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The definition of a physical height system is an essential geodetic application of gravity field modeling. It is desirable to get global height systems consistent at a level of a few centimeters or better, for both scientific and practical reasons. To achieve this goal, a high-resolution and high-precision model for regional gravity field refinement is of great importance, especially for mountainous areas where the gravity field varies rapidly or in developing countries where only sparse observations are available.This study focuses on the gravity field modeling in Colorado, USA, which is regarded as a very challenging study area, due to the rugged terrain, high elevation and varying gravity field. We apply an approach for regional gravity modeling using spherical radial basis functions (SRBF), developed at DGFI-TUM in the last two decades. The very long-wavelength gravity field component as well as the very short-wavelength component are removed by a global gravity model and a detailed terrain model, respectively, following the so-called "Remove-Compute-Restore" (RCR) procedure. The gravity field is calculated by parameter estimation, from which the final quasi-geoid heights are computed.Two types of real observation data are used in this study. The terrestrial data and the airborne data are combined, and their relative weighting is determined by the method of variance component estimation. Comparisons are made between the combined solution and the solutions of using terrestrial data only as well as using airborne data only. In this presentation, we also include comparisons with other solutions based on different modeling approaches but applied to the same data sets. [ABSTRACT FROM AUTHOR]

The establishment of the International Height Reference System (IHRS) as part of a Global Geodetic Reference System (GGRS) is currently one of the main GGOS activities, as the physical height is proposed to be an Essential Geodetic Variable. Since 2017 the two IAG/GGOS joint working groups (JWG) 0.1.2 "Strategy for the Realization of the IHRS" and JWG 2.2.2 "The 1 cm geoid experiment" work close together to investigate open issues regarding its practical realization. The contribution of JWG 2.2.2 is the comparison of different methodologies for potential value calculation with identical input data (terrestrial and airborne gravity observations in Colorado, United States). Then, differences in the results, which indicate disparities in the computation methodologies, may be analyzed and preferably reduced by specifying a set of minimum calculation guidelines. These guidelines should then be used for the definition of the IHRS, which is composed of gravity potential values at selected reference stations. This contribution highlights the TUM solution, which was submitted in the frame of "the 1 cm geoid experiment" and explains exemplarily at the scenario of Colorado the difficulties of defining a 1 cm geoid, which is defined as the minimum requirement of the IHRS. Thereby, we give special emphasis to the occurring differences between terrestrial, airborne and satellite gravity observations as we regard their combination method as one of the main drivers of the final performance. This is particularly significant in the context of our test area, which includes highly mountainous as well as very flat regions and the requirements for potential value calculation might differ according to the terrain. For the calculation we use a joint Least Squares Collocation approach which includes stochastic information in form of variance/covariance information for all components, enabling us to assign realistic uncertainty estimates to our results. This is of great importance for the reference station definition. In the end, we compare and verify our result either in the general context of all participating groups or by highlighting and explaining the differences to one specific solution. From the results and insights of these experiments, we try to derive generalized conclusions on the realization of the IHRS. [ABSTRACT FROM AUTHOR]