Visibility modelling calculates what an observer could theoretically see in the surrounding region based on a digital model of the landscape. In some cases, it is not necessary, nor desirable, to compute the visibility of an entire region (i.e. a viewshed), but instead it is sufficient and more efficient to calculate the visibility from point to point, or from a point to a small set of points, such as computing the intervisibility of predators and prey in an agent-based simulation. This paper explores how different line-of-sight (LoS) sample ordering strategies increase the number of early target rejections, where the target is considered to be obscured from view, thereby improving the computational efficiency of the LoS algorithm. This is of particular importance in dynamic environments where the locations of the observers, targets and other surface objects are being frequently updated. Trials were conducted in three UK cities, demonstrating a robust fivefold increase in performance for two strategies (hop, divide and conquer). The paper concludes that sample ordering methods do impact overall efficiency, and that approaches which disperse samples along the LoS perform better in urban regions than incremental scan methods. The divide and conquer method minimises elevation interception queries, making it suitable when elevation models are held on disk rather than in memory, while the hopping strategy was equally fast, algorithmically simpler, with minimal overhead for visible target cases. 相似文献
Most rhyolite eruption episodes of Tarawera volcano have emitted several physiochemically distinct magma batches (∼1–10 km3). These episodes were separated on a millennial timescale. The magma batches were relatively homogeneous in temperature and
composition at pumice scale (>4 cm), but experienced isolated crystallisation histories. At the sub-cm scale, matrix glasses
have trace element compositions (Sr, Ba, Rb) that vary by factors up to 2.5, indicating incomplete mixing of separate melts.
Some quartz-hosted melt inclusions are depleted in compatible trace elements (Sr, Ti, Ba) compared to enclosing matrix glasses.
This could reflect re-melting of felsic crystals deeper in the crystal pile. Individual quartz crystals display a variety
of cathodoluminescence brightness and Ti zoning patterns including rapid changes in melt chemistry and/or temperature (∼50–100°C),
and point to multi-cycle crystallisation histories. The Tarawera magma system consisted of a crystal-rich mass containing
waxing and waning melt pockets that were periodically recharged by silicic melts driven by basaltic intrusion.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
Acid-base titrations and electrophoretic mobility measurements were conducted on the thermophilic bacteria Anoxybacillus flavithermus and Geobacillus stearothermophilus at two different growth times corresponding to exponential and stationary/death phase. The data showed significant differences between the two investigated growth times for both bacterial species. In stationary/death phase samples, cells were disrupted and their buffering capacity was lower than that of exponential phase cells. For G. stearothermophilus the electrophoretic mobility profiles changed dramatically. Chemical equilibrium models were developed to simultaneously describe the data from the titrations and the electrophoretic mobility measurements. A simple approach was developed to determine confidence intervals for the overall variance between the model and the experimental data, in order to identify statistically significant changes in model fit and thereby select the simplest model that was able to adequately describe each data set. Exponential phase cells of the investigated thermophiles had a higher total site concentration than the average found for mesophilic bacteria (based on a previously published generalised model for the acid-base behaviour of mesophiles), whereas the opposite was true for cells in stationary/death phase. The results of this study indicate that growth phase is an important parameter that can affect ion binding by bacteria, that growth phase should be considered when developing or employing chemical models for bacteria-bearing systems. 相似文献
A response to Johnston et al.’s criticisms of my earlier paper about the Cold War and geography’s quantitative revolution that I am overly deterministic and fail to prove my case. 相似文献
In the present study, we document the nature of contact-style platinum-group element (PGE) mineralization along >100 km of
strike in the northern lobe of the Bushveld Complex. New data from the farm Rooipoort are compared to existing data from the
farms Townlands, Drenthe, and Nonnenwerth. The data indicate that the nature of the contact-style mineralization shows considerable
variation along strike. In the southernmost portion of the northern Bushveld, on Rooipoort and adjoining farms, the mineralized
sequence reaches a thickness of 700 m. Varied-textured gabbronorites are the most common rock type. Anorthosites and pyroxenites
are less common. Chromitite stringers and xenoliths of calcsilicate and shale are largely confined to the lower part of the
sequence. Layering is locally prominent and shows considerable lateral continuity. Disseminated sulfides may reach ca. 3 modal
% and tend to be concentrated in chromitites and melanorites. Geochemistry indicates that the rocks can be correlated with
the Upper Critical Zone. This model is supported by the fact that, in a down-dip direction, the mineralized rocks transform
into the UG2-Merensky Reef interval. Between Townlands and Drenthe, the contact-mineralized sequence is thinner (up to ca.
400 m) than in the South. Chromitite stringers occur only sporadically, but ultramafic rocks (pyroxenites, serpentinites,
and peridotites) are common. Xenoliths of calcsilicate, shale, and iron formation are abundant indicating significant assimilation
of the floor rocks. Sulfides may locally form decimeter- to meter-sized massive lenses. PGE grades tend to be higher than
elsewhere in the northern Bushveld. The compositions of the rocks show both Upper Critical Zone and Main Zone characteristics.
At Nonnenwerth, the mineralized interval is up to ca. 400 m thick. It consists largely of varied-textured gabbronorites, with
minor amounts of igneous ultramafic rocks and locally abundant and large xenoliths of calcsilicate. Layering is mostly weakly
defined and discontinuous. Disseminated sulfides (<ca. 3 modal %) occur throughout much of the sequence. Geochemistry indicates
that the rocks crystallized mainly from tholeiitic magma and thus have a Main Zone signature. The implication of our findings
is that contact-style PGE mineralization in the northern lobe of the Bushveld Complex cannot be correlated with specific stratigraphic
units or magma types, but that it formed in response to several different processes. At all localities, the magmas were contaminated
with the floor rocks. Contamination with shale led to the addition of external sulfur to the magma, whereas contamination
with dolomite may have oxidized the magma and lowered its sulfur solubility. In addition to contamination, some of the magmas,
notably those of Upper Critical Zone lineage present at the south-central localities, contained entrained sulfides, which
precipitated during cooling and crystallization. 相似文献