REVIEW ON THE SRI LANKA DATUM 1999 (SLD_99)

Group No : 03

GROUP MEMBERS

SAMM JAYAWARDENA FG282 KMP MANURANGA FG289 MMCS MELLAWA FG290 APP SUBHASHINI FG321

SUPERVISED BY

• MR. PGV ABEYRATNE

INTRODUCTION In Sri Lanka, systematic triangulation began in

1857. It was recomputed with some additional

observations in 1890. Using these observation results “new fixing

values” were introduced for geodetic network (Jackson, 1933) and the datum was named as KANDAWALA.

These “new fixing values” were later found to be in serious error.

This result led to geodetic triangulation network into a significance review.

INTRODUCTION(Cont…)

In 1992 studied the triangulation network again.

The angular observations agreed with the Jackson report. But the distances differed in some places.

Therefore that the Kandawala datum still contains horizontal positional error.

A new horizontal control network was established with GPS in 1999 (datum called SLD_99) which is compatible with the demanded accuracy of the cadastral surveys.

SIGNIFICANCE OF RESEARCH Abeyratne et al., 2010 pointed out that the ISMD

origin point, GPS surveys and network adjustments used to form SLD_99 are not optimal and was questioned.

They also suggested that the GPS processing and the network adjustment should be re-examined.

Some evidences are found to be supported the above suggestions when examined the residuals of the adjusted network from the report on Sri Lanka Datum1999.

OBJECTIVES Review on establishing and the network

adjustment of the SLD_99 datum.

Perform and analyze the GPS observations and the network adjustment of the base (ISMD) and the secondary base stations of SLD_99 by scaled down version the geometry of the above layout.

To study the issues of geodetic network adjustment (GeoLab).

GEODETIC INFORMATION What is the Datum?

Datum gives the relationship between the center of the earth and the center of an ellipsoid and consists with some orientation about their axis.

It is typically defined by the size and shape of an ellipsoid.

Reference Ellipsoid A reference ellipsoid is a mathematically-

defined surface that approximates the geoid, the truer figure of the Earth.

GEODETIC INFORMATION (Cont..) Map projection

A map projection is any method of representing the surface of a sphere or other three-dimensional body on a plane.

Forming SLD_99 Transverse Mercator Projection was used.

GEODETIC INFORMATION (Cont..) Datum Transformation

Datum transformation parameters are mathematical tools that are used to convert the coordinates of a reference system to another system.

The transformation parameters can be determined using a number of stations which have coordinates common to both systems.

Two major transformations are The Molodensky (five parameter)Method The Bursa-Wolf (Seven Parameter Similarity) Method

GEODETIC INFORMATION (Cont..)KANDAWALA Datum Fixed coordinates at Kandawala. Orientation was fixed from Kandawala to

Halgastota. Two base lines and the astronomical azimuth

of each base lines were used to form KANDAWALA Datum.

Two base lines were Kandawala – Halgastota and Vavunativu – Tavelamunai.

The Everest ellipsoid was used for the adjustment.

Base line and interior angles were used to calculate other distances between trig stations.

GEODETIC INFORMATION (Cont..) Sri Lankan Datum 99

Used GPS technology New control network consist of

One base station (ISMD) 10 secondary base stations 262 new control stations 32 old (common) network points

Reference Ellipsoid Everest1830 . In 1998, GPS surveying of the whole network

was completed.

GEODETIC INFORMATION (Cont..) Establishment of ISMD base station.

The position of ISMD was obtained by 7 day SPP and by a single baseline observation from the GPS marker at COLA (DORIS network) located at Colombo. The latter was accepted

Single point positioning methodObservation is done by using a single receiver over a relatively long time period in order to compensate inherent errors in GPS observations.

Differential positioning methodPositioning is done with respect to a known point differentially. GPS Marker (DORIS) point was used as base and ISMD as rover.

GEODETIC INFORMATION (Cont..) Data from Base station and Secondary base

stations were adjusted using GeoLab. The results based on this adjustment was failed.

Reasons for that error was not identified. (not described in the Report on Sri Lanka Datum 1999)

Again, the network adjusted by adding distances as observations, which were calculated with GPS coordinates.

It was reported that to reach the expected precision as one into ten million. Anyway there were 47 flag residuals in the finally adjusted network.

GEODETIC INFORMATION (Cont..) DORIS

Doppler Orbitography and Radiopositioning Integrated by Satellite.

High accuracy for heights than GPS. COLA point is currently inactive since 1994. COLA has since been identified as a poor

antenna stability stations. TOPEX/Poseidon and Jason are the two

altimetry satellites used for DORIS observation. Other DORIS satellites are the ERS, Envisat,

SPOT and Cryosat-2 satellites.

GEODETIC INFORMATION (Cont..)

PECULARITIES OF SLD_99 Unstable GPS observation from COLA GPS marker

to ISMD which processed with Leica SKI PRO. Adding observation equations for distances based

on calculations was contradiction with the fundamental of Least Squares principle.

Adjustment was done in two steps. First the base (ISMD) and secondary bases (10) and subsequently other 262 points. Hence the adjustment was done under highly constraints. This would result to be deformed the whole network.

As a result of that we can see the residuals with no trend.

ERRORS IN LATITUDESSecondary base stations

Standard deviation

ERRORS IN LONGITUDESecondary base stations

Standard deviation

TOTAL ERROR IN ALL STATIONS

PECULARITIES OF SLD_99 (Cont..) The coordinates have been used

incorrectly for the Single point positioning GPS survey to position ISMD.

Computed coordinates of the COLA was matched with Abeyratne et al, 2010.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)

PECULARITIES OF SLD_99 (Cont..) Geodetic Survey Unit people proved

different accuracies for the same network.

Less number of control points in Northern and Eastern areas.

PROCESS Base and secondary base station network was

scaled down to check whether there is a relationship for the geometry.

Because angles are not change when scaling down. Then GPS observation was done on those stations

and processed data to perform a network adjustment.

This network adjustment was done by the GeoLab software.

The results of the data obtained (GeoLab programme) was failed. The reason could not identified.

PROCESS (Cont..) GeoLab software used for network adjustment

GeoLab V2001.9.20.0 Program written for adjustment was very

complex. For the network adjustment it required IOB file.

(*.iob) IOB file used to input GPS data and it has

standard format.

PROCESS (Cont..) Sample IOB file used in GeoLab

PADJ NO NONONO YES CONF YES YESYESYES CON m * Control Stations:PLH 111 Control 1 n 6 41 28.1315200 e 80 44 40.128520 409.407 m  * Network Stations: PLH Point 1 n 6 41 27.6709200 e 80 44 39.439510 341.764 m PLH Point 2 n 6 41 27.6683500 e 80 44 38.621720 287.376 m PLH Point 3 n 6 41 28.0898900 e 80 44 38.417950 192.876 m PLH Point 4 n 6 41 27.8511300 e 80 44 37.425290 186.875 m

PROCESS (Cont..)* GPS Measurements 3DD DXYZ Control 1 Point 8 9.100 -3.524 17.559 m CORR CT UPPR 0.5407 ELEM 0.0000004 0.0000006 0.0000001 ELEM 0.0000029 0.0000006 ELEM 0.0000003 ELEM 0.0004 0.0009 0.0003

PROCESS (Cont..)

VIEWS / REMARKS (Cont..) There were some similarities between

results of experiment and the results of Geodetic Survey Unit (GSU).

Both adjustment computations were failed. Sometimes this can happen due to bad

scale matrix. This square matrix should be

Condition number = 1 (value one) Inverse matrix = original matrix

VIEWS / REMARKS (Cont..)STATISTICS SUMMARY

Experiment results SLD_99 resultsResidual Critical

Value 3.6568 1.7248

Number of Flagged Residuals

1 0

Estimated Variance Factor

37.9098 1343.6393

Number of Degrees of Freedom

111 3

Chi-Square Test on the Variance Factor

2.9624e+01 < 1.0000 < 5.0254e+01

4.3119e+02<1.0000<1.8679e+04

******** THE TEST FAILS ********

RECOMMENDATIONS Network should be adjusted using calculated

distances to find any kind of similarities. Do a GPS observation on original points of

SLD_99 and network adjustment. Original GPS data of SLD_99 can re process by

different methods such as MATLAB, GPS Processing software.

Stations which have considerable error should re observe using GPS.

Joining SLD_99 network to the ITRF. Do a free adjustment with a minimal constraint.

REFERENCE P.G.V. Abeyratne, W.E. Featherstone and D.A.

Tantrigoda, 2010 , On the Geodetic Datums in Sri Lanka

http://ids-doris.org/network/sitelogs/station.html?code=COLA, 29/03/2012 ,7.30pm

Jackson, J.E. 1933. Re-computation of the principal triangulation, Surveyor General’s Office, Colombo

Geodetic Survey Unit (2000) Report on Sri Lankan Datum 1999, Survey Department, Colombo

Thank You !