923 research outputs found
The effect of EGM2008-based normal, normal-orthometric and Helmert orthometric height systems on the Australian levelling network
This paper investigates the normal-orthometric correction used in the definition of the Australian Height Datum, and also computes and evaluates normal and Helmert orthometric corrections for the Australian National Levelling Network (ANLN). Testing these corrections in Australia is important to establish which height system is most appropriate for any new Australian vertical datum. An approximate approach to assigning gravity values to ANLN benchmarks (BMs) is used, where the EGM2008-modelled gravity field is used to "re-construct" observed gravity at the BMs. Network loop closures (for first- and second-order levelling) indicate reduced misclosures for all height corrections considered, particularly in the mountainous regions of south eastern Australia. Differences between Helmert orthometric and normal-orthometric heights reach 44 cm in the Australian Alps, and differences between Helmert orthometric and normal heights are about 26 cm in the same region. Normal orthometric heights differ from normal heights by up to 18 cm in mountainous regions >2,000 m. This indicates that the quasigeoid is not compatible with normal-orthometric heights in Australia
Symbolic vector/dyadic multibody formalism for tree-topology systems
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76159/1/AIAA-20780-671.pd
A Synthetic Earth Gravity Model Designed Specifically for Testing Regional Gravimetric Geoid Determination Algorithms
A synthetic [simulated] Earth gravity model (SEGM) of the geoid, gravity and topography has been constructed over Australia specifically for validating regional gravimetric geoid determination theories, techniques and computer software. This regional high-resolution (1-arc-min by 1-arc-min) Australian SEGM (AusSEGM) is a combined source and effect model. The long-wavelength effect part (up to and including spherical harmonic degree and order 360) is taken from an assumed errorless EGM96 global geopotential model. Using forward modelling via numerical Newtonian integration, the short-wavelength source part is computed from a high-resolution (3-arc-sec by 3-arc-sec) synthetic digital elevation model (SDEM), which is a fractal surface based on the GLOBE v1 DEM. All topographic masses are modelled with a constant mass-density of 2,670 kg/m3. Based on these input data, gravity values on the synthetic topography (on a grid and at arbitrarily distributed discrete points) and consistent geoidal heights at regular 1-arc-min geographical grid nodes have been computed. The precision of the synthetic gravity and geoid data (after a first iteration) is estimated to be better than 30 ÎŒ Gal and 3 mm, respectively, which reduces to 1 ÎŒ Gal and 1 mm after a second iteration.The second iteration accounts for the changes in the geoid due to the superposed synthetic topographic mass distribution. The first iteration of AusSEGM is compared with Australian gravity and GPS-levelling data to verify that it gives a realistic representation of the Earthâs gravity field. As a by-product of this comparison, AusSEGM gives further evidence of the northâsouth-trending error in the Australian Height Datum. The freely available AusSEGM-derived gravity and SDEM data, included as Electronic Supplementary Material (ESM) with this paper, can be used to compute a geoid model that, if correct, will agree to in 3 mm with the AusSEGM geoidal heights, thus offering independent verification of theories and numerical techniques used for regional geoid modelling
Combining EGM2008 and SRTM/DTM2006.0 residual terrain model data to improve quasigeoid computations in mountainous areas devoid of gravity data
A global geopotential model, like EGM2008, is not capable of representing the high-frequency components of Earth?s gravity field. This is known as the omission error. In mountainous terrain, omission errors in EGM2008, even when expanded to degree 2,190, may reach amplitudes of10cm and more for height anomalies. The present paper proposes the utilisation of high-resolution residual terrain model (RTM) data for computing estimates of the omission error in rugged terrain. RTM elevations may be constructed as the difference between the SRTM (Shuttle Radar Topography Mission) elevation model and the DTM2006.0 spherical harmonic topographic expansion. Numerical tests, carried out in the German Alps with a precise gravimetric quasigeoid model (GCG05) and GPS/levelling data as references, demonstrate that RTM-based omission error estimatesimprove EGM2008 height anomaly differences by 10cm in many cases. The comparisons of EGM2008-only height anomalies and the GCG05 model showed 3.7 cm standard deviation after a bias-fit. Applying RTM omission error estimates to EGM2008 reduces the standard deviation to 1.9 cm which equates to a significant improvement rate of 47%. Using GPS/levelling data strongly corroborates thesefindings with an improvement rate of 49%. The proposed RTM approach may be of practical value to improve quasigeoid determination in mountainous areas without sufficient regional gravity data coverage, e.g., in parts of Asia, South America or Africa. As a further application, RTMomission error estimates will allow refined validation of global gravity field models like EGM2008 from GPS/levelling data
Doing gender locally: The importance of âplaceâ in understanding marginalised masculinities and young menâs transitions to âsafeâ and successful futures
Observable anxieties have been developing about the position of boys and young men in contemporary society in recent years. This is expressed as a crisis of masculinity, in which place is often implicitly implicated, but is rarely considered for its role in the shaping of young menâs practices, trajectories and aspirations. Drawing on research conducted with young people who accessed a range of social care support services, this article argues that transition means different things for young men in different locales and that local definitions of masculinity are required to better understand young menâs lives and the opportunities available to them. The authors argue that home life, street life, individual neighbourhoods, regions and nations all shaped the young menâs identities and the practices they (and the staff working with them) drew on in order to create successful futures and âsafeâ forms of masculinity. It is suggested that this place-based approach has the potential to re-shape the âcrisisâ discourse surrounding masculinity and the anxieties associated with young men
The Metropolis and Evangelical Life: Coherence and Fragmentation in the âLost City of Londonâ
This article examines the interplay of different processes of cultural and subjective fragmentation experienced by conservative evangelical Anglicans, based on an ethnographic study of a congregation in central London. The author focuses on the evangelistic speaking practices of members of this church to explore how individuals negotiate contradictory norms of interaction as they move through different city spaces, and considers their response to tensions created by the demands of their workplace and their religious lives. Drawing on Georg Simmelâs âThe Metropolis and Mental Lifeâ, the author argues that their faith provides a sense of coherence and unity that responds to experiences of cultural fragmentation characteristic of everyday life in the city, while simultaneously leading to a specific consciousness of moral fragmentation that is inherent to conservative evangelicalism
The apparent British sea slope is caused by systematic errors in the levelling-based vertical datum
The spirit-levellingâbased British vertical datum (Ordnance Datum Newlyn) implies a southânorth apparent slope in mean sea level of up to 53âmm degâ1 latitude, due to the datum falling on heading northwards. Although this apparent slope has been investigated since the 1960s, explanations of its origin have remained inconclusive. It has also been suggested that, rather than a slope, the British vertical datum includes a step of about 240âmm affecting all sites north of about 53°N. In either case, the British vertical datum may be of limited use for any study requiring accurate heights or changes in heights, such as testing geoid models, groundwater and hydrocarbon extraction, the calibration and validation of satellite-based digital terrain models, and the unification of vertical datums internationally. Within the last decade, however, based on an apparent reduction in the slope to only â12âmm degâ1 latitude with respect to recent geoid models, it has been claimed that the British vertical datum does provide a physically meaningful surface for use in scientific applications.In this paper, we reinvestigate the presence of apparent southânorth sea slopes around Britain and reported slopes in the vertical datum, using the EGM2008 global gravitational model, together with mean sea level and GPS data from British tide gauges, GPS ellipsoidal heights of 178 fundamental benchmarks across mainland Britain, and vertical deflection observations at 192 stations. We demonstrate a southânorth slope in the British vertical datum of â(20â25)âmm degâ1 latitude with respect to both mean sea level (corrected for the ocean's mean dynamic topography and the inverse barometer response to atmospheric pressure loading) and the EGM2008 quasigeoid model, while EGM2008 is shown to exhibit a negligible slope of (2 ± 4)âmm degâ1 with respect to mean sea level. It is clear, therefore, that the slope can only arise from systematic errors in the levelling, although we are unable to isolate their exact origin. Using an offset detection method based on a penalized likelihood maximization using the Schwarz Information Criterion, we do not detect a step in the vertical datum affecting all sites north of 53°N, but do identify regional distortions that we attribute to the inhomogeneity in both the levelling data used and the least squares adjustment procedures used to realize the datum. We conclude that the British vertical datum remains unsuitable for scientific purposes
Unification of New Zealand's local vertical datums: iterative gravimetric quasigeoid computations
New Zealand uses 13 separate local vertical datums (LVDs) based on normal-orthometric-corrected precise geodetic levelling from 12 different tide-gauges. We describe their unification using a regional gravimetric quasigeoid model and GPS-levelling data on each LVD. A novel application of iterative quasigeoid computation is used, where the LVD offsets computed from earlier models are used to apply additional gravity reductions from each LVD to that model. The solution converges after only three iterations yielding LVD offsets ranging from 0.24 m to 0.58 m with an average standard deviation of 0.08 m. The so-computed LVD offsets agree, within expected data errors, with geodetically levelled height differences at common benchmarks between adjacent LVDs. This shows that iterated quasigeoid models do have a role in vertical datum unification
Systematic Bias in Helioseismic Measurements of Meridional Circulation Arising from Nonlocal Averaging Kernels
Meridional circulation in the solar convection zone plays a profound role in
regulating the interior dynamics of the Sun and its magnetism. While it is well
accepted that meridional flows move from the equator towards the poles at the
Sun's surface, helioseismic observations have yet to provide a definitive
answer for the depth at which those flows return to the equator, or the number
of circulation cells in depth. In this work, we investigate whether the
discrepancies regarding the nature of the return flow are intrinsic to how
helioseismic observations are made. We examine the seismic signature of
possible meridional flow profiles by convolving time-distance averaging kernels
with the mean flows obtained from 3-D hydrodynamic simulations of the solar
convection zone. At mid and high latitudes, we find that weak flow structures
in the deeper regions of the convection zone can be strongly obscured by signal
from the much stronger surface flows. This contamination is the result of
extended side lobes in the averaging kernels and generates a spurious
equatorward signal of 2--3 m s at those latitudes, and at depth. At low latitudes, however, the flows in the simulations
tend to be stronger and multiple cells across the shell depth can produce a
sufficiently strong seismic signal to survive the convolution process. The
signal associated with the deep equatorward return flow in the Sun is expected
to be weak and in the same sense as the contamination from the surface. Hence,
the return flow needs to exceed -- in magnitude
for reported detections to be considered significant.Comment: Submitted to AAS Journal
The north-south tilt in the Australian Height Datum is explained by the oceanâs mean dynamic topography
Using geodetic and oceanographic data, we show that the apparent north-south slope between the Australian Height Datum (AHD) and the geoid is caused almost completely by the oceanâs time-mean dynamic topography (MDT). This is because the AHD was constrained to zero height at local mean sea level at multiple tide gauges around the Australian continent. Using MDT models and corrected leveling data, almost all of the apparent north-south slope can be removed from the AHD. An auxiliary observation is that a satellite-only MDT model based on only around one year of GOCE data generates results commensurate with geodetic, oceanographic and combined MDT models
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