Unconventional applications with single frequency DGPS

RAJNEESH GUPTA, VIVEK BANSAL, BRIG M C DHAMIJA (RETD)

Even though apparently it falls short of double tertiary class II standard, the fact remains that the average correction per linear km is less than 5 mm and per leg of average distance 8 km (i.e. 107÷13) the error is with in 6 cms for each leg of 8 km, which after adjustment where accurate BM heights are known is acceptable for most surveys relating to highways and pipeline & other infrastructure surveys as well as are adequate for topo surveys. Where VI for contouring is 1 meter or more.

DEM/DTM

Under normal conditions, Single Frequency DGPS stipulates that 15 epoch acquisition of data is adequate to achieve the specified accuracy in X, Y & Z. For any DEM / DTM all threevalues of X, Y & Z are necessary. In our trials, an area of about 20 Acres was taken up for preparation of DEM / DTM. Single frequency DGPS was used in Stop & Go mode for acquisition of xyz for points spaced approx every 5 meters. The Base was established in centre of area and Rovers were initialized using initializing bar. The recording interval was kept at one second so that time for data acquisition at each point is not more than 20 seconds. Besides one Base station, 3 Rovers were utilized for data acquisition in grid pattern. In this configuration, the XYZ for approx 3000 points acquired in one day, which was considered to be very economical out turn as compared with other conventional instruments. Alongwith the grid pattern data acquisition, the surveyors picked up positions of breakpoints as well as features of interest.
Repeat observation was taken on well spread 50 points for comparison. The average ellipsoidal height discrepancy on the repeat points was with in 1cm and the maximum discrepancy in repeat point at extreme ends was with in 2 cms. Standard software with 8 degree polynomial was used for drawing DEM. A check line at random was carried out which showed that all elevations were in agreement with in 1.5 cm.

L Section & Cross Section

In the third trial one L section of 500 meters with cross section at every 50 meters was observed by single frequency DGPS in stop & go Kinematic mode and post processed part of this was checked using the
auto level. The discrepancy in relative ellipsoidal heights for this small area was maximum of 3mm at the extremity.

As Built Survey

The fourth project involving survey of ‘As Built’ of oil pipeline was carried out by surveyors M/s Punj Lloyd for their project Dahej-Uran gas Pipeline (DUPL) for GAIL. The pipeline had been laid over a distance of 132 km with turns, curves, and river crossings. The work involved in co-ordinating turns as well as level grading that the as built pipelines adhered to. This was successfully carried out using single frequency DGPS in leap-frog mode by surveyors of M/s Punj Lloyd with our active involvement and under the continued monitoring by their consultant and resulted in considerable time saving, as compared to the conventional systems.

Discussion

In the absence of MSL bench marks, all elevations over longer distance were converted to orthometric heights using EGM-96. However where ever SOI leveling bench marks are made available a suitable Geoid model with interpolation based on suitable polynomial curve fitting should meet the specifications of project indenters. Presently hardlyany contactor has access to SOI bench marks and they usually commence their work from an arbitrarily established point. However by & large accuracy of single tertiary class II suffices for their projects. Accordingly they are able to meet their client’s requirements with relative difference of ellipsoidal heights with EGM96 orthometric heights datumed to mean value of elevation of beginning point which is derived from SOI TOPO maps, so that the elevation data is not too much in variation with absolute M.S.L. elevation in the project area. Hopefully, this situation would improve with technology awareness and opening up measures that SOI has initiated for data availability as well as availability of INDGEOID on which G&RB, Survey of India is currently working. We hope this Geoid model will be available to private surveyors as well.

The 118 km road line was completed in 3 working days. The DEM observations and processing was completed in 2 days and the L section X section was completed in one working day. The asbuilt pipe line of 130 km (approx) was completed in about 25 days under constant scrutiny of the consultant’s engineers.

Concluding remarks

We may also add a word of caution that as far as possible in road/canal/pipeline projects in generally fl at areas the GPS derived heights should as a rule be done section by section of limited lengths.
In extremely fl at areas where the slope measurement is very critical to the project a few small sections of spirit leveling as checkup may be done. One must make appreciation of the tolerance threshold and if this threshold is single tertiary class II, the relative height information can be confidently derived by single frequency DGPS, in leap frog procedure.

Single Frequency DGPS are very versatile, handy and give precise results. The cost of ownership is much lower than those of Dual Frequency as such ROI is realizable in just one or two survey projects. Dual frequency DGPS of course have their own applications.

Acknowledgements

The initial inspiration & guidance for undertaking this study was provided by Brig (Dr.) B. Nagarajan, Director, Geodetic & Research Branch, Survey of India.

The second impetus has been various articles & regular column in various issues of Co-ordinates by Dr. Muneendra Kumar, Chief Geodesist (Retd.) USNGIA. Dr. Kumar has been a strong proponent of utilizing ellipsoidal heights for engineering and Topo survey projects.

The acknowledgement can not be complete without mentioning Prof. P. Misra whose pearls of wisdom exhorts that merely having high level of knowledge of technology is of no help to any body. The challenge before professionals is to be aware of the problem solving abilities of technology and then utilize it for the same.

References

1 ) Redefinition of Indian Geodetic datums Horizontal and Vertical, Brig (Dr.) B. Nagarajan, R.K. Sawhney, Coordinates Vol. III issue 4 April 2007
2 ) Determining of Local gravimetric Geoid, S.K. Singh, Brig (Dr) B. Nagarajan, P.K. Garg, Co-ordinates Vol. III issue 2, Feb 2007.
3 ) Geoid Vol. III issue, Jan-March 2007.
4 ) GPS Based control points for mapping, Jayanta Kumar Ghosh, Ojaswa Sarmg, Amit Goyal,l 5 ) Co-ordinates Vol. II issue 10 Oct 2006.A Cost Effective GPS leveling method versus conventional method
for typical surveying application. Mr. Metin Soycan, GIS Development Asia Pacific, Vol 10 issue 8 Aug 2006
6 ) Ellipsoidal heights and engineering applications, Muneendra Kumar Ph.D, Co-ordinates Vol. III issue 1, Jan 2006.
7 )Global usage of Ellipsoidal Heights Dr. Muneendra Kumar, Coordinates Vol. issue II July 2005
8 ) Orthometric Heights From GPS-Leveling Observations, Brig. (Dr.) B. Nagarajan, S.K. Singh, GIS Development Vol. 9 issue 4 April 2005.