Cs Watson

An Automated Technique for Generating Georectified Mosaics from Ultra-High Resolution Unmanned Aerial Vehicle (UAV) Imagery, Based on Structure from Motion (SfM) Point Clouds

Unmanned Aerial Vehicles (UAVs) are an exciting new remote sensing tool capable of acquiring high resolution spatial data. Remote sensing with UAVs has the potential to provide imagery at an unprecedented spatial and temporal resolution. The small footprint of UAV imagery, however, makes it necessary to develop automated techniques to geometrically rectify and mosaic the imagery such that larger areas can be monitored. In this paper, we present a technique for geometric correction and mosaicking of UAV photography using feature matching and Structure from Motion (SfM) photogrammetric techniques. Images are processed to create three dimensional point clouds, initially in an arbitrary model space. The point clouds are transformed into a real-world coordinate system using either a direct georeferencing technique that uses estimated camera positions or via a Ground Control Point (GCP) technique that uses automatically identified GCPs within the point cloud. The point cloud is then used to generate a Digital Terrain Model (DTM) required for rectification of the images. Subsequent georeferenced images are then joined together to form a mosaic of the study area. The absolute spatial accuracy of the direct technique was found to be 65–120 cm whilst the GCP technique achieves an accuracy of approximately 10–15 cm.

Development of a UAV-LiDAR System with Application to Forest Inventory

We present the development of a low-cost Unmanned Aerial Vehicle-Light Detecting and Ranging (UAV-LiDAR) system and an accompanying workflow to produce 3D point clouds. UAV systems provide an unrivalled combination of high temporal and spatial resolution datasets. The TerraLuma UAV-LiDAR system has been developed to take advantage of these properties and in doing so overcome some of the current limitations of the use of this technology within the forestry industry. A modified processing workflow including a novel trajectory determination algorithm fusing observations from a GPS receiver, an Inertial Measurement Unit (IMU) and a High Definition (HD) video camera is presented. The advantages of this workflow are demonstrated using a rigorous assessment of the spatial accuracy of the final point clouds. It is shown that due to the inclusion of video the horizontal accuracy of the final point cloud improves from 0.61 m to 0.34 m (RMS error assessed against ground control). The effect of the very high density point clouds (up to 62 points per m2) produced by the UAV-LiDAR system on the measurement of tree location, height and crown width are also assessed by performing repeat surveys over individual isolated trees. The standard deviation of tree height is shown to reduce from 0.26 m, when using data with a density of 8 points perm2, to 0.15mwhen the higher density data was used. Improvements in the uncertainty of the measurement of tree location, 0.80 m to 0.53 m, and crown width, 0.69 m to 0.61 m are also shown.

Evaluating Tree Detection and Segmentation Routines on Very High Resolution UAV LiDAR Data

Light detection and Ranging (LiDAR) is becoming an increasingly used tool to support decision-making processes within forest operations. Area-based methods that derive information on the condition of a forest based on the distribution of points within the canopy have been proven to produce reliable and consistent results. Individual tree-based methods, however, are not yet used operationally in the industry. This is due to problems in detecting and delineating individual trees under varying forest conditions resulting in an underestimation of the stem count and biases toward larger trees. The aim of this paper is to use high-resolution LiDAR data captured from a small multirotor unmanned aerial vehicle platform to determine the influence of the detection algorithm and point density on the accuracy of tree detection and delineation. The study was conducted in a four-year-old Eucalyptus globulus stand representing an important stage of growth for forest management decision-making process. Five different tree detection routines were implemented, which delineate trees directly from the point cloud, voxel space, and the canopy height model (CHM). The results suggest that both algorithm and point density are important considerations in the accuracy of the detection and delineation of individual trees. The best performing method that utilized both the CHM and the original point cloud was able to correctly detect 98% of the trees in the study area. Increases in point density (from 5 to 50 points/m2) lead to significant improvements (of up to 8%) in the rate of omission for algorithms that made use of the high density of the data.

Absolute Calibration of TOPEX/Poseidon and Jason-1 Using GPS Buoys in Bass Strait, Australia

An absolute calibration of the TOPEX/Poseidon (T/P) and Jason-1 altimeters has been undertaken during the dedicated calibration phase of the Jason-1 mission, in Bass Strait, Australia. The present study incorporates several improvements to the earlier calibration methodology used for Bass Strait, namely the use of GPS buoys and the determination of absolute bias in a purely geometrical sense, without the necessity of estimating a marine geoid. This article focuses on technical issues surrounding the GPS buoy methodology for use in altimeter calibration studies. We present absolute bias estimates computed solely from the GPS buoy deployments and derive formal uncertainty estimates for bias calculation from a single overflight at the 40–45 mm level. Estimates of the absolute bias derived from the GPS buoys is −10 ± 19 mm for T/P and +147 ± 21 mm for Jason-1 (MOE orbit) and +131 ± 21 mm for Jason-1 (GPS orbit). Considering the estimated error budget, our bias values are equivalent to other determinations from the dedicated NASA and CNES calibration sites.

Absolute Calibration in Bass Strait, Australia: TOPEX, Jason-1 and OSTM/Jason-2

Updated absolute bias estimates are presented from the Bass Strait calibration site (Australia) for the TOPEX/Poseidon (T/P), Jason-1 and the Ocean Surface Topography Mission (OSTM/Jason-2) altimeter missions. Results from the TOPEX side A and side B data show biases insignificantly different from zero when assessed against our error budget (−15 ± 20 mm, and −6 ± 18 mm, respectively). Jason-1 shows a considerably higher absolute bias of +93 ± 15 mm, indicating that the observed sea surface is higher (or the range shorter), than truth. For OSTM/Jason-2, the absolute bias is further increased to +172 ± 18 mm (determined from T/GDR data, cycles 001–079). Enhancements made to the Jason-1 and OSTM/Jason-2 microwave radiometer derived products for correcting path delays induced by the wet troposphere are shown to benefit the bias estimate at the Bass Strait site through the reduction of land contamination. We note small shifts to bias estimates when using the enhanced products, changing the biases by +11 and +3 mm for Jason-1 and OSTM/Jason-2, respectively. The significant, and as yet poorly understood, absolute biases observed for both Jason series altimeters reinforces the continued need for further investigation of the measurement systems and ongoing monitoring via in situ calibration sites.

TOPEX/Poseidon and Jason-1: Absolute Calibration in Bass Strait, Australia

Updated absolute calibration results from Bass Strait, Australia, are presented for the TOPEX/Poseidon (T/P) and Jason-1 altimeter missions. Data from an oceanographic mooring array and coastal tide gauge have been used in addition to the previously described episodic GPS buoy deployments. The results represent a significant improvement in absolute bias estimates for the Bass Strait site. The extended methodology has allowed comparison between the altimeter and in situ data on a cycle-by-cycle basis over the duration of the dedicated calibration phase (formation flight period) of the Jason-1 mission. In addition, it has allowed absolute bias results to be extended to include all cycles since the T/P launch, and all Jason-1 data up to cycle 60. Updated estimates and formal 1-sigma uncertainties of the absolute bias computed throughout the formation flight period are 0 ± 14 mm for T/P and +152 + 13 mm for Jason-1 (for the GDR POE orbits). When JPL GPS orbits are used for cycles 1 to 60, the Jason-1 bias estimate is 131 mm, virtually identical to the NASA estimate from the Harvest Platform off California calculated with the GPS orbits and not significantly different to the CNES estimate from Corsica. The inference of geographically correlated errors in the GDR POE orbits (estimated to be approximately 17 mm at Bass Strait) highlights the importance of maintaining globally distributed verification sites and makes it clear that further work is required to improve our understanding of the Jason-1 instrument and algorithm behavior.

Impact of solid Earth tide models on GPS coordinate and tropospheric time series

Unmodelled sub-daily periodic signals can propagate into time series of daily geodetic coordinates and tropospheric estimates at various different frequencies. Geophysical interpretations of geodetic products, particularly at seasonal timescales, can therefore be affected by poorly modelled signals in the geodetic analysis. In this study, we use two solid Earth tide models (IERS2003 and IERS1992) and analyses of global GPS data to demonstrate how this process occurs. Aliased annual and semi-annual signals are evident in the vertical component of the GPS time series, with the amplitudes increasing as a function of latitude up to approximately 2.0 and 0.4 mm, respectively. Tropospheric zenith delay estimates show differences at the 2mm level, with a dominant diurnal frequency. These results have significant implications in regard to the geophysical interpretation of GPS time series computed using the outdated IERS1992 model and, more generally, for any mis- or unmodelled periodic signals that affect geodetic sites.

The AuScope Geodetic VLBI Array

The AuScope geodetic Very Long Baseline Interferometry array consists of three new 12-m radio telescopes and a correlation facility in Australia. The telescopes at Hobart (Tasmania), Katherine (Northern Territory) and Yarragadee (Western Australia) are co-located with other space geodetic techniques including Global Navigation Satellite Systems (GNSS) and gravity infrastructure, and in the case of Yarragadee, satellite laser ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) facilities. The correlation facility is based in Perth (Western Australia). This new facility will make significant contributions to improving the densification of the International Celestial Reference Frame in the Southern Hemisphere, and subsequently enhance the International Terrestrial Reference Frame through the ability to detect and mitigate systematic error. This, combined with the simultaneous densification of the GNSS network across Australia, will enable the improved measurement of intraplate deformation across the Australian tectonic plate. In this paper, we present a description of this new infrastructure and present some initial results, including telescope performance measurements and positions of the telescopes in the International Terrestrial Reference Frame. We show that this array is already capable of achieving centimetre precision over typical long-baselines and that network and reference source systematic effects must be further improved to reach the ambitious goals of VLBI2010.

In situ Absolute Calibration and Validation: A Link from Coastal to Open-Ocean Altimetry

The determination of global and regional mean sea level variations with accuracies better than 1 mm/year is an important yet challenging problem, the resolution of which is central to the current debate on climate change and its impact on the environment. To address this, highly accurate time series from both satellite altimetry and tide gauges are needed. In both cases, the desired accuracy represents a significant challenge for the geodetic community. From the perspective of space borne altimetry, systematic errors from the orbit, reference frame and altimeter systems are all important limiting factors and must be minimized in order to derive data products of greatest geophysical value. Indeed, the objective for the overall accuracy of future altimeter systems is 1-cm (RMS) along with a stability of 1 mm/year. From the terrestrial perspective, estimating the vertical velocity of tide gauge sites to sufficient accuracy is also one of the most important and challenging problems in modern geodesy. Essential to reaching these goals in the measurement of mean sea level variation are ultra-precise validation and calibration techniques, including in situ absolute calibration experiments. Most of the present calibration experiments are on or near the coast, reinforcing the need for developing such techniques to unify the altimetric error budget for both open-ocean and local (coastal) conditions.

Coastal Tide Gauge Calibration: A Case Study at Macquarie Island Using GPS Buoy Techniques

Abstract Tide gauges remain the fundamental instrument used to measure water level in the coastal environment. Issues surrounding the calibration and vertical datum control of tide gauges are therefore fundamental in studies involving the determination of absolute sea level and its variation over time. Macquarie Island, located in Australian sub-Antarctic waters (54°30′ S, 158°57′ E), represents one of the few possible locations in the Southern Ocean to observe sea level using traditional tide gauge techniques. The wave and atmospheric climatology of the region, coupled with a rugged coastline, makes the operation of a modern tide gauge installation extremely difficult. To overcome many of these difficulties, researchers use an acoustic gauge operated within an inclined shaft that is drilled through a coastal rocky outcrop. The calibration requirements of the gauge are therefore problematic and require special consideration to enable the accurate calculation of mean sea level and its change over time. We present results from a novel application of a GPS-equipped buoy to achieve an in situ calibration of the tide gauge, solving for scale, vertical offset, and sea state–dependent bias parameters. The methodology provides a new, high precision technique using available instrumentation, allowing users to maximise the oceanographic and geodetic value of tide gauge observations.