Photogrammetric Estimation Research Articles

Field Tests of the Procedure for a Photogrammetric Estimation of Snow-Glacial Surface Albedo

The results of using an original procedure for estimating albedo from the photogrammetric data and exposure parameters by an unmanned aerial vehicle (UAV) in Antarctica are discussed. The difficulties of photogrammetric observations performed under extreme conditions are considered. Conclusions are drawn on the ways to improve the recording equipment and on the course of refining the procedure for calculating albedo values based on photogrammetric materials and metadata.

A simple use of camera traps for photogrammetric estimation of wild animal traits

AbstractThe measurement of physical traits of animals is an important topic in animal ecology and wildlife management, but it is quite difficult to achieve an acceptable level of accuracy, especially for free‐ranging wild animals. As a non‐invasive method, camera trapping has become a common tool for detecting and monitoring wild animals. In this study, we combined camera‐trapping images and a photogrammetry technique to present a camera trap‐based photogrammetric estimation (CTPE) approach for robust measurement of morphological traits of animals. We first tested our method in a captive population of fallow deer (Dama dama) and compared the estimated shoulder heights by CTPE with corresponding hand measurements. The results revealed a remarkable accuracy, with an expected measurement error within ± 5 cm. Second, we mainly demonstrated a field application of CTPE in a wild population of Altai wapiti (Cervus elaphus sibiricus) in the Kanas National Nature Reserve, Xinjiang, China. For wapiti, we found that the average shoulder height of males was 135 ± 9 cm (mean ± sd, n = 30) and that of females was 125 ± 6 cm (n = 37), with all data significantly fitting a normal distribution. We did not find any differences in CTPE results among cameras, between photograph types (colour vs. black and white) or temperatures. No significant difference existed between our results and the hand‐measured shoulder heights of wapiti from deer farms. Finally, we discussed the strengths, limitations and potential extensions of the method. We suggest that the combination of camera trapping and the photogrammetry technique is quite promising and that CTPE has great potential in wildlife field investigations.

Image Data Acquisition for Estimating Individual Trees Metrics: Closer Is Better

Background and Objectives: The recent use of Structure-from-Motion with Multi-View Stereo photogrammetry (SfM-MVS) in forestry has underscored its robustness in tree mensuration. This study evaluated the differences in tree metrics resulting from various related SfM-MVS photogrammetric image acquisition scenarios. Materials and Methods: Scaled tri-dimensional models of 30 savanna trees belonging to five species were built from photographs acquired in a factorial design with shooting distance (d = 1, 2, 3, 4 and 5 m away from tree) and angular shift (α = 15°, 30°, 45° and 60°; nested in d). Tree stem circumference at 1.3 m and bole volume were estimated using models resulting from each of the 20 scenarios/tree. Mean absolute percent error (MAPE) was computed for both metrics in order to compare the performance of each scenario in relation to reference data collected using a measuring tape. Results: An assessment of the effect of species identity (s), shooting distance and angular shift showed that photographic point cloud density was dependent on α and s, and optimal for 15° and 30°. MAPEs calculated on stem circumferences and volumes significantly differed with d and α, respectively. There was a significant interaction between α and s for both circumference and volume MAPEs, which varied widely (1.6 ± 0.4%–20.8 ± 23.7% and 2.0 ± 0.6%–36.5 ± 48.7% respectively), and were consistently lower for smaller values of d and α. Conclusion: The accuracy of photogrammetric estimation of individual tree attributes depended on image-capture approach. Acquiring images 2 m away and with 30° intervals around trees produced reliable estimates of stem circumference and bole volume. Research Highlights: This study indicates that the accuracy of photogrammetric estimations of individual tree attributes is species-dependent. Camera positions in relation to the subject substantially influence the level of uncertainty in measurements.

Unmanned Aerial System (UAS) observations of water surface elevation in a small stream: Comparison of radar altimetry, LIDAR and photogrammetry techniques

Water Surface Elevation (WSE) is an important hydrometric observation, useful to calibrate hydrological models, predict floods, and assess climate change. However, the number of in-situ gauging stations is in decline worldwide. Satellite altimetry, including the recently launched satellite missions (e.g. the radar altimetry missions Cryosat 2, Jason 3, Sentinel 3A/B and the LIDAR mission ICESat-2), can determine WSE only in rivers which are more than ca. 100 m wide. WSE measurements in small streams currently remain limited to the few existing in-situ stations or to time-consuming in-situ surveys. Unmanned Aerial Systems (UAS) can acquire real-time WSE observations during periods of hydrological interest (but with flight limitations in extreme weather conditions), within short survey times and with automatic or semi-automatic flight operations. UAS-borne photogrammetry is a well-known technique that can estimate land elevation with an accuracy as high as a few cm, similarly UAS-borne LIDAR can estimate land elevation but without requiring Ground Control Points (GCPs). However, both techniques face limitations in estimating WSE: water transparency and lack of stable visual key points on the Water Surface (WS) complicate the UAS-borne photogrammetric estimates of WSE, while the LIDAR reflection from the water surface is generally not strong enough to be captured by most of the UAS-borne LIDAR systems currently available on the market. Thus, LIDAR and photogrammetry generally require extraction of the elevation of the “water-edge” points, i.e. points at the interface between land and water, for identifying the WSE. We demonstrate highly accurate WSE observations with a new radar altimetry solution, which comprises a 77 GHz radar chip with full waveform analysis and an accurate dual frequency differential Global Navigation Satellite System (GNSS) system. The radar altimetry solution shows the lowest standard deviation (σ) and RMSE on WSE estimates, ca. 1.5 cm and ca. 3 cm respectively, whilst photogrammetry and LIDAR show a σ and an RMSE at decimetre level. Radar altimetry also requires a significantly shorter survey and processing time compared to LIDAR and especially to photogrammetry.

Volumetric monitoring of cutaneous leishmaniasis ulcers: can camera be as accurate as laser scanner?

ABSTRACTCavity volume is an important clinical index for the assessment of the healing process and effectiveness of treatment applied on chronic ulcers. Recently, 3D scanners have proven to effectively track ulcer’s volume evolution. However, photogrammetry presents itself as a low cost and portable alternative. We conducted a comparative study between photogrammetric and 3D scanner-based volume estimation of small skin ulcers. A total of 24 cutaneous leishmaniasis ulcers’ virtual models were generated using a commercial laser scanner and a full-HD portable camera. The reconstruction from videos was performed using open-source software (VisualSFM). The results revealed a similar performance when compared to 3D Scan-based volume estimation with a median deviation and percentile error of 33.68 mm3 and 21.10%, respectively. In addition, the method proved to effectively follow the same tendency in the assessment of healing ulcers when compared to the 3D-scanner. Moreover, when comparing the two virtual models trough residual differences after ICP registration the mean error and standard deviation was 0.36 ± 0.27 mm.

Assessment of the Accuracy of Close Distance Photogrammetric JRC Data

By using close range photogrammetry, this article investigates the accuracy of the photogrammetric estimation of rock joint roughness coefficients (JRC), a measure of the degree of roughness of rock joint surfaces. This methodology has proven to be convenient both in laboratory and in site conditions. However, the accuracy and precision of roughness profiles obtained from photogrammetric 3D images have not been properly established due to the variances caused by factors such as measurement errors and systematic errors in photogrammetry. In this study, the influences of camera-to-object distance, focal length and profile orientation on the accuracy of JRC values are investigated using several photogrammetry field surveys. Directional photogrammetric JRC data are compared with data derived from the measured profiles, so as to determine their accuracy. The extent of the accuracy of JRC values was examined based on the error models which were previously developed from laboratory tests and revised for better estimation in this study. The results show that high-resolution 3D images (point interval ≤1 mm) can reduce the JRC errors obtained from field photogrammetric surveys. Using the high-resolution images, the photogrammetric JRC values in the range of high oblique camera angles are highly consistent with the revised error models. Therefore, the analysis indicates that the revised error models facilitate the verification of the accuracy of photogrammetric JRC values.

Efficient three-dimensional reconstruction of aquatic vegetation geometry: Estimating morphological parameters influencing hydrodynamic drag

Aquatic vegetation can shelter coastlines from energetic waves and tidal currents, sometimes enabling accretion of fine sediments. Simulation of flow and sediment transport within submerged canopies requires quantification of vegetation geometry. However, field surveys used to determine vegetation geometry can be limited by the time required to obtain conventional caliper and ruler measurements. Building on recent progress in photogrammetry and computer vision, we present a method for reconstructing three-dimensional canopy geometry. The method was used to survey a dense canopy of aerial mangrove roots, called pneumatophores, in Vietnam’s Mekong River Delta. Photogrammetric estimation of geometry required 1) taking numerous photographs at low tide from multiple viewpoints around 1 m2 quadrats, 2) computing relative camera locations and orientations by triangulation of key features present in multiple images and reconstructing a dense 3D point cloud, and 3) extracting pneumatophore locations and diameters from the point cloud data. Step 3) was accomplished by a new ‘sector-slice’ algorithm, yielding geometric parameters every 5 mm along a vertical profile. Photogrammetric analysis was compared with manual caliper measurements. In all 5 quadrats considered, agreement was found between manual and photogrammetric estimates of stem number, and of number × mean diameter, which is a key parameter appearing in hydrodynamic models. In two quadrats, pneumatophores were encrusted with numerous barnacles, generating a complex geometry not resolved by hand measurements. In remaining cases, moderate agreement between manual and photogrammetric estimates of stem diameter and solid volume fraction was found. By substantially reducing measurement time in the field while capturing in greater detail the 3D structure, photogrammetry has potential to improve input to hydrodynamic models, particularly for simulations of flow through large-scale, heterogenous canopies.

Parallel‐laser photogrammetry to estimate body size in free‐ranging mammals

ABSTRACTIt is not always practical to hand‐measure body size of free‐ranging animals. In recent years, parallel‐laser photogrammetry has become increasingly common for obtaining remote estimates of body size. However, it is unknown how well this technique might capture variation in body size of curvilinear features or whether the distance between parallel‐laser calipers is altered when projected onto a curved surface. We describe a photogrammetric system that may be useful for obtaining body‐size measurements from unrestrained large mammals that permit approach. We tested the use of parallel‐laser photogrammetry to estimate the size of curvilinear features in domestic horses (Equus ferus caballus) and identified morphometrics that explained variation in body weight. Despite projecting the lasers onto a curved surface (the barrel of a horse), we achieved accurate photogrammetric estimates of linear hand‐measurements. The curvilinear hand‐measurements also showed strong correlations (R2 ≥ 0.996) with their respective linear photogrammetric estimates, and most photogrammetric estimates had high reliability. Using 3 variables of body size, photogrammetric estimates and hand‐measurements explained 86.0% and 96.2% of the variation in weight, respectively. © 2015 The Wildlife Society.

Forest inventory stand height estimates from very high spatial resolution satellite imagery calibrated with lidar plots

Many areas of forest across northern Canada are challenging to monitor on a regular basis as a result of their large extent and remoteness. Although no forest inventory data typically exist for these northern areas, detailed and timely forest information for these areas is required to support national and international reporting obligations. We developed and tested a sample-based approach that could be used to estimate forest stand height in these remote forests using panchromatic Very High Spatial Resolution (VHSR, < 1 m) optical imagery and light detection and ranging (lidar) data. Using a study area in central British Columbia, Canada, to test our approach, we compared four different methods for estimating stand height using stand-level and crown-level metrics generated from the VHSR imagery. ‘Lidar plots’ (voxel-based samples of lidar data) are used for calibration and validation of the VHSR-based stand height estimates, similar to the way that field plots are used to calibrate photogrammetric estimates of stand height in a conventional forest inventory or to make empirical attribute estimates from multispectral digital remotely sensed data. A k-nearest neighbours (k-NN) method provided the best estimate of mean stand height (R 2 = 0.69; RMSE = 2.3 m, RMSE normalized by the mean value of the estimates (RMSE-%) = 21) compared with linear regression, random forests, and regression tree methods. The approach presented herein demonstrates the potential of VHSR panchromatic imagery and lidar to provide robust and representative estimates of stand height in remote forest areas where conventional forest inventory approaches are either too costly or are not logistically feasible. While further evaluation of the methods is required to generalize these results over Canada to provide robust and representative estimation, VHSR and lidar data provide an opportunity for monitoring in areas for which there is no detailed forest inventory information available.

Long-distance photogrammetric trait estimation in free-ranging animals: A new approach

The acquisition of accurate information on the size of traits in animals is fundamental for the study of animal ecology and evolution and their management. We demonstrate how morphological traits of free-ranging animals can reliably be estimated on very large observation distances of several hundred meters by the use of ordinary digital photographic equipment and simple photogrammetric software. In our study, we estimated the length of horn annuli in free-ranging male Alpine ibex (Capra ibex) by taking already measured horn annuli of conspecifics on the same photographs as scaling units. Comparisons with hand-measured horn annuli lengths and repeatability analyses revealed a high accuracy of the photogrammetric estimates. If length estimations of specific horn annuli are based on multiple photographs measurement errors of <5.5mm can be expected. In the current study the application of the described photogrammetric procedure increased the sample size of animals with known horn annuli length by an additional 104%. The presented photogrammetric procedure is of broad applicability and represents an easy, robust and cost-efficient method for the measuring of individuals in populations where animals are hard to capture or to approach.

Transient luminous events above two mesoscale convective systems: Charge moment change analysis

[1] Charge moment change (ΔMQ) data were examined for 41 positive cloud-to-ground (+CG) lightning discharges that were parents of transient luminous events (TLEs; mainly sprites) over two different storms: 9 May (20 parents) and 20 June 2007 (21). Data were broken down by contributions from the impulse ΔMQ (iΔMQ), within the first 2 ms of the return stroke, and the ΔMQ from the continuing current (CC), which can last tens of ms afterward. Three-dimensional lightning mapping data provided positions for the in-cloud components of the parent +CGs. Charge and charge density neutralized by the strokes were estimated. The 20 June parents were more impulsive than 9 May, with increased iΔMQ and CC amplitude but reduced CC duration. Total ΔMQ values between the two storms were very similar, averaging ∼1800 C km. Estimated charge density on 20 June was nearly twice that on 9 May, consistent with the 20 June storm being more intense with a stronger electrical generator. Lightning metrics were analyzed for 9 high- iΔMQ (>300 C km) +CGs that did not produce an observable TLE on 20 June, and compared to that day's TLE parents. Non-TLE +CGs had reduced CC magnitudes and duration, with less total ΔMQ. Photogrammetric estimates of TLE azimuthal swaths were positively correlated with similar metrics of the in-cloud portions of the parent +CGs, as well with total ΔMQ. The implications of all these results for the ΔMQ theory of sprite initiation, and for the relationship between sprite development and in-cloud discharging, are discussed.

Adaptive coding and modulation scheme for satellite-UMTS TDD systems based on a photogrammetric channel estimation method

The conventional wireless communication systems are designed to overcome the worst-case channel, using the huge amount of redundant bits to assure communications performance and quality of services. Those systems cannot achieve the optimum spectrum and power efficiency. This paper presents an adaptive coding and modulation scheme used in the user terminals of the third-generation satellite communication system. A three-state photogrammetric channel estimation method is introduced for tracing the variations of large-scale environments. The mobile user terminal dynamically switches the suitable coding and modulation schemes according to the result of photogrammetric channel estimator in order to maximize the power efficiency and data throughput. The real measurement data were used to validate our proposed method. The results show that the proposed method not only reduces the system complexity, but also mitigates the power control requirements and increases the data throughput for the land mobile satellite personal communication systems. Copyright © 2006 John Wiley & Sons, Ltd.

Assessment of manual and automated methods for updating stand-level forest inventories based on aerial photography

Anttila, P. 2005. Assessment of manual and automated methods for updating stand-level forest inventories based on aerial photography. Dissertationes Forestales 9. 42 p. Data collection for forest planning in private forestry in Finland is based on periodic, labour-intensive, stand level field inventories. Therefore, to increase guidance for forest owners more cost-efficient methods that replace or supplement field inventories are needed so as to reallocate labour resources. In addition, for active forest owners, up-to-date information is of interest. The aim of this thesis was to test the applicability of different inventory and updating methods that are based on aerial photography for the regional forest inventory of private forests in Finland. Four approaches were chosen: 1) stand level visual interpretation of changes and simulation of stand development, 2) automatic, stand level interpretation of aerial photographs utilising nonparametric estimation, 3) automatic, photogrammetric estimation of mean height, and 4) automatic, monoscopic identification of individual trees. Approaches 1 and 2 are inventory or updating methods, while approaches 3 and 4 can be considered as components of such methods. Approach 1 was found to be appropriate for operative use for estates that have not ordered a forest plan, provided that the strict prerequisites for the inventory area are met. The accuracy of approach 2, as such, was not considered to be high enough for forest planning, but this very inexpensive method could turn out to be applicable, if its accuracy could be improved. Approach 3 was based on image matching and would be most useful as a part of other methods, like template matching in multiple images. Although only identification of trees was tested in approach 4, the applicability of whole single tree based inventory method was appraised. Relatively high costs and low accuracy limit the possibilities to use this method, in practice.

Measuring sperm whales from their clicks: stability of interpulse intervals and validation that they indicate whale length.

Multiple pulses can often be distinguished in the clicks of sperm whales (Physeter macrocephalus). Norris and Harvey [in Animal Orientation and Navigation, NASA SP-262 (1972), pp. 397-417] proposed that this results from reflections within the head, and thus that interpulse interval (IPI) is an indicator of head length, and by extrapolation, total length. For this idea to hold, IPIs must be stable within individuals, but differ systematically among individuals of different size. IPI stability was examined in photographically identified individuals recorded repeatedly over different dives, days, and years. IPI variation among dives in a single day and days in a single year was statistically significant, although small in magnitude (it would change total length estimates by <3%). As expected, IPIs varied significantly among individuals. Most individuals showed significant increases in IPIs over several years, suggesting growth. Mean total lengths calculated from published IPI regressions were 13.1 to 16.1 m, longer than photogrammetric estimates of the same whales (12.3 to 15.3 m). These discrepancies probably arise from the paucity of large (12-16 m) whales in data used in published regressions. A new regression is offered for this size range.