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Blackwell J.P.,National Geodetic Survey
ISCORD 2013: Planning for Sustainable Cold Regions - Proceedings of the 10th International Symposium on Cold Regions Development | Year: 2013

The National Geodetic Survey (NGS) traces its roots back to the first scientific agency of the United States, the Survey of the Coast. In 1807, Congress endorsed the request of President Thomas Jefferson to establish a Federal agency to survey the coasts of the United States in support of growth and development of our young country. While the Survey's name has changed over the years, the need for accurate surveying remains the same. Today's NGS provides the United States and its territories with the most accurate geodetic framework for all types of geodetic control, indispensable to surveying, engineering, and mapping activities. Technology and methodology have evolved over the years, but our mission «to define, maintain, and provide access to the National Spatial Reference System to meet our nation's economic, social, and environmental needs» remains. © 2013 American Society of Civil Engineers. Source

Pearson C.F.,National Geodetic Survey | Pearson C.F.,University of Otago | Snay R.S.,National Geodetic Survey retired | McCaffrey R.,Portland State University
International Association of Geodesy Symposia | Year: 2014

This paper describes a block model of tectonic deformation for the western part of the contiguous United States (i.e. west of longitude 100°W and between latitudes 31°N and 49°N). NOAA’s National Geodetic Survey (NGS) uses velocities predicted by this model as the basis of the horizontal velocity grids incorporated in Horizontal Time-Dependent Positioning (HTDP) software. We model the deformation of this area using 46 rotating blocks. Along with the poles of rotation, we also solve for 38 independent horizontal strain rate tensors and the elastic coupling coefficients on faults that bound adjacent blocks. For the release of HTDP 3.1, we updated estimates of model parameters by using 6,287 GPSderived velocity vectors, that include vectors from the 2009 Plate Boundary Observatory (PBO) solution and the NGS Multiyear CORS (MYCORS) solution, and 330 geological measurements of fault slip rates and/or fault orientation. In general, the fault slip rates and the interseismic coupling coefficients are consistent with the results of previous studies; however, because of the comprehensive nature of this model, we are able to quantitatively map deformation rates over the entire plate boundary zone within the contiguous United States. Slip rates on the faults range from over 30 mm/year for the Cascadia subduction zone and parts of the San Andreas system to near zero for faults adjacent to stable North America. Block rotations play a significant role in accommodating deformation in the Pacific Northwest but make amuch smaller contribution south of CapeMendocino. Because of the variable gradient of the velocity field, HTDP3.1 incorporates a hierarchy of 5 grids with a spacing ranging from 4 nodes per degree to 100 nodes per degree. © Springer-Verlag Berlin Heidelberg 2014. Source

Huang C.-H.,National Chiao Tung University | Hwang C.,National Chiao Tung University | Hsiao Y.-S.,National Chung Hsing University | Wang Y.M.,National Geodetic Survey | Roman D.R.,National Geodetic Survey
Terrestrial, Atmospheric and Oceanic Sciences | Year: 2013

In situ airborne gravity data at altitudes of 11, 6.3, and 1.7 km over a smooth area of Alabama are used to assess gravity accuracy and errors in upward and downward continuations. Analysis of the Alabama free-air anomaly gravity data at crossover points at the three altitudes suggests 1 - 2 mgal accuracy for the dataset. Gravity data at each altitude are then expanded into local 3D Fourier series, to prepare for continuation. This Fourier representation results in continuation errors at few-mgal level in Alabama, even in the extreme case of downward continuation from 11 km to sea level. The result in Alabama inspires an airborne gravity survey over the rough, inaccessible terrain of Tibet. Similar investigations as in Alabama are made in Tibet using EGM08-derived airborne gravity data at flight altitudes of 10, 5, and 0 km. Bouguer anomalies at the 10-km altitude preserve the major tectonic features of Tibet. Downward continuation errors increase with terrain roughness, but the survey can enhance local tectonic features. This study highlights the value of a future Tibetan airborne gravity survey and points out the expected gravity accuracy and spatial resolution from this survey. Source

Wang Y.M.,National Geodetic Survey
International Association of Geodesy Symposia | Year: 2012

Based on the assumption that the ultra-high frequencies of thes gravity field are produced by the topography variations, we compute the omission errors by using 3 arc-second elevation data from the Shuttle Radar Topography Mission (SRTM). It is shown that the maximum omission errors to the geoid are in the range of dm, cm and sub-cm level for grid sizes of 5′′, 2′′ and 1′′ over the contiguous United States (CONUS), respectively. The results suggest that a 1 arc-minute grid size is sufficient for the 1-cm geoid, even for areas with very rough topography. The results also show that the omission errors to gravity are significant even for 1′′ grid size, at which the smoothed-out gravity still reaches tens of mGals. The omission errors to gravity at a 5′′ grid size peaks above 100 mGals, demonstrating the importance of correction of residual terrain to gravity observations in data gridding or block mean value computations.The results are also compared with those based on Kaula s rule. While the omission errors based on Kaula's rule are ± 0. 5 and ± 3. 0 cm for 1′′ and 5′′ grid size, respectively, the RMS values of the omission error in this paper are ± 0. 1 and ± 1. 1 cm. The differences suggest Kaula s rule may overestimate the power of the gravity field at the ultra-high frequency band, which renders the convergence studies of the spherical harmonic series based on Kaula's rule questionable. © Springer-Verlag Berlin Heidelberg 2012. Source

Snay R.A.,National Geodetic Survey
Journal of Surveying Engineering | Year: 2012

In 1986, Canada, Greenland, and the United States adopted the North American Datum of 1983 (NAD 83) to replace the North American Datum of 1927 as their official spatial reference system for geometric positioning. The rigor of the original NAD 83 realization benefited from the extensive use of electronic distance measuring instrumentation and from the use of both TRANSIT Doppler observations and very long baseline interferometry observations. However, the original NAD 83 realization predated the widespread use of the global positioning system and the use of continuously operating reference stations. Consequently, NAD 83 has evolved significantly in the United States since 1986 to embrace these technological advances, as well as to accommodate improvements in the understanding of crustal motion. This paper traces this evolution from what started as essentially a two-dimensional (2D) reference frame and has been progressing toward a four-dimensional (4D) frame. In anticipation of future geodetic advances, the U S. National Geodetic Survey is planning to replace NAD 83 about a decade from now with a newer, more geocentric spatial reference system for geometric positioning. © 2012 American Society of Civil Engineers. Source

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