16,004 research outputs found
Reformulating Space Syntax: The Automatic Definition and Generation of Axial Lines and Axial Maps
Space syntax is a technique for measuring the relative accessibility of different locations in a spatial system which has been loosely partitioned into convex spaces.These spaces are approximated by straight lines, called axial lines, and the topological graph associated with their intersection is used to generate indices of distance, called integration, which are then used as proxies for accessibility. The most controversial problem in applying the technique involves the definition of these lines. There is no unique method for their generation, hence different users generate different sets of lines for the same application. In this paper, we explore this problem, arguing that to make progress, there need to be unambiguous, agreed procedures for generating such maps. The methods we suggest for generating such lines depend on defining viewsheds, called isovists, which can be approximated by their maximum diameters,these lengths being used to form axial maps similar to those used in space syntax. We propose a generic algorithm for sorting isovists according to various measures,approximating them by their diameters and using the axial map as a summary of the extent to which isovists overlap (intersect) and are accessible to one another. We examine the fields created by these viewsheds and the statistical properties of the maps created. We demonstrate our techniques for the small French town of Gassin used originally by Hillier and Hanson (1984) to illustrate the theory, exploring different criteria for sorting isovists, and different axial maps generated by changing the scale of resolution. This paper throws up as many problems as it solves but we believe it points the way to firmer foundations for space syntax
CMB power spectrum estimation using noncircular beams
The measurements of the angular power spectrum of the Cosmic Microwave
Background (CMB) anisotropy has proved crucial to the emergence of cosmology as
a precision science in recent years. In this remarkable data rich period, the
limitations to precision now arise from the the inability to account for finer
systematic effects in data analysis. The non-circularity of the experimental
beam has become progressively important as CMB experiments strive to attain
higher angular resolution and sensitivity. We present an analytic framework for
studying the leading order effects of a non-circular beam on the CMB power
spectrum estimation. We consider a non-circular beam of fixed shape but
variable orientation. We compute the bias in the pseudo- power spectrum
estimator and then construct an unbiased estimator using the bias matrix. The
covariance matrix of the unbiased estimator is computed for smooth,
non-circular beams. Quantitative results are shown for CMB maps made by a
\emph{hypothetical} experiment with a non-circular beam comparable to our fits
to the WMAP beam maps described in the appendix and uses a \emph{toy} scan
strategy. We find that significant effects on CMB power spectrum can arise due
to non-circular beam on multipoles comparable to, and beyond, the inverse
average beam-width where the pseudo- approach may be the method of choice
due to computational limitations of analyzing the large datasets from current
and near future CMB experiments.Comment: 23 pages, 12 eps figures, uses RevTeX 4. Matches version accepted to
Phys. Rev. D. Corrected minor typographical error in the final expression
[eqn (3.23)] (post publication
The Evolution of Substructure in Galaxy, Group and Cluster Haloes I: Basic Dynamics
The hierarchical mergers that form the haloes of dark matter surrounding
galaxies, groups and clusters are not entirely efficient, leaving substantial
amounts of dense substructure, in the form of stripped halo cores or
`subhaloes', orbiting within these systems. Using a semi-analytic model of
satellite dynamics, we study the evolution of haloes as they merge
hierarchically, to determine how much substructure survives merging and how the
properties of individual subhaloes change over time. We find that subhaloes
evolve, due to mass loss, orbital decay, and tidal disruption, on a
characteristic time-scale equal to the period of radial oscillations at the
virial radius of the system. Subhaloes with realistic densities and density
profiles lose 25-45 per cent of their mass per pericentric passage, depending
on their concentration and on the circularity of their orbit. As the halo
grows, the subhalo orbits also grow in size and become less bound. Based on
these general patterns, we suggest a method for including realistic amounts of
substructure in semi-analytic models based on merger trees. We show that the
parameters in the resulting model can be fixed by requiring self-consistency
between different levels of the merger hierarchy. In a companion paper, we will
compare the results of our model with numerical simulations of halo formation.Comment: 20 pages, 20 figures; submitted to MNRA
Circular economy for climate neutrality: Setting the priorities for the EU. CEPS Policy Brief No 2019/04, 22 November 2019
The previous Commission policy on resources management was part of the priority for jobs and growth and
economic competitiveness. The circular economy will be no less important for the new political priority of
climate neutrality; it will become one of the indispensable elements for meeting the EUâs ambitions. EU
climate policy and the circular economy are by and large complementary and mutually reinforcing. The
circular economy is more than just another âproduct standardsâ policy.
In order for this to happen,
âą there is a need for a framework that is able to systematically address trade-offs, such as between the
circular and the bioeconomy, but also between material efficiency and energy use, as well as
âą a mechanism to steer and monitor progress, touching upon the question of whether and if so, how to
increase ambition and develop tools to monitor progress, for example via targets, and
âą the new Commission will need to develop and then scale up successful products and processes to create
opportunities for new value chains while addressing risks, such as dependency on raw materials.
Circular economy products for the foreseeable future will require both technology push and market pull
policies. Both the circular economy and low-carbon economy will require new and often yet unknown
business models. This will also require new methods of regulation.
The principal challenge will be to create âlead marketsâ for the circular economy in combination with lowcarbon
products. Many ideas for this exist. They include, for example, âcarbon contracts for differenceâ,
carbon budgets for projects, consumption charges, taxes and tax exemptions, sustainable finance, product
standards and public procurement. Ideas now need to be tested to see whether they could work in practice.
Finally, the EU circular economy will need to be underpinned by a robust and transparent carbon accounting
system. If effective, such as system can at the same time act as a catalyst for investment in the circular
economy and low-carbon products and processes
Evaluation of Motion Artifact Metrics for Coronary CT Angiography
Purpose
This study quantified the performance of coronary artery motion artifact metrics relative to human observer ratings. Motion artifact metrics have been used as part of motion correction and bestâphase selection algorithms for Coronary Computed Tomography Angiography (CCTA). However, the lack of ground truth makes it difficult to validate how well the metrics quantify the level of motion artifact. This study investigated five motion artifact metrics, including two novel metrics, using a dynamic phantom, clinical CCTA images, and an observer study that provided groundâtruth motion artifact scores from a series of pairwise comparisons. Method
Five motion artifact metrics were calculated for the coronary artery regions on both phantom and clinical CCTA images: positivity, entropy, normalized circularity, Fold Overlap Ratio (FOR), and LowâIntensity Region Score (LIRS). CT images were acquired of a dynamic cardiac phantom that simulated cardiac motion and contained six iodineâfilled vessels of varying diameter and with regions of soft plaque and calcifications. Scans were repeated with different gantry start angles. Images were reconstructed at five phases of the motion cycle. Clinical images were acquired from 14 CCTA exams with patient heart rates ranging from 52 to 82 bpm. The vessel and shading artifacts were manually segmented by three readers and combined to create groundâtruth artifact regions. Motion artifact levels were also assessed by readers using a pairwise comparison method to establish a groundâtruth reader score. The Kendall\u27s Tau coefficients were calculated to evaluate the statistical agreement in ranking between the motion artifacts metrics and reader scores. Linear regression between the reader scores and the metrics was also performed. Results
On phantom images, the Kendall\u27s Tau coefficients of the five motion artifact metrics were 0.50 (normalized circularity), 0.35 (entropy), 0.82 (positivity), 0.77 (FOR), 0.77(LIRS), where higher Kendall\u27s Tau signifies higher agreement. The FOR, LIRS, and transformed positivity (the fourth root of the positivity) were further evaluated in the study of clinical images. The Kendall\u27s Tau coefficients of the selected metrics were 0.59 (FOR), 0.53 (LIRS), and 0.21 (Transformed positivity). In the study of clinical data, a Motion Artifact Score, defined as the product of FOR and LIRS metrics, further improved agreement with reader scores, with a Kendall\u27s Tau coefficient of 0.65. Conclusion
The metrics of FOR, LIRS, and the product of the two metrics provided the highest agreement in motion artifact ranking when compared to the readers, and the highest linear correlation to the reader scores. The validated motion artifact metrics may be useful for developing and evaluating methods to reduce motion in Coronary Computed Tomography Angiography (CCTA) images
The evolution of substructure II: linking dynamics to environment
We present results from a series of high-resolution N-body simulations that
focus on the formation and evolution of eight dark matter halos, each of order
a million particles within the virial radius. We follow the time evolution of
hundreds of satellite galaxies with unprecedented time resolution, relating
their physical properties to the differing halo environmental conditions. The
self-consistent cosmological framework in which our analysis was undertaken
allows us to explore satellite disruption within live host potentials, a
natural complement to earlier work conducted within static potentials. Our host
halos were chosen to sample a variety of formation histories, ages, and
triaxialities; despite their obvious differences, we find striking similarities
within the associated substructure populations. Namely, all satellite orbits
follow nearly the same eccentricity distribution with a correlation between
eccentricity and pericentre. We also find that the destruction rate of the
substructure population is nearly independent of the mass, age, and triaxiality
of the host halo. There are, however, subtle differences in the velocity
anisotropy of the satellite distribution. We find that the local velocity bias
at all radii is greater than unity for all halos and this increases as we move
closer to the halo centre, where it varies from 1.1 to 1.4. For the global
velocity bias we find a small but slightly positive bias, although when we
restrict the global velocity bias calculation to satellites that have had at
least one orbit, the bias is essentially removed.Comment: 14 pages, 14 figures, MNRAS in pres
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