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11.
Recent advances in accessibility research: Representation,methodology and applications 总被引:22,自引:0,他引:22
Mei-Po Kwan Alan T. Murray Morton E. O'Kelly Michael Tiefelsdorf 《Journal of Geographical Systems》2003,5(1):129-138
In this article we examine recent advances in accessibility research and their implications for future studies. We base our
discussion on three intersecting dimensions that are useful for evaluating the contribution of recent studies: representation,
methodology and applications. Various examples are selected to show that research concerned with representation and methodological
problem solving is often applied to issues of broad concern in policy and planning. It is, however, not clear that the simultaneous
treatment of representation, methodological and application issues has ever been fully worked out. The questions raised in
this article may serve as a foundation for addressing issues pertinent to accurate representation, improved model building,
and more rigorous applications in accessibility research.
Received: 9 December 2002 / Accepted: 10 February 2003 相似文献
12.
There is a long history of recognising and interpreting discontinuous change—phase transitions—in urban systems. In this paper,
we use the aggregate retail model as an archetype to explore some new ideas. For example, we argue that the dependence of
paths of development on initial conditions has been understudied, and we offer a new graphical analysis that demonstrates
explicitly their influence on discrete change. We introduce an order parameter, and we plot this on a ‘results grid’ to facilitate
the discovery of possible phase transitions. We illustrate the use of these new developments with an application to London.
We show how retail developers can change the ‘initial conditions’ at a point in time and possibly bring about phase transitions
by their actions. This also shows that it should be possible to account for the history of urban development as a sequence
of initial conditions, illustrating path dependence. Secondly, the model could be deployed in relation to a new shopping zone,
and the tools developed here used to calculate the minimum size of a new development in order for it to compete. We explore
the ‘minimum size’ idea in relation to a new shopping zone in London. 相似文献
13.
14.
The EH and EL enstatite chondrites are the most reduced chondrite groups, having formed in nebular regions where the gas may
have had high C/O and/or pH2/pH2O ratios. Enstatite chondrites (particularly EH) have higher CI- and Mg-normalized abundances of halogens (especially F and
Cl) and nitrogen than ordinary chondrites and most groups of carbonaceous chondrites. Even relative to CI chondrites, EH and
EL chondrites are enriched in F. We have found that literature values for the halogen abundance ratios in EH and EL chondrites
are strongly correlated with the electronegativities of the individual halogens. We suggest that the most reactive halogens
were the most efficient at forming compounds (e.g., halides) that were incorporated into EH-chondrite precursor materials.
It seems plausible that, under the more-oxidizing conditions pertaining to the other chondrite groups, a larger fraction of
the halogens remained in the gas. Nitrogen may have been incorporated into the enstatite chondrites as simple nitrides that
did not condense under the more-oxidizing conditions in the regions where other chondrite groups formed. Literature data show
that unequilibrated enstatite chondrites have light bulk N (δ 15N ≈ −20‰) compared to most ordinary (−5 to +20‰) and carbonaceous (+20 to +190‰) chondrites; this may reflect the contribution
in enstatite chondrites of nitride condensates with δ15 N values close to the proposed nebular mean (~−400‰). In contrast, N in carbonaceous chondrites is mainly contained within
15N-rich organic matter. The major carrier of N in ordinary chondrites is unknown. 相似文献
15.
Alan B. Binder 《Icarus》1983,55(1):188-189
16.
17.
Martin Connors Alan R. Hildebrand Mark Pilkington Carlos Ortiz-Aleman Rene E. Chavez Jaime Urrutia-Fucugauchi Eduardo Graniel-Castro Alfredo Camara-Zi Juan Vasquez John F. Halpenny 《Geophysical Journal International》1996,127(3):F11-F14
The buried Chicxulub impact structure is marked by a dramatic ring of sinkholes (called cenotes if containing water), and adjacent less prominent partial rings, which have been shown to coincide with maxima in horizontal gravity gradients and a topographic depression. These observations, along with the discreteness and spacing of the features, suggest a formation mechanism involving faulting in the outer slump zone of the crater, which would thus have a diameter of approximately 180 km.
An opposing view, based primarily on the interpretation of gravity data, is that (he crater is much larger than the cenote ring implies. Given the association of the known cenote ring with faults, we here examine northern Yucatan for similar rings in gravity, surface features and elevation, which we might expect to be associated with outer concentric faults in the case of a larger, possibly multiring, structure.
No such outer rings have been found, although definite patterns are seen in the distribution of karst features outside the crater rim. We explain these patterns as resulting mainly from deformation related to the block fault zone that parallels tbe shelf edge of eastern Yucatan. 相似文献
An opposing view, based primarily on the interpretation of gravity data, is that (he crater is much larger than the cenote ring implies. Given the association of the known cenote ring with faults, we here examine northern Yucatan for similar rings in gravity, surface features and elevation, which we might expect to be associated with outer concentric faults in the case of a larger, possibly multiring, structure.
No such outer rings have been found, although definite patterns are seen in the distribution of karst features outside the crater rim. We explain these patterns as resulting mainly from deformation related to the block fault zone that parallels tbe shelf edge of eastern Yucatan. 相似文献
18.
Polarization analysis of multi-component seismic data is used in both exploration seismology and earthquake seismology. In
single-station polarization processing, it is generally assumed that any noise present in the window of analysis is incoherent,
i.e., does not correlate between components. This assumption is often violated in practice because several overlapping seismic
events may be present in the data. The additional arrival(s) to that of interest can be viewed as coherent noise. This paper
quantifies the error because of coherent noise interference. We first give a general theoretical analysis of the problem.
A simple mathematical wavelet is then used to obtain a closed-form solution to the principal direction estimated for a transient
incident signal superposed with a time-shifted, unequal amplitude version of itself, arriving at an arbitrary angle to the
first wavelet. The effects of relative amplitude, arrival angle, and the time delay of the two wavelets on directional estimates
are investigated. Even for small differences in angle of arrival, there may be significant error (>10°) in the azimuth estimate. 相似文献
19.
20.
Alan B Blaxland 《Lithos》1977,10(1):1-8
RbSr isotopic analyses of 10'whole-rock samples from the controversial peralkaline Norra Kärr complex of southern Sweden suggest an age (1580±62 m.y.) considerably older than had previously been anticipated, and indicate an initial 87Sr/86Sr ratio of 0.7072±0.0035 (errors at 2σ). The isotopic data are consistent with a primary magmatic origin for the Norra Kärr agpaites, but data from 8 mineral separates show that they have experienced at least one period of metamorphic disturbance since the original intrusion; the last episode of isotopic readjustment must have occurred after 1250 m.y. before present, and is attributed to the Sveconorwegian (Grenville) metamorphism. 相似文献