8,479 research outputs found
A worldwide model for boundaries of urban settlements
The shape of urban settlements plays a fundamental role in their sustainable
planning. Properly defining the boundaries of cities is challenging and remains
an open problem in the Science of Cities. Here, we propose a worldwide model to
define urban settlements beyond their administrative boundaries through a
bottom-up approach that takes into account geographical biases intrinsically
associated with most societies around the world, and reflected in their
different regional growing dynamics. The generality of the model allows to
study the scaling laws of cities at all geographical levels: countries,
continents, and the entire world. Our definition of cities is robust and holds
to one of the most famous results in Social Sciences: Zipf's law. According to
our results, the largest cities in the world are not in line with what was
recently reported by the United Nations. For example, we find that the largest
city in the world is an agglomeration of several small settlements close to
each other, connecting three large settlements: Alexandria, Cairo, and Luxor.
Our definition of cities opens the doors to the study of the economy of cities
in a systematic way independently of arbitrary definitions that employ
administrative boundaries
A universal approach for drainage basins
Drainage basins are essential to Geohydrology and Biodiversity. Defining
those regions in a simple, robust and efficient way is a constant challenge in
Earth Science. Here, we introduce a model to delineate multiple drainage basins
through an extension of the Invasion Percolation-Based Algorithm (IPBA). In
order to prove the potential of our approach, we apply it to real and
artificial datasets. We observe that the perimeter and area distributions of
basins and anti-basins display long tails extending over several orders of
magnitude and following approximately power-law behaviors. Moreover, the
exponents of these power laws depend on spatial correlations and are invariant
under the landscape orientation, not only for terrestrial, but lunar and
martian landscapes. The terrestrial and martian results are statistically
identical, which suggests that a hypothetical martian river would present
similarity to the terrestrial rivers. Finally, we propose a theoretical value
for the Hack's exponent based on the fractal dimension of watersheds,
. We measure for Earth, which is close to
our estimation of . Our study suggests that Hack's law can
have its origin purely in the maximum and minimum lines of the landscapes.Comment: 20 pages, 6 Figures, and 1 Tabl
Magnetic-field effects in defect-controlled ferromagnetic Ga_{1-x}Mn_xAs semiconductors
We have studied the magnetic-field and concentration dependences of the
magnetizations of the hole and Mn subsystems in diluted ferromagnetic
semiconductor Ga_{1-x}Mn_xAs. A mean-field approximation to the hole-mediated
interaction is used, in which the hole concentration p(x) is parametrized in
terms of a fitting (of the hole effective mass and hole/local moment coupling)
to experimental data on the Tc critical temperature. The dependence of the
magnetizations with x, for a given temperature, presents a sharply peaked
structure, with maxima increasing with applied magnetic field, which indicates
that application to diluted-magnetic-semiconductor devices would require
quality-control of the Mn-doping composition. We also compare various
experimental data for Tc(x) and p(x) on different Ga_{1-x}Mn_xAs samples and
stress the need of further detailed experimental work to assure that the
experimental measurements are reproducible.Comment: RevTeX 4, 3 two-column pages, 4 colour figures; to appear in J Appl
Phy
A smartphone-based multi-sensor wireless platform for cycling performance monitoring
In recent years there has been a significant evolution regarding applications for mobile
devices that provide location-based services. The mobile devices available on the market
already provide a set of integrated sensors and it is also possible to acquire data from
external sensors. This chapter presents the development and results concerning a mobile
sensing platform applied to cycling which performs data collection using both sensors
integrated in the smartphone and multiple wireless sensor nodes, which are used to
acquire relevant performance parameters. The data collected by the developed mobile app
is stored in a local database and also uploaded to a remote database, where it can be
accessed later using the mobile app or a web browser. This mobile app allows users to
share data with friends, join or create events, locate friends, consult graphs and access
past routes in a map. Based on these functionalities, this system aims to provide detailed
feedback regarding the user performance and enhance the enjoyment of the cyclists.This work has been supported by FCT (Fundação para a Ciência e Tecnologia)
in the scope of the project: UID/EEA/04436/2013.info:eu-repo/semantics/publishedVersio
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