13th International Research Conference General Sir John Kotelawala Defence University Built Environment and Spatial Sciences Sessions
Paper ID: 276 Review on National Geodetic Control Network - Sri Lankan Datum 1999 (SLD_99)
KP Manuranga1#, PGV Abeyratne2, NV Wickramathilake3
1KDU Southern Campus, Sri Lanka 2Sabaragamuwa University, Sri Lanka 3KDU Southern Campus, Sri Lanka
Abstract: Any country in the world has its own geodetic coordinate system and it is Introduction very useful in all types of surveying activities. Accuracy of the geodetic control network is Any country the national geodetic control very important in every aspect. In Sri Lanka network is the most important part of their (Early named as Ceylon), the systematic surveying and mapping activity. Sri Lanka triangulation process began in 1857 and (Early named as Ceylon) is an island, which is completed in 1885. This network was located in an Indian ocean and having a total recomputed with some additional area of 65,610 Km2. In Sri Lanka, the observation done in 1890 due to systematic triangulation began in 1857 with inconsistencies occurring mainly in the the measurement of Kandawala to minor triangulation. But later found that, the Halgasthota base line and the measurements new introduced fixing values have serious were made using 8 and 13-inch vernier error. A new horizontal control network was theodolite. This process was completed in established in 1999 with using Global 1885. This network was connected with the Positioning System (GPS) and there were Indian triangulation network by using included thirty two (32) old network points narrow chain in 1887. This narrow chain was to calculate the transformation parameters run on the Batticaloa – Trincomalie – Manner between the old local datum (Kandawala – Delft area. In 1890, this network was Datum) and the new horizontal network. But recomputed with some additional new system and old system gives different observations due to inconsistencies coordinates for same control points. occurring mainly in the minor triangulation. The minor triangulation is the triangulation In this study, Kandawala network is of first order point triangulation. Using compared with new SLD_99 network to find results of these observations the “new fixing differences between these two networks. values” were introduced and these “new Scaled out figure of the ten secondary control fixing values” were later found to be in points of SLD_99 were observed with GPS serious error. Therefore, this result led to observations and analyzed to perform a significance review of Sri Lankan geodetic network adjustment and for comparison. triangulation network. After that the studies Keywords: GPS, SLD_99, Kandawala Datum, were carried out by the Survey Department Network Adjustment, Triangultion of Ceylon after 1930 and they used new technologies to evaluate the reliability of the
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13th International Research Conference General Sir John Kotelawala Defence University Built Environment and Spatial Sciences Sessions network that formed the Kandawala Datum when examine the residuals of the adjusted and computed values were included in network from the “Report on Sri Lanka Jacksons report and whole process was Datum 1999” some evidences are found to be completed in 1933 (Jackson, 1933). support the above suggestions. After 1996, this network was surveyed by Peculiarities in the national geodetic datum using GPS and Sri Lankan Datum 1999 of a country would drastically affect the (SLD_99) was defined as the new datum. This surveying and mapping system activities in a SLD_99 datum has been used since 1999 for country. It is necessary to identify and rectify surveying and mapping purposes in Sri those to upgrade the national network in par Lanka and both systems (Kandawala and with the international acceptance. SLD_99) were used to the requirement and The main objective of this research is a availability. thorough review on established and the When defining the Kandawala horizontal adjustment of the Sri Lankan national datum, the origin was fixed at Kandawala and geodetic network SLD_99. And also it is the orientation was fixed to Halgasthota from suggested that to carry out an experiment to Kandawala. For the adjustment of the mimic the GPS observations and the network network Everest ellipsoid was used and adjustment of 10 base/secondary base following jackson’s values for semi major (a) stations of SLD_99 by scaled down geometry and semi minor (b) axis were used for of the national network along with adjustment (Jackson, 1933). performing a GPS network adjustment The limitations of this research are, it is difficult a = 2091 2931.80 ft (6374261.613 meter) to carry out GPS observations at the places b = 2085 3373.58 ft (6356108.572 meter) where suspected to be problematic in the SLD_99 datum was introduced and released national network and also the original GPS for survey and mapping purposes in 1999 data (raw data) which were observed for and GPS monument in Institute of Surveying performing national Datum SLD_99 by and Mapping Diyathalawa (ISMD) was used Geodetic Survey Unit in Survey Department as the origin for GPS observations and the of Sri Lanka is not available. network was adjusted by using these Methodology observations. Thirty-two (32) old points in Firstly, the Report on Sri Lanka Datum 1999 Kandawala network were also included for was thoroughly examined to find the this network for determining the accuracy requirements that used to generate transformation parameters between those the SLD99 Datum.Then using the report data, two networks. New network is defined with an experiment was suggested to repeat the the transformation from the WGS 84 GPS observations and perform a network (Geodetic Survey Unit, 2000). adjustment of base and secondary base Abeyratne et al., 2010 pointed out that ISMD stations of scaled down version (1:5000) the origin point, GPS surveys and network geometry of layout of SLD_99 network (base adjustments that used to form the new points were set out at a place where suitable datum (SLD_99) are not optimal and have for making GPS observations). been questioned. They also suggested that Coordinates of base and secondary base the GPS processing and the network stations were collected from Report on Sri adjustment should be re-examined. And also
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13th International Research Conference General Sir John Kotelawala Defence University Built Environment and Spatial Sciences Sessions
Lanka Datum 1999. The ellipsoidal distances between ISMD base points to secondary base AA04 stations were converted to plane distances (04) by using C++ program and this program was used to calculate all azimuths from ISMD base to all other secondary stations. When selecting the location for GPS observation AA05 some considerations were taken to minimize AA03 (03) (05) the obstacle for GPS observation and selected AA02 AA06 ground area was large enough to relocate the (02) scaled down figure of the original network. (06) AA0 7 Tree canopy and other obstacles such is high (07) rise buildings, electrical utilities and water AA10 (10) bodies should be less to get the maximum AA01 ISMD (11) precision in GPS observations. Considering (01) those facts, the playground of Saari Puthra AA08 Maha Vidyalaya – Imbulpe was selected to (08) carry out the GPS observations. Figure 1 AA09 shows the established figure on the ground. (09) obtained from International Doris service Seventeen (17) numbers of network GPS (IDS) and comparison of the GPS marker at observation sessions were carried out in Survey General Office (SGO) shown in table 1. those points to cover the whole base lines same as used for obtained the SLD 99 network. Due to the time frame given thirty minutes observations was done in each Figure 1. Scaled Down all base/ secondary GPS station. observation points
Observed GPS data was processed using When the observed GPS data was processed LEICA Geo office software and processed to used network adjustment it was obtained data was used for network adjustment was variance factor as 37.9098 and the test was written using Geolab software for adjust the failed. GPS network. Then comparison was done between the results obtained from Report on Sri Lanka Datum 1999 shows, the adjustment and available in the Report on Sri same type of adjustment was used by Lanka Datum 1999. Geodetic Survey Unit for Doris point at SGO and ISMD station and it was also failed with Results variance factor 1343.6393. Residuals of the adjustment of SLD_99 were Analysis re-plotted and analyzed. The resultant residuals for both in latitudes and longitudes Figure 2 shows the unpredictable trend of were shown in figure 2. propagation of error throughout the network. Since there are 46 flag residuals in The geodetic coordinates of GPS maker were the Report on Sri Lanka Datum 1999 it calculated and compared. Here the Cartesian clearly shows a possible numerical coordinates of WGS 84 coordinates were instability in the adjustment of SLD_99
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13th International Research Conference General Sir John Kotelawala Defence University Built Environment and Spatial Sciences Sessions datum and the situation is clearly shows in dataset for the network adjustment and to the figure 2. The international standard check whether there is a relationship for the allows none flag residuals in a national geometry and the size. When change the geodetic network. scale, the original shape of the network is remained due to angles are not going to be Computed coordinates of the GPS marker changed. However, the adjustment with show in table 1, confirms and further adding distances was failed and further questioned the values of WGS 84 coordinates analysis of the adjustment could not be made at the ISMD. Though, it would not be affected as done by Report on Sri Lanka Datum 1999. to the local surveys, it gives difficulties when computation done with global data. For example, local GPS coordinates are different when working with the Earth geo potential models (EGMs) which are based on WGS 84.
Secondary
base stations Standard
deviation
Figure 2: Total resultant residuals – both in latitudes and longitudes
Table 1. Comparison of GPS marker coordinates
Coordinate IDS values SGO values Difference
Longitude 79° 52' 26.314640" E 79° 52' 26.3102" E 0.004440" (0.137 m)
Latitude 06° 53' 30.861133" N 06° 53' 30.8699" N 0.008767" (0.270 m)
Ellip. Height -75.692 m -76.238 m 0.454 m
Base and secondary base station network was scaled down to get a similar type of GPS
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13th International Research Conference General Sir John Kotelawala Defence University Built Environment and Spatial Sciences Sessions suggesting that the input variances should be may be having significant residuals as we can tightened. However, as this adjustment is see in the figure 2. used for comparison purposes, this result The effect of heights has not been studied was accepted. One residual was flagged. during the experiment and observed the Variance factor obtained is 37.9098, but this behavior of the residuals of the horizontal value should be close to 1.0, when corrected coordinates only. This experiment was done or removed any measurements containing due to unavailability of the original GPS data any mistakes or large systematic errors. In belonged to SLD_99. contrast to that the same type of adjustment Concerning all the observations made, it is was carried out by Geodetic Survey Unit, Sri questionable the accuracy mentioned in the Lanka (GSU) for DORIS and ISMD stations Report on Sri Lanka Datum 1999 with all the and it was also failed with variance factor of peculiarities noted during the study. 1343.6393. Following recommendations can be drawn. It Conclusion is suggested to re-adjust the SLD_99 network As a first problem with the realization of using the original raw data under minimal SLD_99, COLA has since been identified as constraints. This allows the network to be one of 17 stations with poor antenna stability adjusted freely. However, the effect of and was not recommended for the DORIS adjustment with stages which performed by core network. As such, the base-station SLD_99 should thoroughly be studied before coordinates for the differential GPS baseline the implementation. The data used for the to ISMD may not be as precise as they might field experiment in this study can be used to be. test the effect on the adjustment with adding equations for distances and also be The original adjustment in SLD_99 was not compared the reliability with the adjustment successful only with GPS data and found to be under minimal constraints. acceptable when equations added for distances to the adjustment model. This is A fresh GPS survey should be carried out in fundamentally incorrect with Least Squares the areas where showing high residuals by principle. This situation was not tested using different bases and the result should be during the study due to some practical compared with the SLD_99. inconveniences. References The northern part of the network was not A comparison of existing coordinate densified due to the situation existed during transformation models and parameters in the concerned period. Therefore, four Australia, School of Surveying and Land secondary base stations (Jaffna, Puttalam, Information, Curtin University of Technology. Anuradhapura, and Trincomalee) were
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13th International Research Conference General Sir John Kotelawala Defence University Built Environment and Spatial Sciences Sessions
Abeyratne, P.G.V., Featherstone, W.E. and Jackson, J.E. 1933. Re-computation of the Tantrigoda, D.A. 2010. On the Geodetic Datums of principal triangulation, Surveyor General’s Office, Sri Lanka Colombo DORIS System, Available at
Empirical Evaluation of Published Transformation Parameters from the Australian Geodetic Datums (AGD66 and AGD84) to the Acknowledgement Geocentric Datum of Australia (GDA94). Available at I would like to express my gratitude towards
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