Accuracy Of Ground Control Points Research Articles

A METHOD FOR ESTIMATING THE ACCURACY OF THE TIE POINTS CLOUD: THE CASE STUDY OF PALAZZO DE FALCO

Abstract. In the context of cultural heritage documentation and conservation, an accurate 3D model is essential to properly describe an artifact of historical relevance and provide the expert user with verified metric data. In this paper we propose a workflow aimed at estimating errors in the image orientation steps, which is useful for improving the accuracy of the sparse cloud (composed of Tie Points). Some parameters provided as input to the processing software, often overlooked by the user in favour of the default ones, are analysed. Specifically, tests are carried out using the commercial software package Agisoft Metashape and involve the analysis of three parameters: (i) the accuracy of measuring the Ground Control Points (GCPs) provided by the user; (ii) the accuracy of the Tie Points (TPs) computed by the software; and (iii) the accuracy of collimating the GCPs on the image. The accuracy of the GCPs is estimated by implementing the formulas of error propagation in MATLAB, considering the three concurring sources of error: the coordinate measurement with the Total Station (TS), the coordinate transformation from the local system to the UTM/ETRF00 System, and the GNSS measurement for estimating the transformation parameters. The transformation parameters are calculated using the Bursa Wolf method. The collimation accuracy of GCPs on the image is estimated by the reprojection error of each single GCPs on the image plane. The accuracy of the GCPs will be estimated by the standard error per unit weight (SEUW), accurate GCPs being expected to have a SEUW value close to unity. SEUW is also the overall indicator for estimating the accuracy of the TPS cloud because it is influenced, in different ways, by all three parameters analysed. Test Area is a historic building located in the town of Fisciano, a few kilometres from the city of Salerno, known as Palazzo De Falco. The results highlight how these parameters, if not properly considered, can significantly affect the final 3D model.

Block Adjustment With Relaxed Constraints From Reference Images of Coarse Resolution

As the direct geo-locating accuracy of spaceborne optical images is limited by the uncertainty of the exterior orientation parameters, precise ground control points (GCPs), which are difficult or expensive to obtain, are commonly required to improve the geometric accuracy in practical applications. In this article, we propose a novel block adjustment (BA) method to make use of the GCPs automatically collected from reference images of coarse resolution (Landsat-8 or Sentinel-2), which are publicly available. Different from the conventional BA methods, the proposed one treats the GCPs of low accuracy as relaxed constraints instead of directly minimizing the error between the geometric models and GCPs, and only guarantees that the GCPs are satisfied by the geometric models with a prior accuracy. In addition, an automated method is introduced to estimate the proper GCP accuracy for the proposed BA. The experimental results of three testing sites in China using three different types of spaceborne images, i.e., Gaofen-1 (GF-1) panchromatic (PAN), ZY-3 nadir (NAD), and SPOT-5 high resolution geometric (HRG) whose spatial resolutions are around 2 m, show that the accuracy of 1–2 pixel can be achieved for these high-resolution images when only coarse reference images (spatial resolution of 15 and 10 m) were used as ground control. The results also show that the inaccurate GCPs are not likely to undermine the geometric consistency of images in the proposed BA, and BA with relaxed constraints can even achieve better tie points (TPs) accuracy than BA without ground control. This article provides a practical way to utilize inaccurate ground control and balance the tradeoff between GCPs and TPs.

Development of Operational Applications for TerraSAR-X

In the course of the TerraSAR-X mission, various new applications based on X-Band Synthetic Aperture Radar (SAR) data have been developed and made available as operational products or services. In this article, we elaborate on proven characteristics of TerraSAR-X that are responsible for development of operational applications. This article is written from the perspective of a commercial data and service provider and the focus is on the following applications with high commercial relevance, and varying operational maturity levels: Surface Movement Monitoring (SMM), Ground Control Point (GCP) extraction and Automatic Target Recognition (ATR). Based on these applications, the article highlights the successful transition of innovative research into sustainable and operational use within various market segments. TerraSAR-X’s high orbit accuracy, its precise radar beam tracing, the high-resolution modes, and high-quality radiometric performance have proven to be the instrument’s advanced characteristics, through, which reliable ground control points and surface movement measurements are obtained. Moreover, TerraSAR-X high-resolution data has been widely exploited for the clarity of its target signatures in the fields of target intelligence and identification. TerraSAR-X’s multi temporal interferometry applications are non-invasive and are now fully standardised autonomous tools to measure surface deformation. In particular, multi-baseline interferometric techniques, such as Persistent Scatter Interferometry (PSI) and Small Baseline Subsets (SBAS) benefit from TerraSAR-X’s highly precise orbit information and phase stability. Similarly, the instrument’s precise orbit information is responsible for sub-metre accuracy of Ground Control Points (GCPs), which are essential inputs for orthorectification of remote sensing imagery, to locate targets, and to precisely georeference a variety of datasets. While geolocation accuracy is an essential ingredient in the intelligence field, high-resolution TerraSAR-X data, particularly in Staring SpotLight mode has been widely used in surveillance, security and reconnaissance applications in real-time and also by automatic or assisted target recognition software.

ANALYSIS AND CORRECTION OF SYSTEMATIC HEIGHT MODEL ERRORS

Abstract. The geometry of digital height models (DHM) determined with optical satellite stereo combinations depends upon the image orientation, influenced by the satellite camera, the system calibration and attitude registration. As standard these days the image orientation is available in form of rational polynomial coefficients (RPC). Usually a bias correction of the RPC based on ground control points is required. In most cases the bias correction requires affine transformation, sometimes only shifts, in image or object space. For some satellites and some cases, as caused by small base length, such an image orientation does not lead to the possible accuracy of height models. As reported e.g. by Yong-hua et al. 2015 and Zhang et al. 2015, especially the Chinese stereo satellite ZiYuan-3 (ZY-3) has a limited calibration accuracy and just an attitude recording of 4 Hz which may not be satisfying. Zhang et al. 2015 tried to improve the attitude based on the color sensor bands of ZY-3, but the color images are not always available as also detailed satellite orientation information. There is a tendency of systematic deformation at a Pléiades tri-stereo combination with small base length. The small base length enlarges small systematic errors to object space. But also in some other satellite stereo combinations systematic height model errors have been detected. The largest influence is the not satisfying leveling of height models, but also low frequency height deformations can be seen. A tilt of the DHM by theory can be eliminated by ground control points (GCP), but often the GCP accuracy and distribution is not optimal, not allowing a correct leveling of the height model. In addition a model deformation at GCP locations may lead to not optimal DHM leveling. Supported by reference height models better accuracy has been reached. As reference height model the Shuttle Radar Topography Mission (SRTM) digital surface model (DSM) or the new AW3D30 DSM, based on ALOS PRISM images, are satisfying. They allow the leveling and correction of low frequency height errors and lead to satisfying correction of the DSM based on optical satellite images. The potential of DHM generation, influence of systematic model deformation and possibilities of improvement has been investigated.

ANALYSIS AND CORRECTION OF SYSTEMATIC HEIGHT MODEL ERRORS

The geometry of digital height models (DHM) determined with optical satellite stereo combinations depends upon the image orientation, influenced by the satellite camera, the system calibration and attitude registration. As standard these days the image orientation is available in form of rational polynomial coefficients (RPC). Usually a bias correction of the RPC based on ground control points is required. In most cases the bias correction requires affine transformation, sometimes only shifts, in image or object space. For some satellites and some cases, as caused by small base length, such an image orientation does not lead to the possible accuracy of height models. As reported e.g. by Yong-hua et al. 2015 and Zhang et al. 2015, especially the Chinese stereo satellite ZiYuan-3 (ZY-3) has a limited calibration accuracy and just an attitude recording of 4 Hz which may not be satisfying. Zhang et al. 2015 tried to improve the attitude based on the color sensor bands of ZY-3, but the color images are not always available as also detailed satellite orientation information. There is a tendency of systematic deformation at a Pléiades tri-stereo combination with small base length. The small base length enlarges small systematic errors to object space. But also in some other satellite stereo combinations systematic height model errors have been detected. The largest influence is the not satisfying leveling of height models, but also low frequency height deformations can be seen. <br><br> A tilt of the DHM by theory can be eliminated by ground control points (GCP), but often the GCP accuracy and distribution is not optimal, not allowing a correct leveling of the height model. In addition a model deformation at GCP locations may lead to not optimal DHM leveling. Supported by reference height models better accuracy has been reached. As reference height model the Shuttle Radar Topography Mission (SRTM) digital surface model (DSM) or the new AW3D30 DSM, based on ALOS PRISM images, are satisfying. They allow the leveling and correction of low frequency height errors and lead to satisfying correction of the DSM based on optical satellite images. <br><br> The potential of DHM generation, influence of systematic model deformation and possibilities of improvement has been investigated.

UAV photogrammetry for road surface modelling

Recently, the interest of Unmanned Aerial Vehicle application in photogrammetric environment for roads observation and monitoring has increased in many countries, in Lithuania as well. The experimental object for demonstration of capability and efficiency of aerial vehicle-based remote sensing technology for road data collection was a western bypass of Vilnius. The platform of the model UX5 Trimble with mounted camera Sony NEX-5R was applied for gaining images. The implemented means are mobile and not expensive. Photogrammetric technique with software package Business Center Photogrammetry Module was applied for the modelling of images. The correctness of digital surface model generally depends on camera resolution, flight height and accuracy of ground control points. The coordinates of control points were determined using Global Positioning System Trimble R4. Paper demonstrates results of a new technology application possibilities for linear object (road) mapping and accuracy evaluation of spatial models. The road points positioning accuracy investigation was carried out in consideration with geodetic control measurements. The average root mean square error for the points coordinates is 2.94 cm, and standard deviations – 2.78 cm. Analyzing coincidence or mismatches of Vilnius western bypass project data with photogrammetric product, not significant discrepancies of road section features were determined. The cost consideration of Unmanned Aerial Vehicle in conjunction with photogrammetry employment at experimental object is presented.

An Error Prediction Framework for Interferometric SAR Data

Three of the major error sources in interferometric synthetic aperture radar measurements of terrain elevation and displacement are baseline errors, atmospheric path length errors, and phase unwrapping errors. In many processing schemes, these errors are calibrated out by using ground control points (GCPs) (or an external digital elevation model). In this paper, a simple framework for the prediction of error standard deviation is outlined and investigated. Inputs are GCP position, a priori GCP accuracy, baseline calibration method along with a closed-form model for the covariance of atmospheric path length disturbances, and a model for phase unwrapping errors. The procedure can be implemented as a stand-alone add-on to standard interferometric processors. It is validated by using a set of single-frame interferograms acquired over Rome, Italy, and a double difference data set over Flevoland, The Netherlands.

Error Tracking in Ikonos Geometric Processing Using a 3D Parametric Model

data, different research studies have addressed the potential of Thirteen panchromatic (Pan) and multiband (XS) Ikonos Geo- high-resolution imagery for mapping. A research study at the product images over seven study sites with various environ- National Mapping Agency of Great Britain used simulated 0.2ments and terrain were tested using different cartographic data m and 1-m Pan images derived from 1:7,500-scale aerial photos and accuracies with a 3D parametric model developed at the and XS 4-m images from the Compact Airborne Spectrographic Canada Centre for Remote Sensing, Natural Resources Canada. Images (Ridley et al., 1997). Some results indicated that the The objectives of this study were to define the relationship future high-resolution satellite imagery would have potential between the final accuracy and the number and accuracy of for improving the existing National Topographic Database of input data, to track error propagation during the full geometric Great Britain from 1-m panchromatic images as well as autocorrection process (bundle adjustment and ortho-rectification), mating the detection of topographic feature change from 4-m XS and to advise on the applicability of the model in opera- images. In another research study, a theoretical analysis based tional environments. on in-track and across-track stereo mapping techniques demonWhen ground control points (GCPs) have an accuracy strated that high-resolution satellite imagery could be used for poorer than 3 m, 20 GCPs over the entire image are a good the generation and updating of national mapping products in compromise to obtain a 3- to 4-m accuracy in the bundle the United States, but only if photogrammetric processing were adjustment. When GCP accuracy is better than 1 m, ten GCPs employed (Li, 1998). Based on this theoretical analysis, an are enough to decrease the bundle adjustment error of either evaluation of the potential accuracy of ground points was panchromatic or multiband images to 2 to 3 m. Because GCP performed using Ikonos stereo images simulated from aerial residuals reflect the input data errors (map and/or plotting), photos (Zhou and Li, 2000). Without ground control points these errors did not propagate through the 3D parametric (GCPs), a positioning accuracy of 12 m in the three axes (X, Y, model, and the internal accuracy of the geometric model is and Z ) was achieved, and with 24 GCPs the accuracy was thus better (around a pixel or less). improved to 3mi n planimetry (X and Y ) and 2 m in elevation. Quantitative and qualitative evaluations of ortho-images More recently, preliminary accuracy tests with real images were thus performed with either independent check points or having a viewing angle of 39 and using a3D parametric geooverlaid digital vector files. Generally, the measured errors metric correction model were performed with few precise confirmed the predicted errors or were even slightly better, GCPs (Toutin and Cheng, 2000; Davies and Wang, 2001). The and a 2- to 4-m positioning accuracy was achieved for the results of the two research studies demonstrated positioning ortho-images depending upon the elevation accuracy (DEM accuracies of 2 to 3 m and confirmed the high mapping potenand grid spacing). To achieve a better final positioning tial of Ikonos images. accuracy, such as 1 m, a DEM with an accuracy of 1 to 2 m This paper expands on the above-mentioned preliminary and with a fine grid spacing is required, in addition to well- results related to the geometric processing and ortho-rectificadefined GCPs with an accuracy of 1 m.