Using parallel computation to improve Independent Metropolis--Hastings based estimation

In this paper, we consider the implications of the fact that parallel raw-power can be exploited by a generic Metropolis--Hastings algorithm if the proposed values are independent. In particular, we present improvements to the independent Metropolis--Hastings algorithm that significantly decrease the variance of any estimator derived from the MCMC output, for a null computing cost since those improvements are based on a fixed number of target density evaluations. Furthermore, the techniques developed in this paper do not jeopardize the Markovian convergence properties of the algorithm, since they are based on the Rao--Blackwell principles of Gelfand and Smith (1990), already exploited in Casella and Robert (1996), Atchade and Perron (2005) and Douc and Robert (2010). We illustrate those improvements both on a toy normal example and on a classical probit regression model, but stress the fact that they are applicable in any case where the independent Metropolis-Hastings is applicable.Comment: 19 pages, 8 figures, to appear in Journal of Computational and Graphical Statistic

Coastal Landscapes of South Australia

This book aims to assist people in interpreting coastal landforms in South Australia, revealing how the coast has evolved and is continuing to do so under the influences of a range of processes acting upon a variety of geological settings. South Australian coastal landforms include cliffs, rocky outcrops and shore platforms, mangrove woodlands, mudflats, estuaries, extensive sandy beaches, coastal dunes and coastal barrier systems, as well as numerous near-shore reefs and islands. Geologically, the South Australian coast is very young, having evolved over only 1% of geological time, during the past 43 million years since the separation of Australia and Antarctica. It is also very dynamic, with the current shoreline position having been established from only 7000 years ago. This book is a landmark study into the variable character of the South Australian coast and its long-term evolution

The surface of Mars 4. South polar cap

The south polar cap of Mars occupies a region of cratered terrain. Immediately outside the shrinking cap craters appear no more modified than those in areas farther north that are not annually frost covered. Craters showing through the frost mantle are locally as abundant as elsewhere on Mars. Only in a central region close to the pole are craters sparse. Both far- and near-encounter views reveal a highly irregular pole-cap edge. Photos of the same sector taken six days apart are near duplicates, suggesting that the irregularity is primarily ground controlled. No evidence of the classical polar collar is seen. Within the marginal zone, frost is preserved largely in crater bottoms and on slopes inclined away from the sun. Preferential retention in low spots supports the earlier suggestion that the Mountains of Mitchel may actually be depressions. An argument based on insolation as the prime factor in frost wastage and the narrow width of the marginal zone suggests that slopes of topographic features therein are mostly gentle, on the order of a few degrees. The frost cover of the pole-cap interior may range widely in thickness, obscuring parts of some craters and seemingly enhancing topographic visibility elsewhere, possibly through variations in thickness and reflectivity. Unusually bright areas on the cap surface, and differences in luminance between bright rims and the more somber floors of craters and other depressions, may be due in large part to differences in related frost textures and to the local history of evaporation and sublimation. Irregularly angular depressions within the polecap frost termed ‘etch pits’ may be the product of differential ablation or the undermining by wind of a slabby surficial crust. Encircling the south pole is a region of subdued relief with a paucity of craters, which displays enigmatic quasi-linear markings believed to be ground features. Although no satisfactory explanation of these markings has been formulated, it seems likely that this region has been occupied repeatedly by perennial masses of CO_2 ice, formed and maintained during those phases of the martian precessional cycle that resulted in short cool summers in the southern hemisphere. Such ice masses may play a role in producing the unusual features of the central polar region. Physical relationships suggest a local maximum frost thickness as great as tens of meters. The possibility should be kept in mind that remnants of perennial CO_2 ice of still greater thickness may exist locally, for example, in the ‘etch pit’ area

Coastal Landscapes of South Australia

This book aims to assist people in interpreting coastal landforms in South Australia, revealing how the coast has evolved and is continuing to do so under the influences of a range of processes acting upon a variety of geological settings. South Australian coastal landforms include cliffs, rocky outcrops and shore platforms, mangrove woodlands, mudflats, estuaries, extensive sandy beaches, coastal dunes and coastal barrier systems, as well as numerous near-shore reefs and islands. Geologically, the South Australian coast is very young, having evolved over only 1% of geological time, during the past 43 million years since the separation of Australia and Antarctica. It is also very dynamic, with the current shoreline position having been established from only 7000 years ago. This book is a landmark study into the variable character of the South Australian coast and its long-term evolution

The surface of Mars 1. Cratered terrains

Mariner 6 and 7 pictures show that craters are the dominant landform on Mars and that their occurrence is not correlated uniquely with latitude, elevation, or albedo markings. Two distinct morphological classes are recognized: small bowl-shaped and large flat-bottomed. The former show little evidence of modifications, whereas the latter appear generally more modified than lunar upland craters of comparable size. A regional maria/uplands dichotomy like the moon has not yet been recognized on Mars. Crater modification on Mars has involved much greater horizontal redistribution of material than in the lunar uplands. It is possible that there are erosional processes only infrequently active. Analysis of the natures and fluxes of bodies that have probably impacted the moon and Mars leads to the likelihood that most of the large flat-bottomed craters on Mars have survived from the final phases of planetary accretion. Significant crater modification, however, has taken place more recently on Mars. Inasmuch as the present small bowl-shaped craters evidence little modification, the postaccretion crater-modification process on Mars may have been primarily episodic rather than continuous. The size-frequency distribution of impacting bodies that produced the present small Martian bowl-shaped craters differs from that responsible for post-mare primary impacts on the moon by a marked deficiency of large bodies. Survival of crater topography from the end of planetary accretion would make any hypothetical earthlike phase with primitive oceans there unlikely. The traditional view of Mars as an earthlike planetary neighbor in terms of its surface history is not supported by the picture data

The surface of Mars 2. Uncratered terrains

Mariner 6 and 7 photographs reveal two types of uncratered terrain on Mars. These are descriptively termed chaotic and featureless. Chaotic terrain is younger than cratered terrain and displays features strongly suggestive of slump and collapse. The speculation is offered that it may be an expression of geothermal developments within Mars that only recently have begun to affect the surface. Featureless terrain, identified only within the large circular area Hellas, is devoid of any discernible topographic forms larger than the limit of resolution, about 500 meters. Manner 7 data indicate that Hellas is a topographically low and structurally old basin. Smoothness of its floor could be the product of a recent event or of continuous processes that obliterate craters. Local processes of high efficacy, unusual surface materials, or both, are probably involved. Through its chaotic terrain the martian surface displays a development that does not seem to be recorded, at least in the form of preserved recognizable evidence, on the moon or earth

Real-time crowd control of existing interfaces

Crowdsourcing has been shown to be an effective approach for solving difficult problems, but current crowdsourcing systems suffer two main limitations: (i) tasks must be repackaged for proper display to crowd workers, which generally requires substantial one-off programming effort and support infrastructure, and (ii) crowd workers generally lack a tight feedback loop with their task. In this paper, we introduce Legion, a system that allows end users to easily capture existing GUIs and outsource them for collaborative, real-time control by the crowd. We present mediation strategies for integrating the input of multiple crowd workers in real-time, evaluate these mediation strategies across several applications, and further validate Legion by exploring the space of novel applications that it enables

The surface of Mars 3. Light and dark markings

The Mariner 6 and 7 pictures have provided significant clues to the nature of the light and dark markings on Mars, but do not yet provide an adequate foundation for any complete explanation of the phenomena. They display detail never before seen or photographed and demonstrate that there is no network of dark lines (i.e. canals) on the planet. A variety of shapes and of boundaries between major markings are recorded in the pictures. No substantial correlation of albedo markings with cratered or chaotic terrain has been recognized; featureless terrain conceivably may be genetically related to light areas. Within and surrounding the dark area Meridiani Sinus there is evidence of local topographic control of albedo markings; light material is found in locally low areas. Also, characteristic patterns of local albedo markings are exhibited by craters there. Aeolian transportation of light material with deposition locally in low areas is suggested as an explanation of these markings and may be useful as a working hypothesis for subsequent exploration. Across some light/dark boundaries crater morphologies are unchanged; across others craters in the light area appear smoother. If there is a relationship between cratered terrain modification and surface albedo it is an indirect one