction parameters developed for elevations
Revision of Survey Results of Bench Marks
The target accuracy of the correction parameters for elevations used for the revision of Survey Results of triangulation stations was 10cm to 20cm and those parameters cannot be applied in the revision of Survey Results of bench marks that requires elevation accuracy of 0.1mm to 1mm. Therefore, the revision of Survey Results of bench marks was, in essence, to be carried out through resurvey. Network adjustment with multiple reference points was calculated under the condition that the coordinates of the Origin of the Japanese Vertical Control Network and another bench mark at the northern margin of the revision area which was less affected by the Tohoku Earthquake were fixed.
Revised Survey Results of bench marks obtained by network adjustment calculations based on these two reference points were released on October 31, 2011 with those of triangulation points.
Following the Tohoku Earthquake, notable crustal deformation occurred over a wide area of eastern Japan, which led to the halt in the publication of Survey Results of GEONET stations, triangulation stations and Survey Results of bench marks in the affected area.
In order to contribute to swift restoration/ reconstruction of the disaster-stricken area, and to ensure stable provision of Survey Results for many years to come, the amount of future deformation was estimated by taking post-seismic deformation monitored by GEONET stations into account. After discussions with regard to an appropriate time for re-publication, revised Survey Results of GEONET stations were released on May 31, 2011.
Precise geodetic surveys were conducted at some of triangulation stations, and Survey Results for the remaining triangulation stations were revised through calculations based on the correction parameters.
The revision of the Survey Results of bench marks was, in essence, achieved through resurvey. Leveling was conducted and network adjustment was calculated based on the revised elevation for the Origin of the Japanese Vertical Control Network.
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