Chul-Soo Yang
Chief Researcher, Cadastral Technology Research Institute,
Korea Cadastral Survey Corporation
Sang-Soo Kim
Vice Director, Ministry of Home Affairs, Korea
ABSTRACT
Several performance tests to confirm validity of GPS measurements show that GPS is effective not only on surveying geodetic control points but also on surveying cadastral control points. Using GPS, many points can be observed in a short period of time with enough accuracy. It is remarkable that even in urban area intermingled with buildings, over 95% of the observations on cadastral supplementary control points surveyed using a rapid static technique have good correspondences with the results obtained by EDMs and theodolites. The repeatability of GPS survey is better than RMS of 1 cm in these tests. For control point surveying, RMS differences between the GPS employed WGS84 system and the national datum that refers to the Bessel ellipsoid is 24cm in the national scale, and 15cm in local scales, respectively.
1. INTRODUCTION
The integration of GPS and GIS becomes a powerful tool for the preparation of spatial data. Although GPS may not work in every situation due to GPS signal noises and/or due to terrain blockages, the combination of a real-time kinematic GPS technique and a differential GPS technique efficiently provides users precise positions. This combination can increase productivity, reduce costs, and still produce results with horizontal positional accuracies as good as those obtainable by conventional EDM surveying. In addition, GPS surveying can well accommodate the characteristics of the structures of cadastre and the objects of cadastral survey. Therefore, it is expected that GPS surveying will prevail in a short time period.
Employing GPS surveying requires well-established regional and global geodetic networks. They are essential for the preparation of consistent regional and global spatial data. Problems arise in using WGS84 as a datum when mappings are carried in the local datum since there is no single closed mathematical formula for converting the data from WGS84 datum to local ones or vice-versa. This will be discussed through the paper.
2. FUNDAMENTAL NETWORK OF KOREA
The Korean Datum adopts the Bessel ellipsoid as a reference with its origin fixed at Tokyo, Japan. The network consists of 189 first-order triangulation points, 1,102 second-order points, 3,045 third-order points, and 11,753 fourth-order points surveyed from 1910 to 1924. Unfortunately, 80% of these triangulation points were lost during the 3-year Korean War in 1950-1953. The re-establishment and updating of these points have been greatly demanded in Korea.
The Korean Datum is different from WGS84 (Defense Mapping Agency, 1987) or ITRF (IERS Terrestrial Reference System) that GPS employs as the reference frame. While the request for the coordinates referenced to the existing datum are still large, demands for the well-accepted global coordinate systems have been also growing. Recently, a move to adopt geocentric datum with GRS80 as the ellipsoid has been underway in Korea surveying community.
3. GPS NETWORK
In Korea, serious precise surveying by GPS has started with the establishment of the first continuously operating observation station in 1994. At the end of 1998, more than 20 permanent GPS stations will be established over the country by Ministry of Home Affairs (MOHA) and Korea Cadastral Survey Corporation (KCSC). The distances among the stations range from 50 km in urban areas to 100km in rural and mountainous areas. One of the stations already in operation by Korea Astronomy Observatory (KAO) has been included in the IGS GPS network since 1996.
Figure 1 shows one of the continuously operating GPS stations operated by MOHA and KCSC. The station is equipped with the latest dual-frequency P-code receiver. A receiver, a modem, and a power supply are installed inside the pillar. The collected data are transferred to the data center with the high-speed modem through public telephone lines. We provide the data to users in the RINEX (Receiver Independent data Exchange) format through Bulletin Board System (BBS).
The MOHA, Ministry of Science and Technology, and National Geography Institute have cooperative works in establishing and operating GPS networks for surveying and monitoring crustal movements. The network will be the backbone of the newly established precise geodetic network in conjunction with the conventional triangulation network.
<Figure 1> Permanent GPS station operated by Ministry of Home Affairs and Korea Cadastral Survey Corporation
4. CADASTRAL SURVEY BY GPS
A move to introduce multi purpose coordinated cadastre has been under way in Korea in recent years. To support the movement, parcel based land information system (PBLIS) and cadastral survey by use of GPS technology are under development.
KCSC, a non-profit public foundation, has tested GPS surveying. Several performance tests to confirm validity of GPS show that GPS is effective not only in surveying geodetic control points but also in surveying cadastral control points. Furthermore, many points can be observed in a short time period with satisfactory accuracies.
It is remarkable that even in urban area intermingled with buildings, over 95% of the observations on cadastral supplementary control points surveyed using a rapid static technique have good correspondences with the results obtained by EDMs and theodolites. The duration of GPS measurements was 3 to 7 minutes. The repeatability of GPS survey is better than 1cm RMS in these experiments. In case of boundary relocation survey, however, a stop-and-go method in rural areas for 7~10 second occupation showed 85% of the observations have good results (Yang et al., 1997).
To minimize problems in receiving GPS signals due to noises and/or due to terrain blockages, latest processing techniques and equipments have to be integrated. In addition, conversion of GPS data in WGS84 into local datum has to be solved.
5. DATUM TRANSFORMATION
5.1 National Scale
Datum conversion enables us to combine GPS measurements with the existing conventional measurements. It has advantages in establishing minor control points and finding outlier points in local datum since they can be done with efforts much less than when the conventional method is used. This has led many countries to establish converting processes from a global datum to local ones.
Thirty-six triangulation points are used to determine 7 conversion parameters between the Korean datum and WGS-84. The seven parameters are three origin shifts, three rotations, and a scale factor. They are estimated by Least squares methods. The apparent differences between the Korean Datum and WGS84 horizontal coordinates for the same point are approximately 10.61 second in latitude and 7.79 second in longitude (Yang et al., 1998). In this process, the geoid height referenced to the local ellipsoid is calculated with consideration of the datum shift of Tokyo Datum with respect to WGS84 (D X=146.43m, D Y=-507.89m, D Z=-681.46m) as reported by Geographical Survey Institute of Japan (Tsuchiya and Tsuji, 1996).
Figure 2 shows computed corrections in latitude (solid line) and longitude (broken line) between WGS84 and Korean datum. The estimated seven parameters are:
D X=125.902.97m, D Y=-480.482.60m, D Z=-656.352.31m,
w x=1.730.08sec, w y =-1.930.08sec, w z =-1.850.09sec, and
D S=-6.660.31ppm.
The posterior RMS difference between them is 24cm. This accuracy will result in better than 1-PPM accuracy when relative GPS surveying is carried out from these control points.
<Figure 2> Corrections in latitude and longitude to transform from WGS84 to Bessel
5.2 Local Scale
When 7 transformation parameters are estimated for local areas, they are, especially with no constraint in parameters, significantly different from those for the national scale. This is largely due to the combination of outliers in local data and distorted local surveying networks.
Geographical latitude and longitude of local datum is influenced by vertical deflections different from those at the Tokyo origin. Thus, fixing rotation is not reasonable because vertical deflections are not uniform from one area to another as their amounts and directions are not different. Adopting the scale factor from the nation-wide estimation is desirable to have a reference ellipsoid of a consistent size nationwide. Numerical tests with the fixed scale factor give out parameters consistent with the nation-wide parameters. Also, these parameters give sufficiently small residuals at local minor control points.
6. VERTICAL DATUM
GPS can be very helpful for the height determination. Ellipsoidal heights are given from GPS measurements, and if they are combined with precise local geoid, orthometric heights are easily calculated. This can significantly reduce the efforts to measure the heights from local vertical datum. The accuracy of the calculated orthometric heights depends on the accuracy of the local geoid, and that of the GPS ellipsoidal heights.
Over the southern half of the Korean peninsula, a local geoid model is available. The model is calculated with over 5,000 well-distributed and well-controlled gravity measurements, sea surface heights from various altimeters, and a reference Earth gravity model of the OSU91A (Choi et al., 1997). Figure 3 shows this geoid above the WGS84 ellipsoid. At 71 well-distributed points, the GPS/geoid heights are compared with orthometric heights from the national vertical datum. The RMS difference is about 12cm.
This accuracy is good enough for cadastral surveying. With improved geoid over some erroneous areas in near future, this local geoid can be successfully adopted as a reference for height calculation for cadastral surveying. This geoid can be given out in a grid form, and heights at points can be interpolated if required by users.
<Figure 3> Geoid height in Korea referenced to WGS84 ellipsoid
7. CONCLUDING REMARKS
Cadastral surveying aims at defining and guaranteeing property boundaries, and determining coordinates of all measured points to give information on the size and nature of land use. Using one reference system makes it easier to create boundaries in cadastral maps and to recognize their positions on the ground. For multipurpose cadastre surveys, therefore, GPS positioning is a desirable and adequate method for establishing and strengthening the national and regional geodetic networks. It can provide unified geographic reference frame.
To utilize GPS positioning, well-established control networks and associated procedures are required. Then, coordinates of cadastral points with sufficient accuracies can easily measured. To realize this, the derivation of the consistent relationship between the existing national datum and WGS84 must be one of the most important tasks. From this study, it is found that the transformed results have different residuals, depending on data distribution, their qualities, and sizes of test areas. In KCSC, more efforts are being made to provide a standard for datum transformation and associated procedures that will result in adequate and consistent accuracies over the country.
REFERENCES
Choi, K. S., J. H. Kim, and C. S. Yang, 1997, PNU95 Geoid, The Korean Journal of Geophysical Research, vol.25, No.1
Tsuchiya, A. and H. Tsuji, 1996. Introduction to GPS Surveying (In Japanese), 376p. Japan Surveying Association, Tokyo, Japan.
Yang. C.S., et al., 1997, Test of Cadastral Survey by Use of GPS (In Korean), Technical Report of Cadastral Technology Research Institute, 110p, Korea Cadastral Survey Corporation, Seoul, Korea.
Yang, C. S., K. J. Cho, and J. H. Kim, 1998, Datum Transformation between Korean Geodetic System and WGS84 (In Korean), J. of the Korean Society of Cadastre, Vol.12, No.1