Multispectral UAV monitoring of submerged seaweed in shallow water

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Multispectral proximal remote sensing by unmanned aerial vehicles (UAVS): low-cost sensors application in precision agricolture and coastal monitoring

Negli ultimi anni l’uso di Aeromobili a Pilotaggio Remoto (APR), o semplicemente droni, ha interessato molti campi. La loro estrema versatilità li ha resi efficaci strumenti anche nel campo del telerilevamento, grazie alla crescente disponibilità di sensoristica installabile a bordo, da sistemi LiDAR (Light Detection And Ranging) a camere digitali di vario tipo. Tra queste ultime, è aumentato il numero di camere multispettrali a basso costo capaci di acquisire informazioni spettrali nel campo del vicino infrarosso. Grazie all’elevata risoluzione geometrica (centimetrica) che è possibile raggiungere mediante l’uso di APR, le applicazioni di queste camere multispettrali possono oggi spaziare dall’agricoltura di precisione alla determinazione delle varie coperture al suolo attraverso procedure di classificazione. L’effettiva attendibilità dei dati acquisiti dipende tuttavia sia dalla conoscenza della sensibilità spettrale dei sensori impiegati, sia dalle reali condizioni di illuminazione ambientale al momento dell’acquisizione di ogni singola immagine. In questa tesi di dottorato vengono pertanto analizzati i risultati ottenibili attraverso l’impiego di alcune camere multispettrali a basso costo con particolare riguardo all’agricoltura di precisione e al monitoraggio costiero delle alghe nella Sacca di Goro, sia in termini di accuratezza finale conseguita che di necessità ed efficacia nell’impiego di procedure di calibrazione radiometrica. Dai risultati ottenuti si evince come sia possibile utilizzare con successo camere multispettrali a basso costo sia nell’ambito della caratterizzazione dello stato di salute di una coltivazione che nel monitoraggio algale. Tuttavia è solamente grazie a procedure rigorose di calibrazione radiometrica che diventa possibile effettuare un monitoraggio nel tempo, grazie alla possibilità di calcolare valori di riflettanza accurati ed attendibili. In mancanza di dispositivi quali sensori di irradianza ambientale o pannelli a riflettanza nota risulta solamente possibile effettuare computi di indici spettrali con una validità temporale limitata al singolo rilievo.In the last few years the use of Unmanned Aerial Vehicles (UAVs), or simply drones, has involved many fields. Their extreme versatility has made them effective devices also in the field of remote sensing, thanks to the increasing availability of instrumentation that can be installed onboard, from LiDAR (Light Detection And Ranging) systems to digital cameras of different types. Among these, the number of low-cost multispectral cameras able to detect spectral information in the near infrared region has increased significantly. Thanks to the high geometric resolution (at a centimetre level) made possible by the use of UAVs, the applications of these cameras may now space from the precision farming (or precision agriculture) to the detection and characterization of the different ground cover types. However, the effective reliability of the data sensed depend on both the knowledge of the spectral sensitivity of the used sensors and the actual environmental light conditions at the time of each image acquisition. Therefore, this PhD thesis analyses the results retrievable by the use of some low-cost multispectral cameras with particular regard to the precision farming and the coastal monitoring of the submerged seaweed in the lagoon of Goro, both in terms of final accuracy and effectiveness in using radiometric calibration procedures. The results highlight how multispectral low-cost cameras can be successfully used to assess the health status of crops as well as detecting the seaweed. However, it is only applying rigorous radiometric calibration procedures that becomes possible to perform a monitoring over time, computing accurate and reliable reflectance values. In lack of devices such as downwelling light sensors or known reflectance panels it is only possible to compute spectral indices with a temporal validity limited to the single survey

UAV Approach for Detecting Plastic Marine Debris on the Beach: A Case Study in the Po River Delta (Italy)

Anthropogenic marine debris (AMD) represent a global threat for aquatic environments. It is important to locate and monitor the distribution and presence of macroplastics along beaches to prevent degradation into microplastics (MP), which are potentially more harmful and more difficult to remove. UAV imaging represents a quick method for acquiring pictures with a ground spatial resolution of a few centimeters. In this work, we investigate strategies for AMD mapping on beaches with different ground resolutions and with elevation and multispectral data in support of RGB orthomosaics. Operators with varying levels of expertise and knowledge of the coastal environment map the AMD on four to five transects manually, using a range of photogrammetric tools. The initial survey was repeated after one year; in both surveys, beach litter was collected and further analyzed in the laboratory. Operators assign three levels of confidence when recognizing and describing AMD. Preliminary validation of results shows that items identified with high confidence were almost always classified properly. Approaching the detected items in terms of surface instead of a simple count increased the percentage of mapped litter significantly when compared to those collected. Multispectral data in near-infrared (NIR) wavelengths and digital surface models (DSMs) did not significantly improve the efficiency of manual mapping, even if vegetation features were removed using NDVI maps. In conclusion, this research shows that a good solution for performing beach AMD mapping can be represented by using RGB imagery with a spatial resolution of about 200 pix/m for detecting macroplastics and, in particular, focusing on the largest items. From the point of view of assessing and monitoring potential sources of MP, this approach is not only feasible but also quick, practical, and sustainable

Quality Assessment of Photogrammetric Models for Façade and Building Reconstruction Using DJI Phantom 4 RTK

Aerial photogrammetry by Unmanned Aerial Vehicles (UAVs) is a widespread method to perform mapping tasks with high-resolution to reconstruct three-dimensional (3D) building and façade models. However, the survey of Ground Control Points (GCPs) represents a time-consuming task, while the use of Real-Time Kinematic (RTK) drones allows for one to collect camera locations with an accuracy of a few centimeters. DJI Phantom 4 RTK (DJI-P4RTK) combines this with the possibility to acquire oblique images in stationary conditions and it currently represents a versatile drone widely used from professional users together with commercial Structure-from-Motion software, such as Agisoft Metashape. In this work, we analyze the architectural application of this drone to the photogrammetric modeling of a building with particular regard to metric survey specifications for cultural heritage for 1:20, 1:50, 1:100, and 1:200 scales. In particular, we designed an accuracy assessment test signalizing 109 points, surveying them with total station and adjusting the measurements through a network approach in order to achieve millimeter-level accuracy. Image datasets with a designed Ground Sample Distance (GSD) of 2 mm were acquired in Network RTK (NRTK) and RTK modes in manual piloting and processed both as single façades (S–F) and as an overall block (4–F). Subsequently, we compared the results of photogrammetric models generated in Agisoft Metashape to the Signalized Point (SP) coordinates. The results highlight the importance of processing an overall photogrammetric block, especially whenever part of camera locations exhibited a poorer accuracy due to multipath effects. No significant differences were found between the results of network real-time kinematic (NRTK) and real-time kinematic (RTK) datasets. Horizontal residuals were generally comparable to GNSS accuracy in NRTK/RTK mode, while vertical residuals were found to be affected by an offset of about 5 cm. We introduced an external GCP or used one SP per façade as GCP, assuming a poorer camera location accuracy at the same time, in order to fix this issue and comply with metric survey specifications for the widest architectural scale range. Finally, both S–F and 4–F projects satisfied the metric survey requirements of a scale of 1:50 in at least one of the approaches tested

Esperienze di rilevamento fotogrammetrico e multispettrale da APR per la stima dei danni in agricoltura

La sempre maggiore frequenza di eventi metereologici “estremi” pone la necessità di disporre di un metodo di stima dei danni subìti dalle colture in grado di fornire risultati oggettivi, accurati e a costi contenuti allo stesso tempo. L’operazione di stima, infatti, non deve incidere economicamente in modo significativo sull’entità del risarcimento che le compagnie di assicurazione devono corrispondere all’agricoltore assicurato. Il recente sviluppo del cosiddetto telerilevamento di prossimità da APR sembra offrire una soluzione interessante sia dal punto di vista tecnologico che da quello economico. In questo studio vengono prese in esame due colture estensive e un danno tipico che esse possono subire a causa dell’azione combinata del vento, della pioggia o della grandine: il cosiddetto “allettamento delle piante”. La base per stabilire l’entità economica del danno risiede nella quantificazione della superficie delle zone danneggiate e del livello di allettamento delle piante stesse

UAVs for Structure-From-Motion Coastal Monitoring: A Case Study to Assess the Evolution of Embryo Dunes over a Two-Year Time Frame in the Po River Delta, Italy

Coastal environments are usually characterized by a brittle balance, especially in terms of sediment transportation. The formation of dunes, as well as their sudden destruction as a result of violent storms, affects this balance in a significant way. Moreover, the growth of vegetation on the top of the dunes strongly influences the consequent growth of the dunes themselves. This work presents the results obtained through a long-term monitoring of a complex dune system by the use of Unmanned Aerial Vehicles (UAVs). Six different surveys were carried out between November 2015 and December 2017 in the littoral of Rosolina Mare (Italy). Aerial photogrammetric data were acquired during flight repetitions by using a DJI Phantom 3 Professional with the camera in a nadiral arrangement. The processing of the captured images consisted of the reconstruction of a three-dimensional model using the Structure-from-Motion (SfM). Each model was framed in the European Terrestrial Reference System (ETRS) using GNSS geodetic receivers in Network Real Time Kinematic (NRTK). Specific data management was necessary due to the vegetation by filtering the dense cloud. This task was performed by both performing a slope detection and a removal of the residual outliers. The final products of this approach were thus represented by Digital Elevation Models (DEMs) of the sandy coastal section. In addition, DEMs of Difference (DoD) were also computed for the purpose of monitoring over time and detecting variations. The accuracy assessment of the DEMs was carried out by an elevation comparison through especially GNSS-surveyed points. Relevant cross sections were also extracted and compared. The use of the Structure-from-Motion approach by UAVs finally proved to be both reliable and time-saving thanks to quicker in situ operations for the data acquisition and an accurate reconstruction of high-resolution elevation models. The low cost of the system and its flexibility represent additional strengths, making this technique highly competitive with traditional ones

A Low-Cost GNSS Prototype for Tracking Runners in RTK Mode: Comparison with Running Watch Performance

GNSS positioning is widely use in every kind of application. Nowadays, low-cost GNSS modules are becoming available to apply the Real-Time Kinematic mode in those applications in which a centimeter-level accuracy would be appreciated for a precise positioning. In this work, we developed a prototype for collecting data in RTK mode with a single-frequency multi-constellation device during some physical tests performed by a professional runner. Prior to do this, we assessed the accuracy in estimating the distance actually covered during a walking on a signalized line. Also, we verified the capability to detect short sprints of about 12–15 s. Finally, we compared the results of our prototype with a Polar M430 running watch during three Cooper tests and a Kosmin test. The comparison highlighted that the running watch overestimated the total distance systematically and did not describe the performance of the athlete accurately in time. The distance overestimation was +4.7% on average using the running watch, whereas our prototype system exhibited an error level of about 0.1%

Monitoraggio di area lagunare tramite UAVS e telerilevamento multispettrale

Il rilevamento fotogrammetrico basato su sistemi UAV rappresenta da diversi anni una valida alternativa alle tecniche più tradizionali di rilievo terrestre (tramite stazione totale o GNSS), di rilievo fotogrammetrico da aereo o di rilievo satellitare. In particolare, nelle zone difficili da raggiungere, come ad esempio le aree lagunari o deltizie, i vantaggi di un sistema UAV sono soprattutto legati alla velocità di acquisizione del dato, alla precisione ed al grado di dettaglio elevati ed ai costi relativamente limitati. Gli svantaggi si possono sostanzialmente riassumere nel grande numero di immagini rapidamente acquisite e quindi nella grande mole di dati da elaborare; svantaggio che comporta spesso la riduzione delle aree rilevate soprattutto nel caso di rilievo alle grandi e grandissime scale. Nell’esempio del presente lavoro si analizzano tutti questi aspetti in alcuni case studies nella Sacca di Goro – Comacchio (Provincia di Ferrara). Si tratta di un’area lagunare di particolare interesse sia per gli aspetti naturalistici sia per gli aspetti economici (coltivazione di mitili). A causa dei continui apporti di sedimenti da parte della vicina foce del fiume Po, negli ultimi anni, sono stati realizzati numerosi rilievi di monitoraggio di alcune zone della laguna tramite immagini satellitari ad alta risoluzione oltre a rilievi batimetrici e terrestri. Nel caso specifico si sono utilizzati UAVS di tipo commerciale (Phantom 3 – PRO), con camera fotografica standard, nel rilevamento fotogrammetrico di alcune aree della Sacca di Goro che presentano una rapida evoluzione. Il rilievo è stato realizzato anche con UAVS dotato di camera multispettrale (Micasense RedEdge) utilizzata per studiare l’evoluzione della vegetazione di alcune aree interne della Sacca di Goro

IL RILEVAMENTO MULTISPETTRALE NELL’AGRICOLTURA DI PRECISIONE: COMPARAZIONE DEI RISULTATI CON SENSORI A BASSO COSTO

La necessità di ottimizzare e ridurre l’uso di composti chimici, sia fertilizzanti che pesticidi, nonché la salvaguardia delle risorse idriche, riconosciuta anche a livello europeo, pongono oggi nuove ed incalzanti sfide all’agricoltura. Negli ultimi anni, il crescente impiego di Aeromobili a Pilotaggio Remoto sta fornendo, in una moltitudine di settori, strumenti nuovi e tecniche innovative sia di monitoraggio che di intervento, con costi sempre minori. L’applicazione degli APR nella cosiddetta “agricoltura di precisione” costituisce il punto di incontro tra questi due “mondi”, consentendo una rapida ed accurata mappatura dello stato di salute della coltivazione, con una elevata risoluzione geometrica e con notevoli vantaggi in termini operativi ed economici. Il presente studio mostra i primi risultati ottenuti attraverso l’impiego di alcuni tra i sensori multispettrali a più basso costo attualmente disponibili in commercio allo scopo di produrre mappe di vigore della coltivazione, punto fondamentale di partenza per ogni ulteriore analisi agronomica ed intervento mirato di prescrizione. Il confronto mette in evidenza alcuni limiti e vantaggi delle tecnologie analizzate, dal sensore mono-ottica a quello multi-ottica con sensore di luminosità, senza prescindere dall’onere in termini di post-processamento dei dati acquisiti

Modeling and deterioration mapping of façades in historical urban context by close-range ultra-lightweight UAVs photogrammetry

Deterioration mapping of façades may be a challenging task in historical urban contexts, often characterized by narrow streets and tall buildings. In these situations, the application of Terrestrial Laser Scanning or Terrestrial Photogrammetry does not provide satisfactory results. Furthermore, the use of Unmanned Aerial Vehicles, the so-called “drones”, is generally forbidden in crowded places for safety reasons unless they are lighter than a weight imposed by the regulations in force in each country. This article presents the application of a very small and ultra-lightweight drone (less than 300 g) equipped with a low-cost camera, particularly suitable for operating in very narrow spaces. A close range photogrammetric survey performed to analyze a huge historical building façade in Bologna (Italy) pointed out a good accuracy level, confirmed by the validation of the results carried out by a 3D laser scanner survey. Moreover, a map of the degradation has been realized to support further restoration analysis, demonstrating that this low-cost, fast, and easy-to-use survey technique can be applied to other similar urban contexts at the same time