The Experts below are selected from a list of 10497 Experts worldwide ranked by ideXlab platform

Josef Broucek - One of the best experts on this subject based on the ideXlab platform.

  • photogrammetry using uav mounted gnss rtk Georeferencing strategies without gcps
    Remote Sensing, 2021
    Co-Authors: Martin Stroner, Rudolf Urban, Jan Seidl, Tomas Reindl, Josef Broucek
    Abstract:

    Georeferencing using ground control points (GCPs) is the most common strategy in photogrammetry modeling using unmanned aerial vehicle (UAV)-acquired imagery. With the increased availability of UAVs with onboard global navigation satellite system–real-time kinematic (GNSS RTK), Georeferencing without GCPs is becoming a promising alternative. However, systematic elevation error remains a problem with this technique. We aimed to analyze the reasons for this systematic error and propose strategies for its elimination. Multiple flights differing in the flight altitude and image acquisition axis were performed at two real-world sites. A flight height of 100 m with a vertical (nadiral) image acquisition axis was considered primary, supplemented with flight altitudes of 75 m and 125 m with a vertical image acquisition axis and two flights at 100 m with oblique image acquisition axes (30° and 15°). Each of these flights was performed twice to produce a full double grid. Models were reconstructed from individual flights and their combinations. The elevation error from individual flights or even combinations yielded systematic elevation errors of up to several decimeters. This error was linearly dependent on the deviation of the focal length from the reference value. A combination of two flights at the same altitude (with nadiral and oblique image acquisition) was capable of reducing the systematic elevation error to less than 0.03 m. This study is the first to demonstrate the linear dependence between the systematic elevation error of the models based only on the onboard GNSS RTK data and the deviation in the determined internal orientation parameters (focal length). In addition, we have shown that a combination of two flights with different image acquisition axes can eliminate this systematic error even in real-world conditions and that Georeferencing without GCPs is, therefore, a feasible alternative to the use of GCPs.

  • evaluation of the Georeferencing accuracy of a photogrammetric model using a quadrocopter with onboard gnss rtk
    Sensors, 2020
    Co-Authors: Martin Stroner, Rudolf Urban, Jan Seidl, Tomas Reindl, Josef Broucek
    Abstract:

    Using a GNSS RTK (Global Navigation Satellite System Real Time Kinematic) -equipped unmanned aerial vehicle (UAV) could greatly simplify the construction of highly accurate digital models through SfM (Structure from Motion) photogrammetry, possibly even avoiding the need for ground control points (GCPs). As previous studies on this topic were mostly performed using fixed-wing UAVs, this study aimed to investigate the results achievable by a quadrocopter (DJI Phantom 4 RTK). Three image acquisition flights were performed for two sites of a different character (urban and rural) along with three calculation variants for each flight: Georeferencing using ground-surveyed GCPs only, onboard GNSS RTK only, and a combination thereof. The combined and GNSS RTK methods provided the best results (at the expected level of accuracy of 1-2 GSD (Ground Sample Distance)) for both the vertical and horizontal components. The horizontal positioning was also accurate when Georeferencing directly based on the onboard GNSS RTK; the vertical component, however, can be (especially where the terrain is difficult for SfM evaluation) burdened with relatively high systematic errors. This problem was caused by the incorrect identification of the interior orientation parameters calculated, as is customary for non-metric cameras, together with bundle adjustment. This problem could be resolved by using a small number of GCPs (at least one) or quality camera pre-calibration.

Tor Arne Johansen - One of the best experts on this subject based on the ideXlab platform.

  • real time Georeferencing of thermal images using small fixed wing uavs in maritime environments
    Isprs Journal of Photogrammetry and Remote Sensing, 2019
    Co-Authors: Hakon Hagen Helgesen, Frederik Stendahl Leira, Torleiv H Bryne, Sigurd Morkved Albrektsen, Tor Arne Johansen
    Abstract:

    Abstract This article considers real-time Georeferencing using a fixed-wing unmanned aerial vehicle (UAV) with a thermal camera. A flexible system for direct Georeferencing is proposed without the need for ground reference points. Moreover, as the system is tailored for highly maneuverable and agile fixed-wing UAVs, no restrictions on the motion are assumed. The system is designed with a solution for accurate time synchronization between sensors. This feature enables tracking of objects with low uncertainty. Sensors for navigation, permitting estimation of the UAV pose with a nonlinear observer, are employed in addition to a thermal camera. The estimated UAV pose is utilized in Georeferencing to acquire Earth-fixed coordinates of objects. The main examples studied in this research are Georeferencing of a static object and of a moving marine vessel. To obtain the desired accuracy, thermal camera calibration and compensation of mounting misalignment errors are discussed. The entire system is validated in two independent field experiments with a thorough analysis of the results. Georeferencing of a static object is conducted with centimeter accuracy when the average position of all measurements is used. The position of a moving marine vessel is obtained with mean accuracy of two meters.

  • a ligth weight thermal camera payload with Georeferencing capabilities for small fixed wing uavs
    International Conference on Unmanned Aircraft Systems, 2015
    Co-Authors: Frederik Stendahl Leira, Kenan Trnka, Thor I Fossen, Tor Arne Johansen
    Abstract:

    This paper discusses the design and implementation of a light-weight thermal camera payload for small fixed-wing UAVs with a pan/tilt mechanism and Georeferencing based on a simple autopilot's GPS and IMU. Further, an effective and simple method to accurately calibrate a thermal camera is developed and tested. In order to evaluate the accuracy of the Georeferencing algorithm, a test flight was conducted. Georeferencing 80 images of an object showed that the average of the 80 images were only 1.6m from the object's actual position when operating at 50 – 100m altitude above ground. Although the average Georeferencing error was estimated to be around 7m, this accuracy is considered sufficient for most of the intended object tracking and surveillance applications.

Clive S Fraser - One of the best experts on this subject based on the ideXlab platform.

  • interior orientation error modelling and correction for precise Georeferencing of satellite imagery
    ISPRS - International Archives of the Photogrammetry Remote Sensing and Spatial Information Sciences, 2012
    Co-Authors: Chunsun Zhang, Clive S Fraser, Shijie Liu
    Abstract:

    To exploit full metric quality of optical satellite imagery, precise Georeferencing is necessary. A number of sensor orientation models designed to exploit the full metric potential of images have been developed over the past decades. In particular, generic models attract more interest as they take full account of the physical imaging process by adopting time dependant satellite orbit models and interior orientation (IO) information provided by the satellite imagery vendors. The quality of IO parameters varies for different satellites and has significant impact on the Georeferencing performance. Self-calibration approaches have been developed, however such approaches require a significant amount of ground control with good point distribution. In addition, the results are not always stable due to the correlation between the model parameters. In this paper, a simple yet efficient method has been proposed to correct the IO errors by detailed examination and efficient modelling of the IO error distribution in the focal plane. The proposed correction method, used in conjunction with a generic sensor model, significantly improves the metric performance of satellite images, leading to sub-pixel Georeferencing accuracy.

  • Georeferencing performance of THEOS satellite imagery
    The Photogrammetric Record, 2011
    Co-Authors: Shijie Liu, Clive S Fraser, Chunsun Zhang, M Ravanbakhsh, Xiaohua Tong
    Abstract:

    This paper reports on the application of a generic physical sensor orientation model for evaluation of the Georeferencing performance of 2 m resolution imagery from the Thailand Earth Observation System (THEOS) satellite. Within the generic sensor orientation model, orbit and attitude data are employed to describe the satellite trajectory, which is further modelled by splines. The satellite orbit and sensor attitude errors are then compensated via sensor orientation adjustment using a modest number of ground control points (GCPs), resulting in improved Georeferencing. The generic sensor model and the integration of the THEOS orientation parameters into the model are first described. The presence of errors in the satellite line-of-sight data, which result in imprecise sensor interior orientation are then discussed. Such errors can be effectively accounted for through modelling via a cubic polynomial, leading to sub-pixel Georeferencing accuracy. An account is then given of an experimental evaluation of THEOS Georeferencing conducted in a well-established testfield near Melbourne, Australia. The results demonstrate that sub-pixel 2D geopositioning accuracy is readily achievable with single THEOS images and within strips of up to three images, with as few as six GCPs to effect an orbit adjustment. However, accuracy decreases to near the 2-pixel level over a strip length of five images.

  • Georeferencing accuracy of geoeye 1 stereo imagery experiences in a japanese test field
    NETWORKING THE WORLD WITH REMOTE SENSING, 2010
    Co-Authors: Y Meguro, Clive S Fraser
    Abstract:

    High-resolution satellite imagery (HRSI) is being increasingly employed for large-scale topographic mapping, and especially for geodatabase updating. As the spatial resolution of HRSI sensors increases, so the potential Georeferencing accuracy also improves. However, accuracy is not a function of spatial resolution alone, as it is also dependent upon radiometric image quality, the dynamics of the image scanning, and the fidelity of the sensor orientation model, both directly from orbit and attitude observations and indirectly from ground control points (GCPs). Users might anticipate accuracies of, say, 1 pixel in planimetry and 1-3 pixels in height when using GCPs. However, there are practical and in some cases administrative/legal imperatives for the Georeferencing accuracy of HRSI systems to be quantified through well controlled tests. This paper discusses an investigation into the Georeferencing accuracy attainable from the GeoEye-1 satellite, and specifically the 3D accuracy achievable from stereo imagery. Both direct Georeferencing via supplied RPCs and indirect Georeferencing via ground control and bias-corrected RPCs were examined for a stereo pair of pansharpened GeoEye-1 Basic images covering the Tsukuba Test Field in Japan, which contains more than 100 precisely surveyed and image identifiable GCPs. Salient aspects of the investigation are discussed, including aspects of sensor orientation via bias-corrected RPCs, whether GCPs are required, and the relationship between geolocation accuracy and the number and quality of GCPs. The paper discusses the results obtained, which indicated that the direct Georeferencing accuracy obtained was within that specified for GeoEye-1, namely a 2m Circular Error 90% (CE90) in planimetry and a 3m Linear Error 90% (LE90) in height. The use of a few GCPs improved geopositioning accuracy to around 0.35m (0.7 pixel) in planimetry and 0.7m (1.4 pixel) in height.

  • a strip adjustment approach for precise Georeferencing of alos optical imagery
    IEEE Transactions on Geoscience and Remote Sensing, 2009
    Co-Authors: Franz Rottensteiner, T Weser, Alun Lewis, Clive S Fraser
    Abstract:

    Precise Georeferencing is one of the prerequisites for orthoimage generation from high-resolution satellite imagery. This requires the availability of a small number of GCPs that have to be visible in each scene. In this paper, it is shown how the number of GCPs required for the precise Georeferencing of Advanced Land Observation Satellite (ALOS) imagery can be reduced by up to 90% using a generic pushbroom sensor model and strip adjustment while still achieving an accuracy of better than 1 pixel. A fully automatic work flow requiring an existing digital orthophoto and a digital elevation model (DEM) is also presented. Using an orthophoto mosaic generated from Landsat-7 panchromatic imagery for automatic GCP measurement, pixel-level accuracy can be achieved for images from the Advanced Visible and Near Infrared Radiometer type 2 (AVNIR-2) instrument. For images from the Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM), the accuracy of the automated procedure is only about 2 pixels due to the poor resolution of the orthoimage, compared with PRISM.

  • Georeferencing accuracy of geoeye 1 imagery
    Photogrammetric Engineering and Remote Sensing, 2009
    Co-Authors: Clive S Fraser, M Ravanbakhsh
    Abstract:

    GeoEye-1, launched in September 2008, is the latest in a series of commercial high-resolution Earth observation satellites. With its ground sample distance (GSD) of 0.41m for the panchromatic band, GeoEye-1 offers the highest resolution yet available to the spatial information industry. However, for commercial users, image products are down-sampled to 0.5m GSD. Specifications for GeoEye-1 quote an accuracy in geolocation of better than 3m without ground control, specifically 2m and 2.5m Circular Error 90% (CE90) in planimetry for stereo and mono, respectively, and 3m Linear Error 90% (LE90) in height for stereo coverage (GeoEye, 2009). GeoEye-1 will thus constitute a suitable source of imagery for large scale topographic mapping, to scales of 1:5,000 and possibly larger.

Khalil Almanasir - One of the best experts on this subject based on the ideXlab platform.

  • a review of the use of terrestrial laser scanning application for change detection and deformation monitoring of structures
    Survey Review, 2016
    Co-Authors: Wallace Mukupa, Gethin Wyn Roberts, Craig M. Hancock, Khalil Almanasir
    Abstract:

    Change detection and deformation monitoring is an active area of research within the field of engineering surveying and other overlapping areas such as structural and civil engineering. This paper reviews the application of terrestrial laser scanning in the monitoring of structures and discusses registration and Georeferencing of scan data. Past terrestrial laser scanning research work has shown trends in addressing issues such as accurate registration and Georeferencing of scans, error modelling, point cloud processing techniques for deformation analysis, scanner calibration and detection of millimetre deformations. However, several issues are still open to investigation such as robust methods of point cloud processing for detecting change and deformation, incorporation of measurement geometry in deformation measurements, design of data acquisition and quality assessment for precise measurements and modelling the environmental effects on the performance of laser scanning. A three-stage process model for ...

Geert Verhoeven - One of the best experts on this subject based on the ideXlab platform.

  • mapping by matching a computer vision based approach to fast and accurate Georeferencing of archaeological aerial photographs
    Journal of Archaeological Science, 2012
    Co-Authors: Geert Verhoeven, Michael Doneus, Christian Briese, Frank Vermeulen
    Abstract:

    Abstract To date, aerial archaeologists generally apply simple rectification procedures or more expensive and time-consuming orthorectification algorithms to correct their aerial photographs in varying degrees for geometrical deformations induced by the topographical relief, the tilt of the camera axis and the distortion of the optics. Irrespective of the method applied, the Georeferencing of the images is commonly determined with ground control points, whose measurement and identification is a time-consuming operation and often limits certain images from being accurately georeferenced. Moreover, specialised software, certain photogrammetric skills, and experience are required. Thanks to the recent advances in the fields of computer vision and photogrammetry as well as the improvements in processing power, it is currently possible to generate orthophotos of large, almost randomly collected aerial photographs in a straightforward and nearly automatic way. This paper presents a computer vision-based approach that is complemented by proven photogrammetric principles to generate orthophotos from a range of uncalibrated oblique and vertical aerial frame images. In a first phase, the method uses algorithms that automatically compute the viewpoint of each photograph as well as a sparse 3D geometric representation of the scene that is imaged. Afterwards, dense reconstruction algorithms are applied to yield a three-dimensional surface model. After Georeferencing this model, it can be used to create any kind of orthophoto out of the initial aerial views. To prove the benefits of this approach in comparison to the most common ways of Georeferencing aerial imagery, several archaeological case studies are presented. Not only will they showcase the easy workflow and accuracy of the results, but they will also prove that this approach moves beyond current restrictions due to its applicability to datasets that were previously thought to be unsuited for convenient Georeferencing.