Usefulness of VRML building models in a direction finding context

This paper describes an experiment which aims to examine the effectiveness and efficiency of a Virtual Reality Modelling Language (VRML) building model compared with equivalent architectural plans, for direction finding purposes. The effectiveness and efficiency issues being primarily investigated were number of tasks completed overall and task completion times. The experiment involved a series of tasks where participants had to find a number of locations/objects in a building unknown to them at the outset of the experiment. Statistically significant results are presented for the benefit of the research community, law enforcement officers and fire fighters where it is clear that in this context, the VRML model led to better task completions than the equivalent architectural plans. Regarding the task completion times, no statistical significance was found. Given the current climate of security issues and terrorist threats, it is important that law enforcement officers have at their disposal the best information possible regarding the layout of a building, whilst keeping costs down. This also applies to fire fighters when rescuing victims. This experiment has shown that a VRML model leads to better task completions in direction finding

A double main sequence turn-off in the rich star cluster NGC 1846 in the Large Magellanic Cloud

We report on HST/ACS photometry of the rich intermediate-age star cluster NGC 1846 in the Large Magellanic Cloud, which clearly reveals the presence of a double main sequence turn-off in this object. Despite this, the main sequence, sub-giant branch, and red giant branch are all narrow and well-defined, and the red clump is compact. We examine the spatial distribution of turn-off stars and demonstrate that all belong to NGC 1846 rather than to any field star population. In addition, the spatial distributions of the two sets of turn-off stars may exhibit different central concentrations and some asymmetries. By fitting isochrones, we show that the properties of the colour-magnitude diagram can be explained if there are two stellar populations of equivalent metal abundance in NGC 1846, differing in age by approximately 300 Myr. The absolute ages of the two populations are ~1.9 and ~2.2 Gyr, although there may be a systematic error of up to +/-0.4 Gyr in these values. The metal abundance inferred from isochrone fitting is [M/H] ~ -0.40, consistent with spectroscopic measurements of [Fe/H]. We propose that the observed properties of NGC 1846 can be explained if this object originated via the tidal capture of two star clusters formed separately in a star cluster group in a single giant molecular cloud. This scenario accounts naturally for the age difference and uniform metallicity of the two member populations, as well as the differences in their spatial distributions.Comment: 9 pages, 8 figures, accepted for publication in MNRAS. A version with full resolution figures may be obtained at http://www.roe.ac.uk/~dmy/papers/MN-07-0441-MJ_rv.ps.gz (postscript) or at http://www.roe.ac.uk/~dmy/papers/MN-07-0441-MJ_rv.pdf (PDF

On the Exponentials of Some Structured Matrices

In this note explicit algorithms for calculating the exponentials of important structured 4 x 4 matrices are provided. These lead to closed form formulae for these exponentials. The techniques rely on one particular Clifford Algebra isomorphism and basic Lie theory. When used in conjunction with structure preserving similarities, such as Givens rotations, these techniques extend to dimensions bigger than four.Comment: 19 page

Site Fidelity and Association Patterns of Bottlenose Dolphins (\u3ci\u3eTursiops truncatus\u3c/i\u3e in the Mississippi Sound

The current study examined the site fidelity and association patterns of a community of 678 wild bottlenose dolphins (Tursiops truncatus) in the Mississippi Sound (Sound) over a three-year period (May 2004 - April 2007). Using photoidentification techniques, 74% (n = 498) of the identified dolphins were classified as transients, while 10% (n = 71) were classified as year-round residents, and 16% (n = 109) were classified as seasonal residents based on their sighting histories. Thirty-nine select dolphins (n = 17 seasonal residents, n = 22 yearround residents) that were sighted five or more times over the study period were used to calculate the coefficients of association (COAs) using the Half-weight index. Non-zero COAs ranged from 0.10 to 0.91 (M = 0.25), with a majority (91 %) falling below 0.40. Select dolphins had an average of 55.6 associates, and 21% of the associations between two dolphins were repeated associations. Social networking analyses were used to investigate the substructure of this network. The network was filtered such that only associations greater than the mean COA were represented, and only individuals with more than one association were included (n = 36). The Girvan-Newman algorithm revealed three distinct communities within the network. A randomized test of autocorrelation provided evidence that the dolphins in this network do not preferentially associate with individuals of the same residency classification. However, individuals of high degree (number and weight of network neighbors) were more likely to associate more closely with other individuals of high degree. As individuals with high measures of centrality (degree and betweenness) were removed from the network, the network began to break apart, but not prior to the removal of several individuals, suggesting the structure of the network is maintained by multiple individuals. Networks created for each of the three barrier islands and the channel revealed distinct differences in social structure at those locations. Network centrality measures were also calculated for a group of dolphins sighted two or more times before and after Hurricane Katrina, to examine its effects on the social structure of dolphins in the Sound. Most of the measures of centrality were significantly higher after the hurricane, suggesting that the dolphins in the network were more strongly connected at this time