THE CRITERION FOR PROPAGATION OF THE POSITIVE STREAMER AND ITS APPLICATION

Through analysing the upward leader from a tall structure under storm condition,it is concluded,in this paper,that the intense electric field in a limited,small region provides the necessary condition for the natural lightning discharge to occur.Such a “local intense field” could make it possible that a weak streamer grows and continues to travel in ambient field of only several tens of kilovolts per meter,which otherwise,could propagate only in the ambient field of several hundreds of kilovolts per meter. Based on the positive streamer theory and energy-balance limitation,we have deduced a criterion for de- tcrmining whether the positive streamer propagates continuously or not. Using this criterion,we derive the striking distance for tall structures,and the electric environment for ar- tificially triggering lightning by rocket launching with grounding wire.Both results agree with observations.

An empirical comparison of four technology-mediated travel survey methods

The increasing demana for advanced modelling methods, which can reflect complex travel activities of individuals, requires enhanced travel data collection methods. The introduction of GPS-assisted data collection methods has provided an alternative to the conventional methods of travel data collection. GPS-assisted data collection methods improve the accu- racy of data collection and enable capturing more details of individuals＇ travel behaviour. Recent technological advancements in smartphone-based positioning technologies and communication facilities have opened up new opportunities to apply smartphones as the media of GPS-assisted data collection. Although, different GPS-assisted methods have been employed recently, their performance has not been widely evaluated in real-world experi- ments compared to traditional data collection methods. Accordingly, this paper evaluates the performance of three GPS-assisted methods, namely handheld GPS tracking, smart- phone-based GPS tracking and smartphone-based prompted-recall data collection methods, in conjunction with the web-based data collection to shed light on different aspects of GPS- assisted data collection methods. These methods are compared in terms of the quality and accuracy of the collected data, the demographic attributes of participants and the specifi- cations of labelled trips. The results show that an appropriate employment of smartphones enhances the accuracy of data collection. It is also found that putting an extra burden on participants during a travel data collection survey results in lower trip-rates and poor data quality. Finally, it is found that the application of smartphone-assisted data collection methods help reporting non-motorised trips more accurately.