Western Nepal Topographic Mapping Project (WNTMP) – Phase I
Final Report on Control Surveys
Volume- III Definition of Gravimetric Geoid
HMG Survey Department of Nepal
FM – International FINNMAP
This report includes following details regarding gravity observations and data:
List of observations of western Nepal:
Gravity observations were done in GPS points.
Number of gravity observed stations/points: 56 stations.
Number of gravity stations specified as old British and other points: 43 stations.
List of observations of eastern Nepal:
Gravity observations were done in GPS points.
Number of gravity observed stations/points: 43 stations.
Number of gravity stations specified as other gravity stations: 10 stations.
Number of gravity stations specified as old British points: 95 points.
Note: For numbering of stations, latitude, longitude, ellipsoidal height, orthometric height, and gravity values – see Annex J and Annex K of WNTMP report.
“Surface Gravity Information of Nepal and Its Role in Gravimetric Geoid Determination and Refinement” – By Niraj Manandhar published in Nepalese Journal of Geoinformatics -9, 2067.
Gravity base was established in the country during 1981-84 British Military Survey.
The gravity reference system: ISGN 1971
Instruments used: LaCoste Romberg Model G gravimeter
Number of Stations observer: 21 out of 36 (MODUK)
Fundamental gravity base at Tribhuvan International Airport (TIA), Kathmandu was established designated as KATHMANDU J. IN 1981 gravity transfer from ISGN71 station BANGKOK to Kathmandu was made. At this time 45 gravity stations are surveyed at 35 different locations, mostly at the airports and accessible places around the country.
KATHMANDU J = 978661.22+-0.047 mgal
Total 375 gravity detail stations were established till now which includes WNTMP + ENTMP.
First order gravity network of Nepal:
According Niraj sir on his this article:
Number of first order gravity points: 36 points, 25 points in airport and airstrips and remaining 11 in government buildings, army barracks, and police stations.
Nagarkot Geodetic Observatory, Nepal
Observations, Corrections, and Results
Gravity ties to Kathmandu and Simara airports.
National Geodetic Survey, USA
HMG Survey Department, Nepal
University of Colorado University, Boulder, USA
• To establish a reference datum for the local gravity network in Nepal.
• To establish points that may be remeasured to reveal changes of elevation in future years.
The Fundamental Absolute Gravity Stations (FAGS) was established in 1991 at Nagarkot in assistance with above three organizations.
The absolute gravity value measured at Nagarkot (FAGS -1). The corrected value of the FAGS -1 indoor point at ground level is 978494834.7 +-6.7 microgal.
The gravity gradient at floor level (0 to 0.43m) was 4.4194 microgal/cm.
Relative ties were made to three GPS points: Nagarkot, Kathmandu airport, and Simara airport. The relative differences from FAGS-1 to these points are as follows:
Nagarkot FAGS-1 978494834.7+-6.7 microgal
Nagarkot GPS -0.691+-0.002 mgal
KATHMANDU J +166.469+-0.005 mgal
SIMARA J +368.599 +- 0.017 mgal
SIMARA GPS +368.706 +- 0.013 mgal
The ties were undertaken using a pair of Model D LaCoste Romberg gravimeters. For Nagarkot GPS point which is less than 10m from the brick building where GPS measurements were made.
The absolute accuracy of the 1991 measurements is +- 6 microgals or approximately +- 1.5cm in elevation.
Airborne Gravimetry in Nepal
Article Title: “Geoid of Nepal from Airborne Gravity Survey”
Niraj Manandhar, Kalyan Gopal Shrestha, Rene Forsberg
Earth on the Edge: Science for a Sustainable for a Planet, IAG Symposia 139, Springer-Verlag Berlin Heidelberg 2014
An airborne gravity survey of Nepal was carried out December 2010 in cooperation between DTU-Space, Survey Department of Nepal, and National Intelligence Agency- NGA USA.
• To provide data for a new national geoid model, which will in turn support GPS surveying and national geodetic infrastructure.
• To provide gravity information for future global gravity field EGM 2020.
• To provide training of physical geodesy to Nepalese geodesists.
• To make an improved estimate of the geoid at Sagarmatha allowing an independent height determination by GPS.
• To collect airborne data which will provide an independent validation of GOCE and EGM08 in most mountainous part of the Earth.
Airborne gravity survey details:
Aircraft : Beech King Air aircraft by Danish Company COWI.
57 flightlines @ spacing of 6 nautical miles.
13 flight days. Dec 4-17, 2010
Coverage of border regions of the country towards india and china was not possible. The survey operations were a major challenges due to excessive jet streams at altitude as well as occasional excessive mountains waves. Even a reflight attempts did not provide useful data due to persistent wind effects of Annapurna and Mustang valley regions. Surface gravimetry data of those regions should be used. The reported results appeared accurate to few mgal – despite the large 400 mgal + range of gravity anomaly changes from indian plains to Tibetan plateu.
Instrument installed on aircraft: Lacoste Romberg S –type gravimeter which was controlled by Ultrasys Control System with a number of GPS receivers onboard the aircraft and on ground providing the necessary kinematic positioning.
Checkan –AM gravimeter alongside LCR gravimeter with the special goal to augment the LCR gravity data in expected turbulent conditions.
The airborne gravity reference point at Kathmandu airport tied to NOAA absolute gravity measurement at Survey Department and at Geodetic Observatory Nagarkot. The actual gravity values at altitude differ by 1000’s of mgals from reference values due to the height changes and Eotvos corrections.
The overall accuracy of airborne gravity data collected was 3.3 mgal. The preliminary geoid was computed and the accuracy of computed geoid was likely at the 10-20cm in the interior of Nepal but higher near the border due to lack of data in China and India. This preliminary geoid was compared to eight reasonable GPS-levelling precise data in the Kathmandu valley. It is seen that a significant geoid improvement has been obtained relative to the EGM-08 geoid.
The computed height of Sagarmatha which confirms the official Survey Department height 8848 m within range of +- 1m, based on an ellipsoid height of the snow summit of 8821.40 +- 0.03m.
Present Works in Gravity: Refinement of Geoid around Sagarmatha region
There has been planning for the purpose of refinement of geoid around Sagarmatha regions for the computation of orthometric elevation of Sagarmatha from GPS heightening and geoidal undulations. To refine the geoid around Sagarmatha, we need dense/extra surface gravimetry data which will improve the past geoid computed from 2010 airborne gravity geoid. The surface gravimetry observations were planned to take on 5km*5km grids, around that region.