Our research focusses on the problem of finding nearby peers in the Internet. We focus on one particular approach, Beacon Triangulation that is widely used to solve the peer-finding problem. Beacon Triangulation is based on relative distances of nodes to some special nodes called beacons. The scheme gives an error when a new node that wishes to join the network has the same relative distance to two or more nodes. One of the reasons for the error is that two or more nodes have the same distance vectors. As a part of our research work, we derive the conditions to ensure the uniqueness of distance vectors in any network given the shortest path distribution of nodes in that network. We verify our analytical results for G(n, p) graphs and the Internet. We also derive other conditions under which the error in the Beacon Triangulation scheme reduces to zero. We compare the Beacon Triangulation scheme to another well-known distance estimation scheme known as Global Network Positioning (GNP).

An ad hoc network is formed by a group of self-configuring nodes, typically deployed in two or three dimensional spaces, and communicating with each other through wireless or some other media. The distinct characteristics of ad hoc networks include the lack of pre-designed infrastructure, the natural correlation between the network topology and geometry, and limited communication and computation resources. These characteristics introduce new challenges and opportunities for de- signing ad hoc network applications. This dissertation studies various optimization problems in ad hoc network information retrieval and transmission. Information stored in ad hoc networks is naturally associated with its location. To effectively retrieve such information, we study two fundamental problems, range search and object locating, from a distance sensitive point of view, where the retrieval cost depends on the distance between the user and the target information. We develop a general framework that is applicable to both problems for optimizing the storage overhead while maintaining the distance sensitive retrieval requirement. In addition, we derive a lowerbound result for the object locating problem which shows that logarithmic storage overhead is asymptotically optimal to achieve linear retrieval cost for growth bounded networks. Bandwidth is a scarce resource for wireless ad hoc networks, and its proper utilization is crucial to effective information transmission. To avoid conflict of wireless transmissions, links need to be carefully scheduled to satisfy various constraints. In this part of the study, we first consider an optimization problem of end-to-end on- demand bandwidth allocation with the single transceiver constraint. We study its complexity and present a 2-approximation algorithm. We then discuss how to estimate the end-to-end throughput under a widely adopted model for radio signal interference. A method based on identifying certain clique patterns is proposed and shown to have good practical performance.

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This study looks at the difference in accessibility to green spaces within the four largest Scottish cities. Having access to green spaces provides several physical and mental health benefits while also providing important ecosystem services. Previous studies show that the frequency of use of a green space declines once the distance surpasses 300 m to an access point. Geographic Information Systems (GIS) were used to analyse the service area of an access point to a green space, from which the rate of accessibility is established. The study also analyses the difference in accessibility between Euclidean and network distance. It is found that the Euclidean difference underestimates the distance needed to reach an access point and that 300 m recommendation by Euclidean distance is more closely resembles 500 m network distance. This study recommends that a distinction be made between which measurement metric is used when stating distances regarding accessibility, in order to create a more consistent approach.

The p-median problem is often used to locate p service centers by minimizing their distances to a geographically distributed demand (n). The optimal locations are sensitive to geographical context such as road network and demand points especially when they are asymmetrically distributed in the plane. Most studies focus on evaluating performances of the p-median model when p and n vary. To our knowledge this is not a very well-studied problem when the road network is alternated especially when it is applied in a real world context. The aim in this study is to analyze how the optimal location solutions vary, using the p-median model, when the density in the road network is alternated. The investigation is conducted by the means of a case study in a region in Sweden with an asymmetrically distributed population (15,000 weighted demand points), Dalecarlia. To locate 5 to 50 service centers we use the national transport administrations official road network (NVDB). The road network consists of 1.5 million nodes. To find the optimal location we start with 500 candidate nodes in the network and increase the number of candidate nodes in steps up to 67,000. To find the optimal solution we use a simulated annealing algorithm with adaptive tuning of the temperature. The results show that there is a limited improvement in the optimal solutions when nodes in the road network increase and p is low. When p is high the improvements are larger. The results also show that choice of the best network depends on p. The larger p the larger density of the network is needed.

There exists a debate regarding where to build new housing in increasingly denser cities today. Often it is the green spaces that must be sacrificed which lead to conflicts among decision makers and citizens. Although, some sources claim (Byrne et al., 2010; Van Herzele & Wiedemann, 2003) that it is not only the fact that there is a green area close to you that matters; but rather which values that piece of green land offers you as a citizen. Values of parks can be described as the features or attributes the park area possess that inspire people to go to and stay there. In Stockholm, Sweden these values are connected to sociotopes, a delimited area containing a set of user values with social meaning (Ståhle, 2006). User values thus describes an activity or an experience that is present at a location.  There exists a lack of and a desire for more detailed mapping of user values of green areas as well as a potential need for finding an efficient method for aiding in citizen dialogue when green areas are planned for urban development. The purpose of this thesis is thus to try to incorporate geographical information science (GIS) in urban planning by investigating if it is possible to measure the physical accessibility of user values of a green area before and after its urban development in lines with the recommendations on accessibility from the municipality. Then, try to create a visual tool to be used in the designing phase of urban planning and in citizen dialogue when developing a new urban area. To do this, a case study was done of Årstafältet, a green area in Stockholm that is planned for urban development and improvement of existing and creation of new user values. By using GIS, the user values were defined spatially, and geographical data based on the study area currently as well as after the development where found or created based on descriptions of the new area in planning documents. Accessibility was defined as the physical distance a resident must walk from their home to the closest access point of a user value. Based on previous research, 1000m was deemed the largest distance a resident can walk to be considered a potential user of a user value. An access analysis was made for all residents within the study area to the closest access point of every user value.  The results of the analysis were visualized in two ways, one regarding urban design and another regarding citizen dialogue in the form of bivariate maps and a GIS web application. Apart from the visual maps, some numerical results regarding distance, distance change and number of accessed user values were calculated.   The maps point out the areas that are mostly affected both in the negative and positive sense.  More research needs to be done to decide the best way of deriving and using the numerical measures. Because of the many assumptions and generalizations made in the study it is difficult to make any overall conclusions about the accessibility of user values at Årstafältet. What is more interesting is the reception by people in the field of GIS and urban planning; which was in general positive. They noted upon the important aspect of concretize the design phase of the urban planning process, which often is based on feeling, and create a solid ground to base more informed decisions upon. The visualization methods presented were well received as tools for enabling more people access into the planning process as well as an easy way of exploring geographical data. Also, the possibility to extend this type of access analysis beyond sociotopic user values was deemed as very useful. Lastly, they expressed that this type of analysis is desired by the workers in the field and highly relevant in today’s urban planning process.

Reliability requirements for power systems have been constantly increasing, as customers of electric power desire high power availability. In order to improve continuity of supply in medium voltage (MV) networks, utilities in many countries in Europe, North/South America and Oceania use resonant earthing and allow the operation of the network during a single phasetoearth fault for periods from several seconds up to several hours, since the earthfault current at the fault point in such systems is considerably reduced and is unlikely to damage equipment or create hazardous voltages for people or livestock. Due to the neutral potential displacement during the phasetoearth fault, the healthy phases can rise up to 1.73 times the nominal values in the entire network, overstressing insulation and increasing the probability of a second fault from another phasetoearth fault somewhere in the network. The situation of two simultaneously active phasetoearth faults on different phases at different locations in a network is commonly called a crosscountry or doubleearth fault. The current through the earth then becomes many times higher than in case of a single phasetoearth fault in a resonantearthed network. Few studies about CrossCountry faults in MV systems have been done so far, particularly with real fault data and simulations. This thesis has as main objectives to: improve understanding of how crosscountry faults behave in resonantearthed systems; show how the double fault situation can be analytically calculated; and study what difficulties these faults can pose to traditional distance protection. This is done by: analyzing recordings of real faults, obtained from utilities in Scandinavia; modelling the fault situations analytically; simulating such systems with real system data; and examining in particular how faultedphase selection and distance protection behave during these faults. The developed mathematical model was compared with a simple simulated system. The results showed that the modelling produces accurate results. Analysis of the real faults showed that crosscountry faults cannot be equated directly to more traditional singlephase or doublephase faults on particular feeders, since voltages and currents can behave in atypical manners. Finally, during the comprehensive simulations of such network, limitations of traditional distance protection were identified. Phasetophase loops can face difficulties in properly detecting crosscountry faults even when one of the two simultaneous faults is relatively close to the measuring point. Phasetoearth loops can only be reliable when faults are solid or with very low resistance and in conditions where zero sequence current is present, while for multiple infeeds can also face difficulties. / Kraven på elkraftsystemets pålitlighet ökar hela tiden, då kundernas krav på tillgänglighet hela tiden stiger. Som ett led i att förbättra leveranssäkerheten på mellanspänningsnivån har ett flertal elnätsbolag i Europa, Nord och Sydamerika samt Oceanien börjat använda spoljordning. Genom detta kan jordfel under drift tillåtas, på allt ifrån sekundbasis till timmar. På detta sätt undviks driftavbrott vid jordfel. Spoljordning gör att jordfelsströmmen reduceras vilket minimerar risken för person, djur och egendomsskador. Under ett enfasigt jordfel höjs spänningen i de två opåverkade faserna till maximalt 1.73 gånger den nominella spänningen, detta i hela det aktuella nätet. Spänningshöjningen riskerar att ge skador på isolaringen, vilket gör att risken för uppkomsten av ett andra jordfel någonstans i samma nätavsnitt ökar. Att två enfasiga jordfel på olika platser inträffar samtidigt i samma nät, kallas dubbelt jordfel, på engelska crosscountry fault. Strömmen genom jord i ett spoljordat nät blir i detta fallet mycket högre än för ett enfasigt jordfel. Än så länge har bara ett fåtal studier med verkliga data och simuleringar gällande dubbla jordfel på mellanspänningsnivå utförts. Detta examensarbete syftar till att: förbättra förståelsen för hur dubbla jordfel uppkommer i spoljordade nät; beskriva hur dubbla jordfel kan beräknas analytiskt; och visa på vilka svårigheter dubbla jordfel kan ge för distansskydd. Detta görs genom: att analysera verkliga fel uppkomna i Skandinavien; analytisk modellering av dessa situationer; simulering med data från verkliga elnät; undersökning av hur felbehäftade delar och dess distansskydd beter sig vid fel. Den utvecklade matematiska modelleringsmetoden är jämförd med ett enkelt system som var simulerat, och resultaten av jämförelsen visar att modellen ger ett överensstämmande och noggrant resultat. Analysen av verkliga fel visar att dubbla jordfel inte direkt kan likställas med traditionella enfas- eller tvåfasfel för ett och samma utgående fack, då spänning och strömmar kan ha olika beteende. Avslutningsvis kunde begränsningar hos traditionella distansskydd upptäckas, tack var en omfattande simulering av nyss nämnda elnät. Fas- fas slingan kan se svårigheter att detektera dubbla fel även när ett av dessa två är nära till mätpunkten. Fas- jord slingan är endast pålitlig när felresistansen är försumbar, eller i fall där felresistansen är låg, samt i fall när nollföljdsströmmen är närvarande. Flera felströmsinmatningar kan likaså ställa till svårigheter.

This research work concentrate on developing digital soil maps to support field based plant phenotyping research. We have developed soil organic matter content (OM), cation exchange capacity (CEC), natural soil drainage class, and tile drainage line maps using topographic indices and aerial imagery. Various prediction models (universal kriging, cubist, random forest, C5.0, artificial neural network, and multinomial logistic regression) were used to estimate the soil properties of interest.