25,926 research outputs found
A system to geometrically rectify and map airborne scanner imagery and to estimate ground area
A system of computer programs were developed which performs geometric rectification and line-by-line mapping of airborne multispectral scanner data to ground coordinates and estimates ground area. The system requires aircraft attitude and positional information furnished by ancillary aircraft equipment, as well as ground control points. The geometric correction and mapping procedure locates the scan lines, or the pixels on each line, in terms of map grid coordinates. The area estimation procedure gives ground area for each pixel or for a predesignated parcel specified in map grid coordinates. The results of exercising the system with simulated data showed the uncorrected video and corrected imagery and produced area estimates accurate to better than 99.7%
Minimising biases in Full Configuration Interaction Quantum Monte Carlo
We show that Full Configuration Interaction Quantum Monte Carlo (FCIQMC) is a
Markov Chain in its present form. We construct the Markov matrix of FCIQMC for
a two determinant system and hence compute the stationary distribution. These
solutions are used to quantify the dependence of the population dynamics on the
parameters defining the Markov chain. Despite the simplicity of a system with
only two determinants, it still reveals a population control bias inherent to
the FCIQMC algorithm. We investigate the effect of simulation parameters on the
population control bias for the neon atom and suggest simulation setups to in
general minimise the bias. We show a reweighting scheme to remove the bias
caused by population control commonly used in Diffusion Monte Carlo [J. Chem.
Phys. 99, 2865 (1993)] is effective and recommend its use as a post processing
step.Comment: Supplementary material available as 'Ancillary Files
Projections of Australian obstetricians ceasing practice and the reasons
The document attached has been archived with permission from the editor of the Medical Journal of Australia (10 January 2008). An external link to the publisher’s copy is included.Objectives: To assess the intentions of Australia's specialist obstetricians to cease practice and their reasons for abandoning this specialty. Design: A structured questionnaire posted to Fellows of the Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG), issued 11 July 2001 with a return date of 31 July 2001 (in practice, responses were accepted up to 31 August 2001). Participants: Australian specialists holding a Fellowship of the RANZCOG. Main outcome measures: Demographic data (eg, age, sex); type and location of practice; past, current and intended future obstetric practice; reasons for stopping practice; cost of indemnity premiums; experience of litigation and its influence on practice; and experience in giving medicolegal opinion. Results: The response rate was 74% (829/1116), with 826 responses fulfilling our selection criteria. The median number of years since admission as a Fellow was 17 (range, 1–47 years), and 19% (158/817) of respondents were women (9 people did not specify their sex). Of the 826 respondents, 596 (72%) were currently practising obstetrics, 548 (66%) intended to still be practising after one year, 365 (44%) intended to be practising after five years, and 196 (24%) intended to be practising after 10 years. The median indemnity premium in 2001–02 was 156 000) for practising obstetricians. The main reasons given for ceasing obstetrics were intention to specialise in gynaecology, fear of litigation, high indemnity costs, family disruption, and long working hours. About two-thirds of respondents (557/818) had experienced the threat of litigation, and almost all (768/803) desired some type of "no-fault" indemnity scheme. Thirty-three of the 314 respondents who had given medicolegal opinions accounted for 71% of the total number of opinions. Many of these were non-practising obstetricians who were not accredited RANZCOG expert witnesses. Conclusion: There will soon be a shortage of experienced practising obstetricians in Australia.Alastair H MacLennan and Michael K Spence
Model of surface instabilities induced by stress
We propose a model based on a Ginzburg-Landau approach to study a strain
relief mechanism at a free interface of a non-hydrostatically stressed solid,
commonly observed in thin-film growth. The evolving instability, known as the
Grinfeld instability, is studied numerically in two and three dimensions.
Inherent in the description is the proper treatment of nonlinearities. We find
these nonlinearities can lead to competitive coarsening of interfacial
structures, corresponding to different wavenumbers, as strain is relieved. We
suggest ways to experimentally measure this coarsening.Comment: 4 pages (3 figures included
A Robust Numerical Method for Integration of Point-Vortex Trajectories in Two Dimensions
The venerable 2D point-vortex model plays an important role as a simplified
version of many disparate physical systems, including superfluids,
Bose-Einstein condensates, certain plasma configurations, and inviscid
turbulence. This system is also a veritable mathematical playground, touching
upon many different disciplines from topology to dynamic systems theory.
Point-vortex dynamics are described by a relatively simple system of nonlinear
ODEs which can easily be integrated numerically using an appropriate adaptive
time stepping method. As the separation between a pair of vortices relative to
all other inter-vortex length scales decreases, however, the computational time
required diverges. Accuracy is usually the most discouraging casualty when
trying to account for such vortex motion, though the varying energy of this
ostensibly Hamiltonian system is a potentially more serious problem. We solve
these problems by a series of coordinate transformations: We first transform to
action-angle coordinates, which, to lowest order, treat the close pair as a
single vortex amongst all others with an internal degree of freedom. We next,
and most importantly, apply Lie transform perturbation theory to remove the
higher-order correction terms in succession. The overall transformation
drastically increases the numerical efficiency and ensures that the total
energy remains constant to high accuracy.Comment: 21 pages, 4 figure
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