Satellite imagery has traditionally been used to collect crop statistics, but its low resolution and registration accuracy limit agricultural analytics to plant stand levels and large areas. Precision agriculture seeks analytic tools at near single plant level, and this work explores how to improve aerial photogrammetry to enable inter-day precision agriculture analytics for intervals of up to a month.Our work starts by presenting an accurately registered image time series, captured up to twice a week, by an unmanned aerial vehicle over a wheat crop field. The dataset is registered using photogrammetry aided by fiducial ground control points (GCPs). Unfortunately, GCPs severely disrupt crop management activities. To address this, we propose a novel inter-day registration approach that only relies once on GCPs, at the beginning of the season.The method utilises LoFTR [1], a state-of-the-art image-matching transformer. The original LoFTR network was trained using imagery of outdoor urban areas. One of our contributions is to extend LoFTR’s training method, which uses matching images of a static scene, to a dynamic scene of plants undergoing growth. Another contribution is a thorough evaluation of our registration method that integrates intraday crop reconstruction with earlier-day scans in a seven degree-of-freedom alignment. Experimental results show the advantage of our approach over other matching algorithms and demonstrate the importance of retraining using crop scenes, and a training method customised for growing crops, with an average registration error of 27 cm across a season.

Research on learning suitable feature descriptors for Computer Vision has recently shifted to deep learning where the biggest challenge lies with the formulation of appropriate loss functions, especially since the descriptors to be learned are not known at training time. While approaches such as Siamese and triplet losses have been applied with success, it is still not well understood what makes a good loss function. In this spirit, this work demonstrates that many commonly used losses suffer from a range of problems. Based on this analysis, we introduce mixed-context losses and scale-aware sampling, two methods that when combined enable networks to learn consistently scaled descriptors for the first time.

The booming interest in Unmanned Aerial Vehicles (UAV s) is fed by their potentially great impact, however progress is hindered by their limited perception capabilities. While vision-based odometry was shown to run successfully onboard UAV s, loop-closure detection to correct for drift or to recover from tracking failures, has so far, proven particularly challenging for UAVs. At the heart of this is the problem of viewpoint-tolerant place recognition; in stark difference to ground robots, UAVs can revisit a scene from very different viewpoints. As a result, existing approaches struggle greatly as the task at hand violates underlying assumptions in assessing scene similarity. In this paper, we propose a place recognition framework, which exploits both efficient binary features and noisy estimates of the local 3D geometry, which are anyway computed for visual-inertial odometry onboard the UAV. Attaching both an appearance and a geometry signature to each `location', the proposed approach demonstrates unprecedented recall for perfect precision as well as high quality loop-closing transformations on both flying and hand-held datasets exhibiting large viewpoint and appearance changes as well as perceptual aliasing.

Relocalization is a vital process for autonomous robot navigation, typically running in the background of sequential localization and mapping to detect loops in the robot's trajectory. Such loop-closure detections enable corrections for drift accumulated during the estimation processes and even recovery from complete localization failures. In this work, we present a novel approach loosely integrated with a keyframe-based SLAM system to perform loop-closure detection in urban scenarios for autonomous robot navigation. Generating a mesh of the current robot's surroundings in real-time using monocular and inertial cues, the proposed method estimates the most salient plane in the current view, enabling the creation of the corresponding orthophoto for this plane. Evaluating image similarity on orthophotos forms a much better conditioned problem for relocalization, minimizing effects from viewpoint changes. Employing binary image descriptors and tests on their relative constellation in the image, the proposed approach exhibits robustness also to illumination and situational variations common in real scenes, overall resulting to significant improvement in loop-closure detection performance in urban scenes with respect to the state of the art.

Recently, image representations derived from Convolutional Neural Networks (CNNs) have been demonstrated to achieve impressive performance on a wide variety of tasks, including place recognition. In this paper, we take a step deeper into the internal structure of CNNs and propose novel CNN-based image features for place recognition by identifying salient regions and creating their regional representations directly from the convolutional layer activations. A range of experiments is conducted on challenging datasets with varied conditions and viewpoints. These reveal superior precision-recall characteristics and robustness against both viewpoint and appearance variations for the proposed approach over the state of the art. By analyzing the feature encoding process of our approach, we provide insights into what makes an image presentation robust against external variations.