ISG

   International Service for the Geoid

   

Search in ISG Website



Links:










      

      

Colorado Experiment


The main objective of the 1-cm geoid experiment in Colorado was the estimation and comparison of geoid undulations, height anomalies and potential values using the same input data and different methodologies for the gravity field (geoid and quasi-geoid) modelling. The comparison of the results allowed the identification of the differences caused by disparities in the computation methodologies. To get as similar and compatible results as possible, this experiment also included the definition of a set of basic processing requirements (see Sánchez et al. 2018). The input data were provided by the US National Geodetic Survey (NGS) and comprised terrestrial and airborne gravity data, a digital terrain model, and GNSS/levelling data for an area of about 500 km x 800 km in Colorado, USA. With these data, fourteen groups from fourteen countries computed the mentioned gravity-field estimates using their own methodologies (and software).After one preliminary run and two iterations, each group provided geoid and quasi-geoid models at a resolution of 1′x1′ for the complete area and geoid heights, height anomalies, and geopotential values at 223 benchmarks of the Geoid Slope Validation Survey 2017 (GSVS17). The methodologies applied in this experiment, the comparison of the results, and the main conclusions are presented in detail the Special Issue “Reference Systems in Physical Geodesy” of the Journal of Geodesy. The basic agreements of the experiment can be found here.

The 1-cm geoid experiment is a joint effort of the Focus Area Unified Height System of the Global Geodetic Observing System (GGOS) and its working group 0.1.2: Strategy for the realization of the IHRS (Sánchez 2019); the IAG Sub-commission 2.2: Methodology for geoid and physical height systems (Ågren and Ellmann 2019); the joint working group 2.2.2: The 1 cm geoid experiment (Wang and Forsberg 2019); and the study group 0.15: Regional geoid/quasi-geoid modelling - Theoretical framework for the sub-centimeter accuracy of the IAG Inter-Commission Committee on Theory – ICCT (Huang and Wang 2019).

Input data over Colorado, USA


Terrestrial Gravity Dataset:

  • File Name: Colorado_gravity_data.dat
  • Description: point file with terrestrial gravity observations based on the NGA/NGS dataset. This is a ‘cleaned’ dataset. Number of points = 59,303.
  • Geographic Limits: 35°N - 40°N, 250°E - 258°E
  • File Format: ASCII with columns associated with: latitude(deg), longitude(deg), orthometric height above MSL(m), gravity(mGal), survey ID, year.
  • File Download: click here [0.8 MB]

GRAV-D Airborne Gravity Dataset:
  • File Name: GRAVD_ms05_median_debiased_1hz.txt
  • Description: point file with airborne gravity observations based on the block MS05 of the GRAV-D survey. This is a de-biased detaset using the max median technique, and then resampled at 1 Hz sampling rate. Number of points = 283,716.
  • Geographic Limits: 34.5°N - 38.9°N, 250.8°E - 258.7°E
  • File Format: ASCII with columns associated with: block, line, time(YYYYMMDDHHMMSS.SSS), latitude(deg), longitude(deg), ellipsoidal height(m), gravity(mGal). Coordinates in the IGS08 system, ellipsoidal height above the GRS80 reference ellipsoid
  • File Download: click here [5.1 MB]

Digital Elevation Model (DEM):
  • File Name: Colorado_dem.grd
  • Description: grid file with topographic heights based on the SRTM v4.1 dataset from https://srtm.csi.cgiar.org/srtmdata/
  • Geographic Limits: 35°N - 38.6°N, 250°E - 258°E
  • File Format: DTU/GRAVSOFT grid format with center cell registration. Text file read from North to South and West to East (the first row is 40°N from 250° to 258°)
  • File Download: click here [82.2 MB]

Global Gravity Model (GGM):
  • File Name: xGEOID17RefB.gfc
  • Description: spherical harmonic coefficients of the gravitational potential, provided by NGS
  • File Format: ICGEM ASCII format (.gfc) with columns associated with: keyword, degree n, order m, cosine coefficient Cnm, sine coefficient Snm
  • File Download: click here [43.9 MB]

Other GGMs with the same file format are available at the ICGEM website

Validation data over Colorado, USA


Historical GPS/Leveling Dataset:

  • File Name: Colorado_GPSBM_v2.2_08Feb18.txt
  • Description: point file with GPS/leveling data from Colorado and surrounding states. Number of points = 509, with 467 benchmarks from the NGS Integrated Database (IDB) and 42 benchmarks from NGS OPUS-Share Tool (https://www.ngs.noaa.gov/OPUS/). Sparse spatial distribution due to the mountainous topography, expected accuracy of geoid undulationms around 4 to 5 cm. Original dataset in NAD83/NAVD88, converted to IGS08 for this experiment.
  • Geographic Limits: 35°N - 40°N, 250°E - 258°E
  • File Format: ASCII with comma separated columns associated with: index, latitude(deg), longitude(deg), IGS08 geodetic height(m), NAVD88 orthometric_height(m), pid, state, designation.
  • File Download: click here [36.1 KB]

GSVS17 GPS/Leveling Dataset:
  • File Name: GSVS17_FieldData.xlsx
  • Description: point file with GPS, leveling, gravity and astrogeodetic deflections of the vertical observations, acquired along a 360 km line (from Durango to Walsenburg), ranging from 1900 to 3300 meters in elevation, with a station spacing of about 1.6 km. Number of points = 223. Expected accuracy of geoid undulations around 1.5 cm. For further details, please see https://geodesy.noaa.gov/GEOID/GSVS17/index.shtml
  • Geographic Limits: 37.2°N - 37.7°N, 252.1°E - 255.5°E
  • File Format: EXCEL with column descriptions in the table header.
  • File Download: click here [64.7 KB]

List of gravimetric geoid and quasi-geoid models


Computation method Acronym Authors           
Institutions Status
DOI
1D FFT with Wong-Gore modification of the Stokes kernel FFTWG V.N. Grigoriadis, et al.
AUTh PUBLIC
Spherical FFT with Wong-Gore modification of the Stokes kernel FFTWG R. Forsberg
DTU PUBLIC  
Windowed Least-Squares Collocation WLSC Barzaghi, et al. POLIMI PUBLIC
Residual Least-Squares Collocation RLSC M. Willberg, et al. IAPG PUBLIC
Spherical Radial Basis Functions SRBF Q. Liu, et al. DGFI PUBLIC  
Least Squares modification of Stokes integral with additive corrections LSMSA J. Ågren KTH PUBLIC  
Least Squares modification of Stokes integral with additive corrections LSMSA M. Varga, et al. GEOF-NTIS PUBLIC  
Least Squares modification of Stokes integral with additive corrections LSMSA M.S. Işık, et al. ITU-GRG PUBLIC
Least Squares modification of Hotine integral with additive corrections LSMHA M.S. Işık, et al. ITU-GRG PUBLIC
Window Remove-Restore technique with Meissl modification of the Stokes kernel WRRT H.A. Abd-Elmotaal Minia PUBLIC  
Remove-Compute-Restore UNB Stokes-Helmert scheme UNBSH K. Matsuo, R. Forsberg GSI/DTU PUBLIC  
Spectral combination approach SCA T. Jiang, et al. CASM PUBLIC  
Spherical harmonic analysis scheme with Wong-Gore modification of the Stokes kernel SHAWG Y.M. Wang, et al. NGS PUBLIC  



References

Ågren J and Ellmann A (2019). Report of the Sub-commission 2.2: Methodology for Geoid and Physical Height Systems, Reports 2015-2019 of the International Association of Geodesy (IAG), Travaux de l'AIG Vol. 41, Commission 2, pages 33-38.

Huang J and Wang YM (2019). Report of Joint Study Group 0.15: Regional geoid/quasi-geoid modelling – theoretical framework for the sub-centimetre, Reports 2015-2019 of the International Association of Geodesy (IAG), Travaux de l'AIG Vol. 41, Inter-Commission Committee in Theory, pages 40-45.

Sánchez L (2019). Report of the GGOS Focus Area “Unified Height System” and the Joint Working Group 0.1.2: Strategy for the Realization of the International Height Reference System (IHRS), Reports 2015-2019 of the International Association of Geodesy (IAG), Travaux de l'AIG Vol. 41, Global Geodetic Observing System (GGOS), pages 42-51.

Sánchez L, Ågren J, Huang J, Wang Y M, Forsberg R (2018). Basic agreements for the computation of station potential values as IHRS coordinates, geoid undulations and height anomalies within the Colorado 1 cm geoid experiment. Version 0.5, October 30, 2018.

Wang YM and Forsberg R (2019). Report of the Joint Working Group 2.2.2: The 1 cm geoid experiment, Reports 2015-2019 of the International Association of Geodesy (IAG), Travaux de l'AIG Vol. 41, Commission 2, pages 56-58.

Jarvis A, Reuter HI, Nelson A, Guevara E (2008). Hole-filled SRTM for the globe: Version 4. Available from the CGIAR-CSI SRTM 90m Database (http://srtm.csi.cgiar.org).