Height Error Research Articles

Passive vision measurement for robust reconstruction of molten pool in wire and arc additive manufacturing

Robust measurement of molten pool in wire and arc additive manufacturing can help better understand the geometrical size of fabricated parts and even offer feedback to control this process. Most of existing detection methods are not sufficient to express full geometric features of molten pool. In this paper, a novel passive vision sensing system using a single camera is designed based on the refraction principle of a biprism. Intrinsic and external parameters of left-right virtual cameras and their relative position are solved through a planar pattern, and an image pair is rectified by transmission projection. A global-based iterative matching algorithm is optimized for calculating the left-right parallax images of the image pair, and coordinates of space points are calculated via the triangle measuring method. The proposed sensing system and algorithms are validated by a standard cylinder. The results show that the width error of the calculated cylinder appearance is approximately 3.02% and the maximum height error is 4.15%. Finally, molten pool geometry in gas metal arc (GMA) additive manufacturing is restructured, proving the effectiveness of the virtual binocular vision system and the accuracy of the reconstruction algorithms.

Nontraditional hypsometric equation

AbstractThis study corrects the hypsometric equation by restoring the nontraditional terms to relax the hydrostatic approximation. The nontraditional terms include one Coriolis term and two metric terms in the vertical momentum equation. The hypsometric equations with and without the nontraditional correction are used to calculate the geopotential height of pressure levels using more than 300,000 selected tropical rawinsonde profiles. With westerlies between two pressure levels, the thickness of the layer increases, which reduces the upward pressure gradient forces to balance the upward nontraditional Coriolis forces; the opposite is true for easterlies. Hence, zonal winds are negatively correlated with traditional geopotential height biases aloft. At 500 hPa, for example, traditional geopotential height error in the tropical troposphere is on the order of at least 0.5 m, which is considerable with respect to geopotential height variability in tropical large‐scale flow, on the order of 10 to 15 m.

Assessing the effects of sea-state related errors on the precision of high-rate Jason-3 altimeter sea level data

Abstract The investigation of ocean surface topography variation by low-resolution mode (LRM) altimeters at length scales shorter than 100 km is limited by cm-scale measurement noise. Spectral analysis of along-track altimeter data has been used to highlight the issue and to illustrate that one significant noise source stems from correlation between significant wave height (SWH) and range errors that is inherent to any waveform retracking algorithm. This paper focuses on improved characterization and reduction of these high-rate correlated errors that arise from this altimeter measurement process. Specifically, it addresses the effects of sea-state related errors in high rate (20-Hz) Jason-3 satellite sea level data that may possibly impact both short-scale and longer-scale data via interplay with the sea state bias (SSB) correction. Among the suite of standard corrections used to extract sea surface height (SSH) from the raw altimeter range, the empirical SSB term is designed to remove correlation between range and SWH measurements, but not explicitly short-scale error between the two. In this paper, we report that the efficacy of the SSB correction varies with wavelength and it does not remove all correlated signal at high wavenumbers. Following several recent studies, an independent high-frequency adjustment (HFA) is developed to remove these residual correlated errors. Both SSB and HFA are applied on SSH estimations at the 20-Hz rate, improving both 20-Hz and 1-Hz data. Jason-3 data are used as a test bed. Both corrections and net results are specific to MLE4 Jason-3 waveform retracking. Because the reported updated SSB models provide nearly unbiased continuity from Jason-1 to Jason-3 SSB time-series and the MLE4 retracking algorithm is the standard for these missions, the Jason-3 HFA solution can be applied to earlier Jason data with implications for improving the combined 16-year Jason record. By design, the HFA does not impact wavelengths greater than 200 km which is the acting domain of the SSB correction. Applying both the SSB and HFA corrections leads to a global SSH variance reduction nearing 35% in average. The SSH denoising approach through the computation of an additional HFA term is applicable for any LRM ocean altimeter.

T2 formula in a highly myopic population, comparison with other methods and description of an improved approach for estimating corneal height

BackgroundTo determine the accuracy of the T2 formula as applied to highly myopic eyes, to compare the T2 formula to the SRK/T and Holladay 1 formulas, and to describe possible ways to improve the estimation of corneal height and prediction error in two settings, the Hadassah Hospital, Ophthalmology Department, Jerusalem, Israel and Clínica Barraquer, Bogotá, Colombia.MethodsIn this retrospective case series, optical biometer measurements were taken for 63 highly myopic patients (> 25 mm) undergoing uneventful crystalline lens phacoemulsification and insertion of an acrylic intraocular lens. Prediction errors were obtained, with estimations of ±0.50 D, ± 1.00 D, and greater than ±2.00 D. A method to improve the corneal height calculation is described.ResultsThe SRK/T formula (mean absolute error [MAE] = 0.418; median absolute error [MedAE] = 0.352) was the most accurate, followed by the T2 (MAE = 0.435; MedAE = 0.381) and Holladay 1 (MAE = 0.455; MedAE = 0.389) formulas. Both the SRK/T and T2 formulas overestimated corneal height, but values were higher with the T2 formula. Corneal height was more precisely estimated using an alternative method that, when combined with axial length optimization, resulted in lower MAE (0.425) and MedAE (0.365) values than when applying the T2 formula alone.ConclusionsThe T2 formula seems to be less accurate than the SRK/T formula in highly myopic eyes. An improved corneal height estimation method is described for the the T2 formula.

Wave Retrieval Under Typhoon Conditions Using a Machine Learning Method Applied to Gaofen-3 SAR Imagery

The spatial resolution of Chinese Gaofen-3 (GF-3) Synthetic Aperture Radar (SAR) image, a pixel size of 500 m at azimuth/flight direction for Global Observation (GLO) and 100 m for Wide ScanSAR (WSC) modes, is relatively coarse, and waves shorter than 100 m are undetectable. In this study, SAR images collected under typhoon conditions by the GF-3 mission are exploited for wave retrieval purposes. In detail, six imagery collected during 2017 and 2018 over the China Seas captured four typhoons. The GLO and WSC modes images are collocated with simulated wave fields using a numeric wave model, called WAVEWATCH-III (WW3) and are processed using a machine learning method to retrieve sea surface waves. In this work, the criterion for the training completion in the process of machine learning is set as 0.15 m of Root mean Square Error (RMSE) for the Significant Wave Height (SWH). Then, the retrieved SWH is compared with measurements collected by the Jason-2 altimeter, showing a 0.61 m RMSE. The proposed algorithm is shown to outperform empirical approaches developed to retrieve the SWH using GF-3 SAR imagery acquired in the GLO and WSC imaging modes.

Measurement analyses and evaluations of sea-level heights using the HY-2A satellite’s radar altimeter

The HY-2A satellite is China’s first independent oceanic dynamic environmental satellite, and has been operating continuously for more than six years. The satellite’s radar altimeter, which is one of the main loads on the satellite, has the ability to realize all-weather and all-day observations of global sea-surface heights, as well as significant wave heights and sea-surface wind speeds. These observed data have been widely used in marine disaster prevention and reduction, along with resource development, maritime security and other fields. In order to achieve a comprehensive understanding of the multi-year overall observational performances of the HY-2A satellite’s radar altimeter, all of the observational data of the IGDR product from October 26, 2012 to August 27, 2017 were selected in this study for a comprehensive evaluation. The height measurement capability of the HY-2A satellite’s radar altimeter was evaluated using self-crossover and Jason-2 crossover methods. The height discrepancies at the self-crossover point of the HY-2A satellite’s ascending and descending orbits were also calculated. It was found that for the HY-2A satellite’s radar altimeter in global waters under the restriction conditions of ascending and descending orbits, the height anomaly differences were within a range of less than 30 cm. The absolute mean error was determined to be 5.81 cm, and the height anomaly standard deviation was 7.76 cm. Under the conditions of the observational areas being limited within a scope of 60° from the Equator, it was determined that the sea-level height anomaly differences were less than 10 cm at the junction of the ascending and descending orbits, the absolute mean error was 3.95 cm. In addition, the sea-level height anomaly standard deviation was observed to be 4.76 cm. Using a mutual cross method with the Jason-2 satellite, it was found that under the conditions of the observational area being within the scope of 66° from the equator, the height anomaly differences at the junction were less than 30 cm, and the absolute mean error of HY-2A and Jason-2 sea level height anomaly was 5.86 cm, with a standard deviation of 7.52 cm. It was observed that, if within the sea area the sea level height anomaly difference was limited to within 10 cm, then the absolute mean error and standard deviation could reach 4.19 cm and 4.98 cm, respectively. It was confirmed that the HY-2A satellite’s radar altimeter had successfully reached the height measurement level of similar international altimeters. Therefore, it had the ability to meet the needs of marine scientific research and ocean circulation inversions.

Modeling and Correcting Cure‐Through in Continuous Stereolithographic 3D Printing

AbstractContinuous stereolithography offers significant speed improvements over traditional layer‐by‐layer approaches but is more susceptible to cure‐through, undesired curing along the axis of exposure. Typically, cure‐through is mitigated at the cost of print speed by reducing penetration depth in the photopolymer resin via the addition of nonreactive light absorbers. Here, a mathematical approach is presented to model the dose profile in a part produced using continuous stereolithography. From this model, a correction method is developed to modify the projected images and produce a chosen dose profile, thereby reducing cure‐through while maintaining print speed. The method is verified experimentally on a continuous stereolithographic 3D printer, and the practicality of various dose profiles is investigated. In optimizing the critical dose parameter, the measured gelation dose Dgel is found to be insufficient for accurate reproduction of features, and an optimal value of Dc = 5Dgel is chosen for the test resin. Using optimized parameters with a high‐absorbance height resin (ha = 2000 µm), feature height errors are reduced by over 85% in a test model while maintaining a high print speed (s = 750 mm h−1).

EVALUATION OF SELECTED COST AGGREGATION METHODS ON HIGH RESOLUTION SATELLITE STEREO IMAGES

Abstract. Dense stereo processing requires a critical step that called cost aggregation or cost optimization. Most of the cost aggregation methods are evaluated on close range stereo images from Middlebury or KITTI datasets. While the effect of cost aggregation on high resolution satellite stereo processing has not yet been sufficiently evaluated. In this paper, three typical cost aggregation methods together with another approach which is a combination of these methods are evaluated on high resolution satellite stereo images and then are compared with LiDAR ground truth. These methods including Semi-Global Matching (SGM), Guided Filtering (GF), iterative GF (IGF), and SGM followed by GF (SGM-GF) with Census and Zero Normalized Cross Correlation (ZNCC) cost functions. Although the Census cost function has a good performance on the border of the objects and low blurring effects, the results of both cost functions, i.e. Census and ZNCC, have same treatment on all stereo methods. Also, in order to make an impartial assessment, for all stereo methods, we do not perform any disparity map refinement. The bad-pixel criteria with an absolute difference height error greater than 2 meters for SGM, GF, IGF, and SGM-GF methods is 36.7%, 34.8%, 33.8%, and 28.6% respectively. Also, the Normalized Median Absolute Difference (NMAD) error for SGM, GF, IGF, and SGM-GF is 1.29, 1.15, 1.06, and 0.94 meters, respectively. Overall, the experimental results on WV III stereo images demonstrate that the SGM method has lower accuracy and SGM-GF method is accurate than other methods.

Removal of Covariant Errors from Altimetric Wave Height Data

The echo waveforms received by conventional radar altimeters are interpreted by retracking algorithms to give estimates of range, wave height, and backscatter strength. However, in response to fading noise on the waveform leading edge, common retrackers, such as MLE-3 and MLE-4, show correlated errors in wave height and range. This correlation is used to develop a correction to the wave height data that reduces the high-frequency variability by ∼22%, without affecting the global distribution of values. This correction also results in a closer matchup of Jason-2 and Jason-3 data during their tandem phase. Although the correction is quite straightforward in practice, the appropriate conversion factor has to be determined for each combination of altimeter and retracker. There are also remaining open questions concerning the needed low-pass filtering.

Advanced DINSAR analysis on dam stability monitoring: A case study in the Germano mining complex (Mariana, Brazil) with SBAS and PSI techniques

This work presents an investigation of detection and monitoring of surface motions using Advanced Differential SAR Interferometry (A-DInSAR) with SBAS (Small Baseline Subset) and PSI (Persistent Scatterer Interferometry) techniques in the Germano iron mining complex region (Mariana, Brazil), after the collapse of the Fundão tailing dam occurred on November 5th, 2015. The research was carried out aiming to provide useful information about ground movement monitoring, planning and risk assessment in the area after the tragic event. The analysis was performed using 46 TerraSAR-X images and a Digital Elevation Model (DEM) obtained by Pleiades data. A-DInSAR processing provided the removal of the atmospheric phase artifacts, baseline related errors, DEM height error estimation and finally the ground displacement determination. The results pointed out that with both techniques it was possible to assess the stability conditions of the Germano mining assets, particularly the main structures of the Germano dam and the detection of small settlements in the mining tailings storage, pointing out that these may be an important components for risk management of open pit mining operations, including tailings dams.

FREE GLOBAL DEM: CONVERTING DSM TO DTM AND ITS APPLICATIONS

Abstract. DTM is a digital model that visualizes height topography. DTM can be used for various survey mapping applications. DTM can be derived from DSM. Currently DSM is widely available but the availability of DTM is still rare, especially in Indonesia. In this study, DSM was obtained from a combination of DSM X SAR, SRTM, and TanDEM-X with the integration method. This DSM integration made height error correction and height reference field to the EGM 2008. This DSM integration has fulfilled the tolerance of vertical accuracy in 1.96σ (95%). This study aims to convert DSM to DTM and discuss some of the DTM applications. The study area of this study is Rote Islands (north and east side), especially in the Dead Sea Lake Area which includes Lake Oemasapoka to Oesu Island. DSM can be converted into DTM by considering several things, namely the canopy tilt angle to topography, the height of the vegetation approach, and the radius defined around the canopy. One way to get this parameter is to make contours on the vegetation in the DSM. After conversion of DSM to DTM, DTM is obtained at ground level. The DTM that has been produced needs to be done a height difference test and the making of longitudinal profile. This longitudinal profile will visualize the conditions of DSM and DTM so that the vegetation height will appear along the profile. The resulting DTM can be used for several applications such as topographic contours, watersheds, water levels, flow accumulation, slope, slope direction, and others.

Target positioning of UAV airborne optoelectronic platform based on nonlinear least squares

The target positioning algorithm of the traditional unmanned aerial vehicle (UAV) airborne optoelectronic platform introduces a large number of angle measurement errors, resulting in low target positioning accuracy. In this paper, a hybrid nonlinear algorithm of least squares and Gauss-Newton is proposed. Firstly, the Gauss-Newton iterative nonlinear target localization algorithm based on laser ranging value is derived. Then the rough solution of linear least square is used as the initial value of the nonlinear Newton iteration method for target location estimation. The algorithm combines the advantages of the simple and easy implementation of the least squares method and the high convergence accuracy of the Gauss-Newton method, and satisfies the requirements of the Gauss-Newton method for the initial value accuracy. Experimental results of measured data show that the longitude error of fixed target positioning results of this method is less than 1.37×10-5 degrees, the latitude error is less than 6.31×10-5 degrees, and the height error is less than 1.78 meters. And the processing time of each positioning is within 6 ms, which meets the requirements of real-time positioning.

Application of convolutional neural networks for low vegetation filtering from data acquired by UAVs

The main advantage of using unmanned aerial vehicles (UAVs) is the relatively low cost of collecting data, especially when using photogrammetry on images of relatively small areas. Additionally, they have high operational flexibility and the results have a high spatial and temporal resolution. To further facilitate the use of UAVs in photogrammetry, we developed an algorithm to filter out points that indicate areas covered in low vegetation (grass, crops) from the generated point cloud. This paper presents a three-layer filtering algorithm based on convolutional neural networks (CNNs) created for this specific purpose. The modular structure of the algorithm makes it easy to expand on and improve. The proposed solution allows errors in the height of digital elevation model (DEM) points caused by the influence of vegetation to be reduced by as much as 60–70% in relation to height errors from the raw data of high grass. At the same time, the solution presented here is practical for low grass because it does not weaken the model. The algorithm significantly reduces the errors in the DEM, as well as the products derived from the DEM.

A New Structural Bump Foil Model With Application From Start-Up to Full Operating Conditions

Abstract This paper presents a new structural bump foil model that can handle all operating conditions from start-up to full speed. The model is based on a nonlinear contact algorithm with friction and gaps. The top foil is modeled as a curved beam while bump foil uses a coupled truss model. The model considers the gaps between the bump foil and the bearing casing, between the bump foil and the top foil and between the rotor and the top foil. Thus, any numerical interference between the rotor and the top foil is avoided. A mixed lubrication model is used for the thin film pressures. Following this algorithm, contact pressures appear if the film thickness is less than three times the equivalent roughness of the rotor and of the top foil. Fluid pressures are calculated from numerical solutions of Reynolds equation while contact pressures, if present, are calculated with the model of Greenwood and Williamson. The model is validated by comparisons with the experimental results obtained for start-up operating conditions of a first-generation foil bearing of 38.1 mm diameter with static loads of 10–50 N. Theoretical predictions of the start-up torque and takeoff speed compare well with experimental results. It is also shown how manufacturing bump height errors can explain the differences between theoretical and experimental predictions. Further validations are presented for the same bearing operating at high speeds (30, 45, and 55 krpm) and heavy static loads (up to 200 N). The calculated minimum film thickness and attitude angle are compared with experimental data from the literature.

A Global Adjustment Method for Photogrammetric Processing of Chang’E-2 Stereo Images

The Chang’E-2 (CE2) lunar orbiter was the second robotic orbiter of the Chinese Lunar Exploration Program, as well as the pioneer robotic orbiter in the soft landing project in the second phase of the program. It used a two-line stereo camera to acquire stereo images with global coverage at a resolution of 7 m. The stereo images have a large potential for producing the best lunar topographic map. However, errors and uncertainties in the interior orientation (IO) and exterior orientation (EO) parameters of the camera seriously affected the accuracy of the global topographic mapping. In this paper, a global adjustment method is proposed to eliminate the effects of these errors. The error models are represented by a Chebyshev polynomial. The polynomial coefficients were estimated as unknowns using five lunar laser ranging retroreflector (LRRR) points as ground control points in the adjustment. The experimental results show that the planimetric and height deviations between the neighboring strips were 5 and 2 m (less than 1 pixel), respectively, which were decreased by 32.6 and 31.5 times, respectively, relative to those derived from the original EO parameters. The large inconsistencies in the CE2 trajectory data were significantly reduced after the adjustment. In comparison with the LRRR positions, the planimetric and height errors ranged from 21 to 97 m and −19 to 10 m, respectively. A new seamless mosaic and high precision absolute position topographic map has been generated using this method.

Height Recomputation of the Korean Vertical Origin Point Using New Mean Sea Level of Incheon Bay

Kim, K.B.; Yun, H.S., and Kim, T. W., 2019. Height recomputation of the Korean vertical origin point using new mean sea level of Incheon Bay. In: Lee, J.L.; Yoon, J.-S.; Cho, W.C.; Muin, M., and Lee, J. (eds.), The 3rd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 91, pp. 396-400. Coconut Creek (Florida), ISSN 0749-0208.The mean sea level (MSL) of Incheon Bay using the long-term tidal observations more than 18.6 years (January 1, 1999 ∼ August 31, 2017) was determined by harmonic analysis. In comparison with the current MSL of Incheon Bay provided by Korea Hydrographic and Oceanographic Agency, the difference of the MSL is 0.035 m. The height of the current Korean vertical original point (KVOP) was reexamined by the newly computed MSL of Incheon Bay and height difference was computed between Tidal Gauge Bench Mark (TGBM) installed near Incheon Harbor and the KVOP. There is a difference of 2.27 cm in between the recalculated height and the current height of the KVOP. In order to check height error in the geodetic leveling network generated from single vertical origin point system based on the KVOP of Incheon, it would be necessary to consider the application of height deviation in multiple vertical origin point systems based on the local (Mokpo, Busan, and Mukho) MSLs.

Evaluation of the impact of high-resolution winds on the coastal waves

This study discusses the impact of high-resolution winds on the coastal waves and analyses the effectiveness of the high-resolution winds in recreating the fine-scale features along the coastal regions during the pre-monsoon season (March–May). The influence of the diurnal variation of winds on waves is studied for the Tamil Nadu coastal region using wind fields from weather research and forecast (WRF) (3 km) and European Centre for Medium-Range Weather Forecasts (ECMWF) (27.5 km). The improvement in the coastal forecast is then quantified with wave rider buoy observations. The high-resolution wind fields simulated fine-scale features like land–sea breeze events and showed good agreement with observation results. The error in the wave height and period is reduced by 8% and 46%, respectively, with the use of high-resolution forcing winds WRF over ECMWF, although the overestimation of wave energy on high frequencies due to overestimated WRF winds remains as a challenge in forecasting. The analysis also shows the importance of accurate wave forecast during a short-duration sudden wind (~12 m/s) occurrence in southern Tamil Nadu near Rameswaram during the pre-monsoon period. Low pressure forms over Tamil Nadu due to the land surface heating, resulting in a sudden increase of winds. High winds and steep waves which cause damage to the property of the coastal community near Rameswaram also were well simulated in the high-resolution forecast system with WRF winds.

High-Resolution Elevation Model of Lop Nur Playa Derived from TanDEM-X

In this paper, a digital elevation model (DEM) was produced for Lop Nur playa produced with the data from TanDEM-X mission. The spatial resolution is 10 m. It covers an area of 38,000 km2for orthometric height from 785 m to 900 m above sea level, which is composed of 42 interferometric synthetic aperture radar (InSAR) scenes. A least-square adjustment approach was used to reduce the systematic errors in each DEM scene. The DEM produced was validated with data from other sensors including Ice, Cloud, and land Elevation Satellite (ICESat) Geoscience Laser Altimeter System (GLAS) and aerial Structure-from-Motion (SfM) DEM. The results show that global elevation root mean square error to GLAS is 0.57 m, and the relative height error to SfM DEM in complicated terrain is 3 m. The excellent height reliability of TanDEM InSAR DEM in Lop region was proved in this paper. A reliable high-resolution basic topographic dataset for researches of Lop Nur was provided.

Inequality, validity of self-reported height, and its implications for BMI estimates: An analysis of randomly selected primary sampling units' data.

Any systematic errors in self-reported height, a measure commonly used in health research, may produce biased BMI estimates and reduce the effectiveness of public health interventions. To our knowledge, none of the studies evaluating the validity of self-reported height explore this issue in cross-national settings. This study analyses data on a sub-set of 750 individuals with information on self-reported and measured height from the Life in Transition Survey (LITS) conducted in 34 European and Central Asian countries in 2016. We make use of the unique design of LITS in which all respondents reported their height, but in one randomly selected primary sampling unit in each country the actual height was also measured, using a portable stadiometer. In addition to analysing individual-level characteristics, using a multiply imputed dataset for missing data and multilevel mixed-effects regressions, we test if macro-level factors are associated with respondents under- or over-reporting their height. We find that on the aggregate level self-reported and measured height estimates are not statistically different, but some socio-demographic groups such as women and those who live in rural areas are likely to overestimate their height. Adjusting for this bias would lead to the higher estimates of the proportion of individuals who are overweight and obese. The results from multilevel analysis also show that macro-level factors do not per se explain the likelihood of misreporting height, but rather some of the effects of individual characteristics are moderated by income inequality.

Non‐uniform phase filter for Tiangong‐2 Interferometric Imaging Radar Altimeter

The interferometric imaging radar altimeter (InIRA) on-board Chinese Tiangong-2 space laboratory is a Ku-band interferometric radar whose major aim is to measure the wide-swath ocean topography. Unlike traditional interferometric synthetic aperture radar (InSAR) aiming at measuring digital elevation model of land surface, Tiangong-2 InIRA adopts small incidence angles and short baseline to get strong and high coherency reflections from ocean surface. The interferometric phase noise is a notable source of errors which limits the accuracy of the ocean height measurements. Compared with traditional InSAR, the height error of Tiangong-2 InIRA shows different distribution pattern due to the absolutely small but relatively large variation incidence angles. This study gives a filtering method for interferometric phase generated with small incidence angles and short baseline. Non-uniform windows are selected according to the relation between the coherence, imaging geometry, and the height variation. Results on Tiangong-2 InIRA data are analysed by comparing the non-uniform filtering method with traditional multi-looking method.