Mapping boulders in rivers is commonly done manually with direct measurements, marking the location and size of each boulder in the field. This method is precise but time consuming and in study areas that are inaccessible by foot or has to turbulent water the danger or expense can prohibit measurements. Using Uncrewed Aerial vehicles (UAV) could prove an effective alternative method, however how accurate UAVs are for mapping boulders in rivers is yet to be determined since the ability to map submerged boulders is limited. The aim of this study is to compare boulder data derived from UAV orthophotos to in-situ field data. Additionally, I investigate if the error between the two methods is controlled by catchment- and reach- scale characteristics. Instream boulders were manually digitized from UAV orthophotos of 8 river reaches in Northern Sweden. Fewer boulders with smaller area were found for UAV, and the error between the two methods correlated with bankfull depth, water level and sediment size distribution. This highlights the importance of water level during UAV flights, since higher water levels resulted in more boulders partially or fully submerged below the water surface decreasing the amount and area of boulders found. Similar correlations between reach-scale channel characteristics were for the UAV data as for the field data showing that data derived from UAV images can be used to better understand boulder- bed rivers. Through mapping and understanding boulder distribution in pre-disturbance reference sites, UAVs can help guide river restoration in a safe, cost and time efficient way.

River restoration is something that is often used to help restore watercourses that were historically used for timber floating. In these restorations, instream wood (IW) plays a big role in increasing biodiversity and habitat heterogeneity in watercourses. Uncrewed Aerial Vehicles (UAVs) have proven to be useful when monitoring changes following river restoration but there is a need for less time-consuming methods to detect IW. This study’s objective was to investigate IW following a river restoration in an alluvial reach in Vargån, Northern Sweden and how well UAVs can be used to do this. Manual digitizing of IW was done in GIS from orthomosaics of three different flight occasions: pre-restoration, directly after and one year after restoration. An object- and color-based automated image thresholding was done to investigate if it could be used to automatically detect IW. The results showed that there were differences in number of IW, volume, width, and length between the different flight occasions. There was also indication that there had been movement of IW as well as changes in cluster composition, with more clusters with a higher number of wood pieces in the latest flight occasion. The automated image thresholding was able to accurately detect IW with an accuracy of 47,4 %, but it had limitations due to natural conditions. However, it showed the possibility of using automated methods to detect IW and with improvements it could become a faster and more accessible way of detecting IW in river monitoring.

In wireless communications, employing pilot symbols enables to estimate the state of the wireless channel at the expense of decreasing the number of symbols available for transmitting data. Addressing this trade-off is particularly challenging when the channel changes rapidly over time, since channel estimates become obsolete over short transmission periods. This master thesis proposes an analytical model to characterize this trade-off and develops an algorithm to find the near-optimal pilot spacing in terms of the achieved over spectral efficiency. This algorithm is simulated in a cellular system that serves uncrewed aerial vehicles. Numerical results indicate that the altitude of the uncrewed aerial vehicle, the Rician factor, the Doppler frequency, and the number of receive antennas influence the overall spectral efficiency and consequently, pilot spacing should take into account these system parameters. / I trådlös kommunikation möjliggör användning av pilotsymboler att uppskatta tillståndet för den trådlösa kanalen på bekostnad av att minska antalet tillgängliga symboler för att överföra data. Att ta itu med denna avvägning är särskilt utmanande när kanalen ändras snabbt över tiden, eftersom kanaluppskattningar blir föråldrade under korta överföringsperioder. Denna masteruppsats föreslår en analytisk modell för att karakterisera denna avvägning och utvecklar en algoritm för att hitta det närmast optimala pilotavståndet i termer av uppnådd över spektral effektivitet. Denna algoritm simuleras i ett cellulärt system som betjänar obemannade flygfarkoster. Numeriska resultat indikerar att höjden för det obemannade luftfartyget, Rician-faktorn, Dopplerfrekvensen, antalet mottagarantenner påverkar den totala spektrala effektiviteten och följaktligen bör pilotavståndet ta hänsyn till dessa systemparametrar.