G13A Towards a New Vertical Datum Daniel R. Roman 1, Xiaopeng Li 2, Simon A. Holmes 3, Vicki A. Childers 4, and Yan M. Wang 1 1. Geosciences Research Division, NOAA's National Geodetic Survey, Silver Spring, MD, United States. 2. Earth Resources Technology, Inc., NOAA's National Geodetic Survey, Silver Spring, MD, United States 3. Stinger Ghaffarian Technologies, Inc., NOAA's National Geodetic Survey, Silver Spring, MD, United States. 4. Observations and Analysis Division, NOAA's National Geodetic Survey, Silver Spring, MD, United States. APPROACH Aerogravity were combined with the GOCO03S model (Mayer-Gürr et al. 2012) to develop a model of the Earth’s gravity field that are independent of NGS’s terrestrial data. The aerogravity profiles were collected under the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) program (Smith 2007) specifically with the intent of developing a cm-level accurate geoid height model. These data are collected between 6-11 km in elevation and at 10 km track spacing. This necessarily limits the lower resolution of the aerogravity to 20 km. In fact, after analysis, it has been determined to be reliable to about 40 km, though future filtering improvements should result in achieving 20 km resolution. The tracks themselves are typically about 500 km long due to aircraft and fuel limitations. This still allows significant spectral overlap with satellite-only models such as GOCO03S built using GRACE and GOCE (Drinkwater et al. 2006) data. The expected resolution of the aerogravity is currently between degrees 100 and about 540 (40 km equivalent) expressed in ellipsoidal harmonics. To permit a smoother transition, the GOCO03S model is tapered to zero between degrees 100 and 200 with the half power point at degree 150. The aerogravity are developed in a harmonic model and blended from zero to full power in the same range, making the effective transition from aerogravity to satellite data at about degree 150. Finally, the EHM is filtered to zero at degree 540 with the half power point at degree 450. The effective range of the aerogravity is then between degrees 150 and 450. To provide a consistent comparison, EGM2008 was similarly adapted. GOCO03S was blended in with a half power point of 150 and then filtered to zero at degree 540 with a half power point at degree 450. Since both this adapted EGM2008 model and the combined NSG satellite-aerogravity model are built on GOCO03S through degree 150, differences seen between the models will emphasize any long to intermediate wavelengths between terrestrial data in EGM2008 and the aerogravity (Figure 1). Much of the terrestrial data that NGS holds was available to the National Geospatial-Intelligence Agency and was used in making EGM2008. Hence a comparison to this grid of differences to the terrestrial points values will highlight potential corrections. Figure 2 provides the comparison of the existing terrestrial data to a EHM developed from the GRAV-D aerogravity. Points are collected in surveys and are thought to have systematic features associated within each survey. Points in surveys as an aggregate were evaluated specifically looking for potential biases or trends. Those points with significant systematic differences were rejected lower right of Figure 2. The good data are shown in Figure 3 are then the retained data that are gridded, while Figure 4 shows the magnitudes of the rejected data. Figure 5 shows the interpolated corrections determined from Figure 1. When these were applied to the rejected points in Figure 4, the resulting “cleaned” data are then shown in Figure 6 and are available for further analysis. Figure 7 shows a grid of all passed and cleaned data. CONCLUSIONS & OUTLOOK Aerogravity have demonstrated that significant systematic effects exist in terrestrial gravity data held at NGS. These data were separated and corrected based on differences between EGM2008’s terrestrial signal and the aerogravity signal in the bandwidth between degrees 150 and 450. The corrected values were then combined with the data that had passed the initial analysis step and then compared along the GSVS 11 profile. A definite improvement was seen though not yet at the desired cm-level of accuracy. Continued refinements should improve both the aerogravity products as well as the cleaned gravity data. Ten years remain to determine a cm-level accurate geoid height model. This will likely be sufficient time but research and development must continue at a vigorous pace to ensure it is done along with the production and collection schedules. REFERENCES Drinkwater MR, R Haagmans, D Muzi, A Popescu, R Floberghagen, M Kern, and M Fehringer (2007) Proceedings of 3rd International GOCE User Workshop, 6-8 November, 2006, Frascati, Italy, ESA SP-627. Mayer-Gürr T, D Rieser, E. Hoeck, JM Brockman, W-D Schuh, I Krasbutter, J Kusche, A Maier, S Krauss, W Hausleitner, O Baur, A Jaeggi, U Meyer, L Prange, R Pail, T fecher, and T Gruber. (2012) The new combined satellite only model GOCO03s. Paper S2-183, GGHS Meeting in Venice, Italy 9-12 OCT Roman DR, M Véronneau, D Avalos, X Li, and J Huang (2013) Integration of gravity data into a seamless transnational height model for North America, Proceedings of the Gravity Geoid and Height Systems, International Association of Geodesy Symposia, Springer-Verlag Heidelberg (in review). Saleh J, X Li, YM Wang, DR Roman, and DA Smith (2012) Error analysis of the NGS’ surface gravity database, J. Geodesy, DOI /s Smith D (2007) The GRAV-D project: gravity for the redefinition of the American Vertical Datum. Available online at: Smith DA (2011) Initial results of the 2011 Geoid Slope Validation Survey, Abstract G52A-04 presented at 2011 Fall Meeting, AGU, San Francisco, Calif., 5-9 Dec. Smith D.A., M Véronneau, D.R. Roman, J. Huang, Y.M. Wang, and M.G. Sideris (2013) Towards the Unification of the Vertical Datum Over the North American Continent. Chapter 36 in: Z. Altamimi and X. Collilieux (eds.), Reference Frames for Applications in Geosciences, International Association of Geodesy Symposia 138, DOI 10./1007/ _36 © Springer-Verlag Heidelberg Tapley BD, S Bettadpur, M Watkins, and C Reigber (2004) The gravity recovery and climate experiment: Mission overview and early results. GEOPHYSICAL RESEARCH LETTERS 31 (9). ABSTRACT The National Geodetic Survey (NGS) is responsible for maintaining and improving the National Spatial Reference System. This paper particularly focuses on developments leading to a new vertical datum to replace the existing North American Vertical Datum of 1988 (NAVD 88). This new model will be developed from a combination of satellite, airborne, and terrestrial gravity data to define a gravimetric geoid height model. In particular, the aerogravity data collected as a part o the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) Project are intended to help achieve the goal of a cm-level accurate geoid model to serve as the new vertical datum. The different data sources have been melded into a single gravity field model consistent across the entire spectrum to about 2 km resolution. A previous comparison developed a localized model over just the southern Texas region, where the Geoid Slope Validation Study for 2011 (GSVS 11) demonstrated that it was possible to achieve the desired accuracy. This new model was developed using methodology consistent at regional to national scales following techniques used to make USGG2009 and USGG2012, but now incorporating aerogravity. This new model proves out the basic concepts behind GRAV-D in that the aeorgravity bridge the spectral gap between satellite and terrestrial data and provide the requisite improvements to the derived gravimetric geoid height model - all without artificially targeting a solution to a specific test area. Additional comparisons were made to tidal bench mark data observed by GPS in combination with ocean topography models to validate the behavior of the model in the coastal regions. INTRODUCTION EGM2008 was built using GRACE (Tapley et al. 2004) gravity field models and a significant terrestrial gravity data set. In parts of the United States, much of this data is suspect. This paper focuses on a data domain solution to cleaning such data. A previous study (Smith 2011) demonstrated that satellite, airborne and terrestrial gravity data could be spectrally merged to produce a gravimetric geoid height model accurate most likely to the cm-level. While this certainly met the desired goal of accuracy, the approach did not resolve issues in the underlying terrestrial data (Saleh et al. 2012), Poorer performing data were detected using established techniques (Roman et al. 2013). New here is that appropriate corrections were determined & applied, and a final comparison then made to determine if this approach is viable for cleaning the extensive terrestrial gravity data set at the National Geodetic Survey (NGS), where nearly two million points are used in modeling. This is very urgent because NGS will adopt a new vertical datum in 2022 based on a gravimetric geoid. A regional model that is acceptable to Canada, Mexico, Central American nations and nations in the Caribbean is also very desirable (Smith et al. 2013). To that end then, resolving systematic errors in the existing NGS gravity data is essential and away forward is needed. The magnitude of these gravity errors is usually just a few mGal’s but their spatial extent results in errors at the dm-level. This work demonstrates that detecting and cleaning such data is feasible though much work remains to be done. Figure 1. Difference between terrestrial and airborne gravity between degrees 150 and 450 ( km spatial resolution). Multi-mGal features spanning 100 km can result in dm-level geoid errors. GOCE cannot resolve such errors. Figure 2. Separation of points in surveys bases on comparisons with aerogravity. Retained data statistics are on left and rejected data on right. Note that data are broken out by survey line. Figure 3. Grid of retained data. Suspect data (black dots in Figure 2) were removed and retained data were gridded. Figure 4. Plot of suspect data not used in Figure 3. Note the bias of nearly 1 mGal in this subset of data; evidence of biased data. Figure 5. Corrections. At each suspect point in Figure 4, a correction was interpolated from the Figure 1 grid. Figure 6. Rejected data (Figure 4) are corrected (Figure 5) and new residuals formed. Note reduction Of mean value from Figure 4. Figure 7. Grid of retained and corrected/cleaned data. COMPARISON TO GSVS 11 LINE In 2011, NGS collected GPS, leveling, terrestrial gravity, astrogeodetic Deflections of the vertical, and gravity gradients all along a 330 km line in South Central Texas (Figure 8). This line of data is intended for geoid studies and was collected under the the Geoid Slope Validation Study for 2011 (GSVS 11) project (Smith 2011). This line of data was collected under the TX09 aerogravity collection region with the specific intent of determining the feasibility of cm-level accuracy in gravimetric geoids. It demonstrated that this was feasible. However, the approach relied strictly on a spectral merging of data sets that had significant systematic effects as is seen in Figure 1. The intent here then is to make multiple comparisons to determine if the cleaning approach employed here was successful. Models for comparison were: (1) the enhanced version of EGM2008 (i.e., with GOCO03S blended in), (2) a gravimetric geoid model developed from all NGS data that used the enhanced EGM2008 model as a reference field, (3) a geoid model built using only those data deemed to be relatively unbiased (i.e., “good”) on the enhanced EGM2008 model (Figure 3), and (4) the good and cleaned data in a geoid model based on the enhanced EGM2008 model (Figure 7). Comparisons were made to 218 GPS/leveling marks (Table 1), which were all minimally constrained to obtain an internal consistency but not be tied to any datum specific biases and trends (known to exist in leveling based on NAVD 88. The comparisons show that dropping suspect data helped somewhat (S.D. improved from 1.6 cm to 1.5 cm), but that cleaning the data and using that extra information did better still (S.D. of 1.3 cm.). This is only a first attempt though and is based on the assumption that the terrestrial signal in EGM2008 is directly related to that of the NGS terrestrial data. Data SetEGM2008EGM all FAA EGM Unbiased FAA EGM Unbiased + Cleaned Average (m) S.D. (m) Table 1. Comparisons to the GSVS 11 GPS/leveling line with interpolated values from geoids determined from EGM2008, EGM2008 plus NGS terrestrial data, EGM2008 plus unbiased data only, and EGM2008 plus unbiased and cleaned gravity data. Removing biased data helped but cleaning and restoring was better. Figure 8. Geoid Slope Validation Study Line for 2011 (GSVS 11). Of primary interest were minimally constrained GPS on leveling on 218 bench marks. These provide an independent comparison for geoid models developed using different techniques.