The mathematics of surface reconstruction

This thesis discusses mathematics engineers use to produce computerized three dimensional im- ages of surfaces. It is self-contained in that all background information is included. As a result, mathematicians who know very little about the technology involved in three dimensional imaging should be able to understand the topics herein, and engineers with no differential geometry background will be able to understand the mathematics. The purpose of this thesis is to unify and understand the notation commonly used by engineers, understand their terminology, and appreciate the difficulties faced by engineers in their pursuitsIt is also intended to bridge the gap between mathematics and engineering. This paper proceeds as follows. Chapter one introduces the topic and provides a brief overview of this thesis. Chapter two provides background information on technology and differential geometry. Chapter three discusses various methods by which normal vectors are estimated. In Chapter four, we discuss methods by which curvature is estimatedIn Chapter six, we put it all together to recreate the surface. Finally, in chapter seven, we conclude with a discussion of future research. Each chapter concludes with a comparison of the methods discussed. The study of these reconstruction algorithms originated from various engineering papers on surface reconstruction. The background information was gathered from a thesis and various differential geometry texts. The challenge arises in the nature of the data with which we work. The surface must be recreated based on a set of discrete points. However, the study of surfaces is one of differential geometry which assumes differentiable functions representing the surface. Since we only have a discrete set of points, methods to overcome this shortcoming must be developed. Two categories of surface reconstruction have been developed to overcome this shortcoming. The first category estimates the data by data by smooth functionsThe second reconstructs the surface using the discrete data directly. We found that various aspects of surface reconstruction are very reliable, while others are only marginally so. We found that methods recreating the surface from discrete data directly produce very similar results suggesting that some underlying facts about surfaces represented by discrete information may be influencing the results

Knowledge and perceptions in participatory policy processes: lessons from the delta-region in the Netherlands

Water resources management issues tend to affect a variety of uses and users. Therefore, they often exhibit complex and unstructured problems. The complex, unstructured nature of these problems originates from uncertain knowledge and from the existence of divergent perceptions among various actors. Consequently, dealing with these problems is not just a knowledge problem; it is a problem of ambiguity too. This paper focuses on a complex, unstructured water resources management issue, the sustainable development—for ecology, economy and society—of the Delta-region of the Netherlands. In several areas in this region the ecological quality decreased due to hydraulic constructions for storm water safety, the Delta Works. To improve the ecological quality, the Dutch government regards the re-establishment of estuarine dynamics in the area as the most important solution. However, re-establishment of estuarine dynamics will affect other uses and other users. Among the affected users are farmers in the surrounding areas, who use freshwater from a lake for agricultural purposes. This problem has been addressed in a participatory decision-making process, which is used as a case study in this paper. We investigate how the dynamics in actors’ perceptions and the knowledge base contribute to the development of agreed upon and valid knowledge about the problem–solution combination, using our conceptual framework for problem structuring. We found that different knowledge sources—expert and practical knowledge—should be integrated to create a context-specific knowledge base, which is scientifically valid and socially robust. Furthermore, we conclude that for the convergence of actors’ perceptions, it is essential that actors learn about the content of the process (cognitive learning) and about the network in which they are involved (strategic learning). Our findings form a plea for practitioners in water resources management to adopt a problem structuring approach in order to deal explicitly with uncertainty and ambiguity

The impact of specific language impairment on working memory in children with ADHD combined type

To access publisher full text version of this article. Please click on the hyperlink in Additional Links fieldThe objective of this study was to examine the impact of comorbid specific language impairment (SLI) on verbal and spatial working memory in children with DSM-IV combined subtype Attention Deficit Hyperactivity Disorder (ADHD-C). Participants were a clinical sample of 812- to 1212-year-old children diagnosed with ADHD-C. A group of ADHD-C with SLI was compared to a group of ADHD-C without SLI, and a group of normal children, matched on age and nonverbal intelligence. The results show that ADHD-C children with SLI scored significantly lower than those without SLI and normal children, on verbal working memory measures only. Both ADHD groups performed normally on spatial working memory measures. It is concluded that working memory deficits are not a specific characteristic of ADHD but are associated with language impairments. The importance of screening for language disorders in studies of neuropsychological functioning in children with ADHD is emphasized

Patches in a side-by-side configuration: a description of the flow and deposition fields

In the last few decades, a lot of research attention has been paid to flow-vegetation interactions. Starting with the description of the flow field around uniform macrophyte stands, research has evolved more recently to the description of flow fields around individual, distinct patches. However, in the field, vegetation patches almost never occur in isolation. As such, patches will influence each other during their development and interacting, complex flow fields can be expected. In this study, two emergent patches of the same diameter (D = 22 cm) and a solid volume fraction of 10% were placed in a side-by-side configuration in a lab flume. The patches were built as an array of wooden cylinders, and the distance between the patches (gap width Delta) was varied between Delta = 0 and 14 cm. Flow measurements were performed by a 3D Vectrino Velocimeter (Nortek AS) at mid-depth of the flow. Deposition experiments of suspended solids were performed for selected gap widths. Directly behind each patch, the wake evolved in a manner identical to that of a single, isolated patch. On the centerline between the patches, the maximum velocity U-max was found to be independent of the gap width Delta. However, the length over which this maximum velocity persists, the potential core L-j, increased linearly as the gap width increased. After the merging of the wakes, the centerline velocity reaches a minimum value U-min. The minimum centerline velocity decreased in magnitude as the gap width decreased. The velocity pattern within the wake is reflected in the deposition patterns. An erosion zone occurs on the centerline between the patches, where the velocity is elevated. Deposition occurs in the low velocity zones directly behind each patch and also downstream of the patches, along the centerline between the patches at the point of local velocity minimum. This downstream deposition zone, a result of the interaction of neighbouring patch wakes, may facilitate the establishment of new vegetation, which may eventually inhibit flow between the upstream patches and facilitate patch merger

Field trial for assessment of avian influenza vaccination effectiveness in Indonesia

The aim of this field study was to determine the efficacy of vaccination against highly pathogenic avian influenza (HPAI) virus strain H5N1 in Indonesia. A limited, prototype clinical trial was performed using a standardised treatment group, in which poultry flocks were vaccinated at least twice with a selected H5N1 vaccine, and a control group comprising flocks treated with nonstandardised procedures chosen by the farmer. Each group consisted of six flocks comprising either layers or native chickens. Haemagglutination inhibition (HI) antibody levels were determined by regular serum sampling, and outbreak surveillance relied on non-Al-vaccinated sentinel birds. After three vaccinations high antibody titres were produced in the treatment group, and the percentage of layers with an HI titre > 40 was approximately 90%. Although no conclusions can be drawn regarding reduction of virus transmission, this study demonstrated that 11 farms remained free from Al during the observation period, and that a surveillance programme based on differentiating infected from vaccinated animals (DIVA) can be implemented

Werk als oplossing?:Narratieven over de waarde van werk ter preventie van en herstel na partnergeweld

Vanuit het idee dat betaald werk leidt tot economische zelfstandigheid, meer keuzevrijheid en een versterking van weerbaarheid is het bevorderen van arbeidsparticipatie van vrouwen een belangrijk speerpunt in het Nederlandse emancipatiebeleid. In de aanpak van geweld tegen vrouwen is deze veronderstelde reeks aan causale verbanden uitgebreid met het idee dat betaald werk vrouwen helpt in het voorkomen van en herstellen na partnergeweld. Wetenschappelijke studies laten echter een paradoxaal verband tussen ‘betaald werk’ en ‘partnergeweld’ zien. Wanneer vrouwen(gaan) werken kan dit bescherming bieden op herhaling van partnergeweld, maar het kan het risico op partnergeweld ook vergroten. Daarnaast is er weinig wetenschappelijk onderzoek verricht naar de waarde van werk in het herstelproces. Dit roept de vraag op hoe beleidsmakers met stelligheid in positieve termen spreken over de waarde van werk in het voorkomen van en herstellen na partnergeweld, terwijl wetenschappelijke onderbouwing daarvoor lijkt te ontbreken. Zijn er vanuit de context van een transitie van een verzorgingsstaat naar een participatiemaatschappij mogelijk verschuivingen opgetreden in het spreken over de waarde van werk? En spreken beleidsuitvoerders in dezelfde termen over de waarde van werk, of uiten zij ook kritiek? Studies naar de waarde van werk zijn van oudsher primair gebaseerd op het mannelijke perspectief. Hoe spreken vrouwen zelf over de waarde van werk in het voorkomen van en herstellen na partnergeweld? En treden er bij de vrouwen of beleidsuitvoerders verschuivingen op in het spreken over de waarde van werk wanneer zij met elkaar in contact komen? Door toepassing van een discoursanalyse op de narratieven van beleidsmakers, beleidsuitvoerders en vrouwelijke slachtoffers van partnergeweld worden deze vragen in dit proefschrift belicht. Om alle verschillende perspectieven te duiden en een antwoord te geven op de overkoepelende vraag ‘wat is de waarde van werk voor vrouwelijke slachtoffers van partnergeweld?’ is een combinatie van theoretische kaders toegepast die de wisselwerking tussen discursieve macht en agency als basis hebben

Wave attenuation at a salt marsh margin: A case study of an exposed coast on the Yangtze estuary

To quantify wave attenuation by (introduced) Spartina alterniflora vegetation at an exposed macrotidal coast in the Yangtze Estuary, China, wave parameters and water depth were measured during 13 consecutive tides at nine locations ranging from 10 m seaward to 50 m landward of the low marsh edge. During this period, the incident wave height ranged from <0.1 to 1.5 m, the maximum of which is much higher than observed in other marsh areas around the world. Our measurements and calculations showed that the wave attenuation rate per unit distance was 1 to 2 magnitudes higher over the marsh than over an adjacent mudflat. Although the elevation gradient of the marsh margin was significantly higher than that of the adjacent mudflat, more than 80% of wave attenuation was ascribed to the presence of vegetation, suggesting that shoaling effects were of minor importance. On average, waves reaching the marsh were eliminated over a distance of similar to 80 m, although a marsh distance of >= 100 m was needed before the maximum height waves were fully attenuated during high tides. These attenuation distances were longer than those previously found in American salt marshes, mainly due to the macrotidal and exposed conditions at the present site. The ratio of water depth to plant height showed an inverse correlation with wave attenuation rate, indicating that plant height is a crucial factor determining the efficiency of wave attenuation. Consequently, the tall shoots of the introduced S. alterniflora makes this species much more efficient at attenuating waves than the shorter, native pioneer species in the Yangtze Estuary, and should therefore be considered as a factor in coastal management during the present era of sea-level rise and global change. We also found that wave attenuation across the salt marsh can be predicted using published models when a suitable coefficient is incorporated to account for drag, which varies in place and time due to differences in plant characteristics and abiotic conditions (i.e., bed gradient, initial water depth, and wave action).