S Robson

Straightforward reconstruction of 3D surfaces and topography with a camera: Accuracy and geoscience application

Topographic measurements for detailed studies of processes such as erosion or mass movement are usually acquired by expensive laser scanners or rigorous photogrammetry. Here, we test and use an alternative technique based on freely available computer vision software which allows general geoscientists to easily create accurate 3D models from field photographs taken with a consumer-grade camera. The approach integrates structure-from-motion (SfM) and multi-view-stereo (MVS) algorithms and, in contrast to traditional photogrammetry techniques, it requires little expertise and few control measurements, and processing is automated. To assess the precision of the results, we compare SfM-MVS models spanning spatial scales of centimeters (a hand sample) to kilometers (the summit craters of Piton de la Fournaise volcano) with data acquired from laser scanning and formal close-range photogrammetry. The relative precision ratio achieved by SfM-MVS (measurement precision : observation distance) is limited by the straightforward camera calibration model used in the software, but generally exceeds 1:1000 (i.e. centimeter-level precision over measurement distances of 10s of meters). We apply SfM-MVS at an intermediate scale, to determine erosion rates along a ~50-m-long coastal cliff. Seven surveys carried out over a year indicate an average retreat rate of 0.70±0.05 m a-1. Sequential erosion maps (at ~0.05 m grid resolution) highlight the spatio-temporal variability in the retreat, with semivariogram analysis indicating a correlation between volume loss and length scale. Compared with a laser scanner survey of the same site, SfM-MVS produced comparable data and reduced data collection time by ~80%.

The accuracy and precision of underwater measurements of length and maximum body depth of southern bluefin tuna (Thunnus maccoyii) with a stereo-video camera system

The accuracy and precision of in situ stereo-video measurements of the snout to fork length (SNFL) (range 830-1412 mm) and maximum body depth (MBD) (range 228-365 mm) of free-swimming southern bluefin tuna (SBT) (Thunnus maccoyii) were tested by filming live fish in sea cages immediately prior to harvest. Stereo-video measurements of the SNFL of 54 fish produced an average error of 1.72 mm (relative error of 0.16%), while an average error of 1.37 mm (relative error of 0.51%) was recorded for measurements of MBD from 47 fish.A procedure was developed to maximise the accuracy and precision of measurements of the SNFL and MBD from a single SBT over sequential images to avoid the underestimation of SNFL and overestimation of MBD due to sinusoidal changes in body form associated with fast swimming.The results demonstrate the potential of stereo-video systems to non-destructively make counts and measurements of tuna and other fish in both wild fisheries and mariculture situations, without the need to capture and handle them

Recent developments in large-scale dimensional metrology

Abstract With ever-more demanding requirements for the accurate manufacture of large components, dimensional measuring techniques are becoming progressively more sophisticated. This review describes some of the more recently developed techniques and the state-of-the-art in the more well-known large-scale dimensional metrology methods. In some cases, the techniques are described in detail, or, where relevant specialist review papers exist, these are cited as further reading. The traceability of the measurement data collected is discussed with reference to new international standards that are emerging. In some cases, hybrid measurement techniques are finding specialized applications and these are referred to where appropriate.

Echolocation call intensity in the aerial hawking bat Eptesicus bottae (Vespertilionidae) studied using stereo videogrammetry.

SUMMARY Aerial hawking bats use intense echolocation calls to search for insect prey. Their calls have evolved into the most intense airborne animal vocalisations. Yet our knowledge about call intensities in the field is restricted to a small number of species. We describe a novel stereo videogrammetry method used to study flight and echolocation behaviour, and to measure call source levels of the aerial hawking bat Eptesicus bottae (Vespertilionidae). Bats flew close to their predicted minimum power speed. Source level increased with call duration; the loudest call of E. bottae was at 133 dB peSPL. The calculated maximum detection distance for large flying objects (e.g. large prey, conspecifics) was up to 21 m. The corresponding maximum echo delay is almost exactly the duration of one wing beat in E. bottae and this also is its preferred pulse interval. These results, obtained by using videogrammetry to track bats in the field, corroborate earlier findings from other species from acoustic tracking methods.

Photogrammetry Methodology Development for Gossamer Spacecraft Structures

Photogrammetry--the science of calculating 3D object coordinates from images--is a flexible and robust approach for measuring the static and dynamic characteristics of future ultra-lightweight and inflatable space structures (a.k.a., Gossamer structures), such as large membrane reflectors, solar sails, and thin-film solar arrays. Shape and dynamic measurements are required to validate new structural modeling techniques and corresponding analytical models for these unconventional systems. This paper summarizes experiences at NASA Langley Research Center over the past three years to develop or adapt photogrammetry methods for the specific problem of measuring Gossamer space structures. Turnkey industrial photogrammetry systems were not considered a cost-effective choice for this basic research effort because of their high purchase and maintenance costs. Instead, this research uses mainly off-the-shelf digitalcamera and software technologies that are affordable to most organizations and provide acceptable accuracy.

Principal point behaviour and calibration parameter models for Kodak DCS cameras

Digital still cameras have been widely adopted for close range photogrammetry and machine vision applications. Due to the advantages of onboard storage of digital images, portability and rapid data processing, digital still cameras are rapidly becoming standard equipment for measurement tasks such as industrial metrology and heritage recording. As for any metric application, the accuracy of the derived object data is dependent, amongst many other factors, on the accuracy of the camera calibration. For the vast majority of photogrammetricapplications, use of the simple case of a block invariant calibration model comprising the primary physical parameters, including the principal point position, is sufficient.However, cameras designed for photojournalismand domestic use, such as the Kodak DCS420 and 460 cameras, are well known for their calibration instability because the design is based on a 35 mm SLR camera body. In particular, previous research has shown that the principal point location is prone to movement during normal handling of the camera, due to the mounting mechanism of the CCD array. This paper reports on an investigation of the physical behaviour of the principal point location and compares different calibration parameter models for the Kodak DCS420 and DCS460 digital still cameras. F RONTISPIECE . A unique remote application of digital photogrammetry used as an inspection tool during the first fully remote shutdown at JET Joint Undertaking, the European Union fusion research project based at Culham, Oxfordshire, England. An article by Macklin et al. describes the work elsewhere in this issue of the PhotogrammetricRecord.

Practical testing of the precision and accuracy of target image centering algorithms

Close range photogrammetry and vision metrology often use signalized points in the form of active or passive targets. Many theoretical and some practical tests of different target image centering algorithms have been carried out. This paper will describe the empirical testing of several such algorithms using real data acquired for industrial measurement projects and camera calibrations. The precision and accuracy of the centering algorithms will be characterized by analysis of self calibrating network solutions using multiple camera stations and a target array. Particular emphasis will be placed on the comparison between centroiding and ellipse fitting to locate target image center.

ACCURACY OF MEASUREMENTS MADE WITH A CYRAX 2500 LASER SCANNER AGAINST SURFACES OF KNOWN COLOUR

Abstract Several commercial manufacturers produce laser scanning systems capable of measuring the surfaces of objects to precisions of the order of a few millimetres at ranges of between 2 and 200 metres. Experience on a number of projects has demonstrated significant variations in the quality of point cloud data as a function of object surface reflectivity. This paper investigates the performance of a Cyrax 2500 laser scanner in making measurements to a variety of surfaces of specified colour characteristics under laboratory conditions. From the data obtained it is evident that significant systematic range discrepancies exist which can be broadly correlated against the colour of each surface with respect to the wavelength of the laser used. Over the distances investigated a high correlation between the point data quality measure returned by the scanner and the range discrepancy was observed, offering the possibility of applying a correction to the data produced by a Cyrax 2500 scanner. Such a correction, which is demonstrated to be applicable for diffusely reflecting surfaces, can be carried out with data available to the end user in order to significantly improve scanner accuracy.

3-D uncertainty-based topographic change detection with structure-from-motion photogrammetry : precision maps for ground control and directly georeferenced surveys

Structure-from-motion (SfM) photogrammetry is revolutionising the collection of detailed topographic data, but insight into geomorphological processes is currently restricted by our limited understanding of SfM survey uncertainties. Here, we present an approach that, for the first time, specifically accounts for the spatially variable precision inherent to photo-based surveys, and enables confidence-bounded quantification of 3-D topographic change. The method uses novel 3-D precision maps that describe the 3-D photogrammetric and georeferencing uncertainty, and determines change through an adapted state-of-the-art fully 3-D point-cloud comparison (M3C2; Lague, et al., 2013), which is particularly valuable for complex topography. We introduce this method by: (1) using simulated UAV surveys, processed in photogrammetric software, to illustrate the spatial variability of precision and the relative influences of photogrammetric (e.g. image network geometry, tie point quality) and georeferencing (e.g. control measurement) considerations; (2) we then present a new Monte Carlo procedure for deriving this information using standard SfM software and integrate it into confidence-bounded change detection; before (3) demonstrating geomorphological application in which we use benchmark TLS data for validation and then estimate sediment budgets through differencing annual SfM surveys of an eroding badland. We show how 3-D precision maps enable more probable erosion patterns to be identified than existing analyses, and how a similar overall survey precision could have been achieved with direct survey georeferencing for camera position data with precision half as good as the GCPs’. Where precision is limited by weak georeferencing (e.g. camera positions with multi-metre precision, such as from a consumer UAV), then overall survey precision can scale as n-½ of the control precision (n = number of images). Our method also provides variance-covariance information for all parameters. Thus, we now open the door for SfM practitioners to use the comprehensive analyses that have underpinned rigorous photogrammetric approaches over the last half-century.

Calibration modeling and stability testing for the Kodak DC200 series digital still camera

This paper reports on initial investigations into appropriate calibration models for and the reliability and stability of the calibration of the Kodak DC200 series cameras. Results of test of different types of digital still cameras are compared, in general, to the DC200 series and then various calibration test of a DC265 camera are presented and analyzed. Block- and photo-invariant camera calibration models are compared to ascertain their suitability for the physical variability of the cameras. In conclusion, this paper makes some recommendations on the potential reliability and stability of the Kodak DC265 camera.