Temporal Processing of News: Annotation of Temporal Expressions, Verbal Events and Temporal Relations
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AbstractThe ability to capture the temporal dimension of a natural language text is essential to many natural language processing applications, such as Question Answering, Automatic Summarisation, and Information Retrieval. Temporal processing is a ¯eld of Computational Linguistics which aims to access this dimension and derive a precise temporal representation of a natural language text by extracting time expressions, events and temporal relations, and then representing them according to a chosen knowledge framework. This thesis focuses on the investigation and understanding of the di®erent ways time is expressed in natural language, on the implementation of a temporal processing system in accordance with the results of this investigation, on the evaluation of the system, and on the extensive analysis of the errors and challenges that appear during system development. The ultimate goal of this research is to develop the ability to automatically annotate temporal expressions, verbal events and temporal relations in a natural language text. Temporal expression annotation involves two stages: temporal expression identi¯cation concerned with determining the textual extent of a temporal expression, and temporal expression normalisation which ¯nds the value that the temporal expression designates and represents it using an annotation standard. The research presented in this thesis approaches these tasks with a knowledge-based methodology that tackles temporal expressions according to their semantic classi¯cation. Several knowledge sources and normalisation models are experimented with to allow an analysis of their impact on system performance. The annotation of events expressed using either ¯nite or non-¯nite verbs is addressed with a method that overcomes the drawback of existing methods v which associate an event with the class that is most frequently assigned to it in a corpus and are limited in coverage by the small number of events present in the corpus. This limitation is overcome in this research by annotating each WordNet verb with an event class that best characterises that verb. This thesis also describes an original methodology for the identi¯cation of temporal relations that hold among events and temporal expressions. The method relies on sentence-level syntactic trees and a propagation of temporal relations between syntactic constituents, by analysing syntactic and lexical properties of the constituents and of the relations between them. The detailed evaluation and error analysis of the methods proposed for solving di®erent temporal processing tasks form an important part of this research. Various corpora widely used by researchers studying di®erent temporal phenomena are employed in the evaluation, thus enabling comparison with state of the art in the ¯eld. The detailed error analysis targeting each temporal processing task helps identify not only problems of the implemented methods, but also reliability problems of the annotated resources, and encourages potential reexaminations of some temporal processing tasks.
PublisherUniversity of Wolverhampton
TypeThesis or dissertation
DescriptionA thesis submitted by Georgiana Marşic.
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Spatial and temporal variations in soil and vegetation dynamics on stabilized desert dunes in Ningxia, ChinaFearnehough, William (University of Wolverhampton, 1998)The dynamics of vegetated dune stabilization were investigated at two field stations of the Institute of Desert Research, Academia Sinica, during the summers of 1993 and 1994. The majority of research was conducted at Shapotou Research Station, where an area of dunes, representing a 37 year stabilization chronosequence, allowed the progress of vegetated dune stabilization to be investigated. Investigations were made into dune vegetation dynamics, aeolian deposition, microphytic crust formation and function, dune surface micromorphology and hydrology. Following stabilization by a combination of straw checkerboards and planted shrubs at Shapotou, aeolian deposits formed a finer textured 'grey sand' layer at the dune surface, which thickened at 1.3 - 1.9 mm y-1. Dust gauging and measurement of aeolian enrichment of the stabilized dune surface revealed a complex pattern of deposition, which varied with shrub cover, topography and distance from the margin of the stabilized dunes. An extensive microphytic crust, comprised of cyanobacteria and bryophytes, formed over the surface of the stabilized dunes. The microphytic crusts proved effective stabilizers, protecting deposited dust and promoting the development of the 'grey sand' layer. The accumulation of 'grey sand' and development of the associated microphytic crust appeared to be controlling features of the dune stabilization process at Shapotou. Dune hydrology altered significantly upon stabilization. Dune moisture is transpired by planted shrubs and subsequent recharge is restricted by the moisture retentive 'grey sand' and microphytic crust. This leads to a progressive desiccation of deeper dune sand and an increase in available moisture at the dune surface. Evaporative losses were increased by the development of the 'grey sand' and microphytic crust, which also significantly decreased infiltration rates. The nocturnal distillation and condensation at the dune surface of subsurface moisture became increasingly significant as dune stabilization progressed. Vegetation responded to these changes in surface stability and hydrology. Shrub cover declined with the desiccation of deeper dune sand, whilst the cover afforded by the microphytic crust and annual species increased in response to the increasing availability of near-surface moisture. Initially straw checkerboards and planted shrubs ensure surface stabilization, but as dune stabilization progresses, the surface stability afforded by the microphytic crust becomes of increasing importance.