Umut G. Sefercik

Performance Estimation of Aster Global DEM Depending upon the Terrain Inclination

A digital elevation model (DEM) is a source of immense three dimensional data revealing topographic characteristics of any region. The performance of a DEM can be described by accuracy and the morphologic conformity. Both depend upon the quality of data set, the used production technique and the roughness of the terrain. The global DEM of ASTER (Advanced Space-borne Thermal Emission and Reflection Radiometer) was released to public utilization as free of charge on June 2009. It covers virtually overall the globe using 1 arc-second posting interval. Especially easy availability renders ASTER Global DEM (GDEM) one of the most popular and considerable global topographic data for scientific applications. From this point of view, the performance of ASTER GDEM has to be estimated for different kinds of topographies. Accordingly, six test fields from Spain (Barcelona) and Turkey (Istanbul and Zonguldak) have been preferred depending upon the terrain inclination. Thus, the advantages and disadvantages of the DEM product have been proved by means of a group of advanced performance analysis. The analyses indicate that the performance of ASTER GDEM is quite satisfying at urban areas because of flat topography. On the other hand, terrain slope has negative effect on the results. Especially steep, mountainous, forestry topographic formations and the regions which have sudden changes at the altitude have lower accuracy.

Comparison of DEM Accuracies Generated by Various Methods

Presently, various different methods are being used for digital elevation model (DEM) generation. DEMs can be generated from topographic maps obtained with terrestrial measurements, photogrammetric flight data or remote sensing technologies. And also a question can be asked that which method is better than others thats why these DEM generation techniques have been analysed and the accuracies of DEMs have been compared in this study. For Zonguldak test field, three types of DEMs have been used. First DEM has been produced by digitized contour lines of 1:25000 scale topographic maps, second one has been produced by photogrammetric flight project data had been made by Zonguldak Municipality in 2005 and last one has been derived from Shuttle Radar Topography Mission (SRTM) X-band data which has used single-pass interferometric synthetic aperture radar (InSAR) technique for DEM generation. In the study, all these DEMs have been compared one by one based on selected reference. The DEM produced with Photogrammetric method has approximately 5.5 m better accuracy for open and flat, 6.5 m better accuracy for forest areas against the DEM generated from 1:25000 scale topographic maps. SRTM X-band DEM is 4 m less accurate for open, 4.5 m less accurate for forest areas against the DEM produced with photogrammetric method and 9.5 m less accurate for open, 11 m less accurate for forest areas against the DEM generated from 1:25000 scale topographic maps. And at the result, it has been clearly seen that the DEM produced by photogrammetric flight project in 2005 has better accuracy than the others.

Geometric conditions of space imagery for mapping

From the test field Zonguldak different high and very high resolution optical satellite images are available like TK350, ASTER, KOMPSAT-1, IRS-IC, SPOT 5, KVR1000, IKONOS and QuickBird. The images partially have been achieved as close to original images (level 1A) and partially projected to a plane with constant object height (level 1B). For some images, based on direct sensor information, a good image orientation is given which for some only has to be improved by a shift in X and Y, while for others only rough orientations are distributed. In addition sometimes the orientation has to be improved by additional parameters to compensate systematic geometric effects. Some orientation information of IKONOS- and QuickBird-images is available also as rational polynomial coefficients (RPCs), describing the relation between the image and the object coordinates by a ration of polynomials.

Geometric evaluation, automated DEM and orthoimage generation from along-track stereo ASTER images

A cloud-free ASTER scene combination covering 61.5km /spl times/ 63km Zonguldak testfield in the north-west Turkey has been analysed. It comprises the nadir and backward views with a base-to-height ratio of 0.6. The pixel size on the ground is 15m. The bundle orientation was executed with the related module of PCI Geomatica V9.1.4 software package and resulted the 3D geo-positioning to an accuracy of about 14m in planimetry and 13m in height. This level of accuracy can be provided using the number of GCPs up to 14 which are distributed over the scene uniformly. Based on the scene orientation, a DEM of the area has been determined by an automatic image matching and PCI system yielded a DEM with 30m cell size. For the validation of extracted DEM, different groups of GCPs selected over the testfield were utilized. In this analysis, GCPs were located in the raster DEM in according to their planimetric coordinates, then the heights are estimated by the bilinear interpolation of the neighboured grid cells. This was done by the program DEMINT and mean square differences was obtained in the range of 12 to 14m. Moreover, matched DEM was checked against reference DEM based on digitised contour lines from the 1:25000 scale topographic maps using program DEMANAL. The discrepancies between the two DEMs were determined as reference DEM minus matched DEM. Then positive biases resulted which show that matched DEM occurred under the reference DEM. These biases appeared also in the superimposition of contours from two DEMs. Image of DZ discrepancies is displayed as a function of grey values as well and highest residuals occurred mainly at the ridges. In the meantime, RMSE-Z from the comparison of both DEMs obtained very close in the range of /spl plusmn/21-22m. Finally, orthoimage was generated using matched DEM and nadir image component of ASTER stereopair without problem. Planimetric accuracy check of this product was realized using the GCPs and shows no systematic error pattern overall.

Productivity of TerraSAR-X 3D Data In Urban Areas: A Case Study In Trento

Abstract TerraSAR-X (TSX) is supposed to be a revolution for synthetic aperture radar (SAR) imaging because it offers the highest resolution spatial data and is suitable for interferometry. This study aims to validate the performance of TSX high-resolution spotlight (HS) mode data in 3D representations of urban areas. Therefore, a digital surface model (DSM) with a 3-m grid was generated over Trento, Italy, and its quality was validated by comprehensive analyses. The TSX HS DSM matches the reference model well, and the absolute vertical accuracy is ±7-8 m. The quality of the topographic estimation is satisfactory due to the suitable normal baseline which yields a more sensitive SAR signal to height differences, transformed into a lower value for the height of ambiguity. The DSM has distortions because of foreshortening, layover and shadows caused by terrain inclination and tall objects on the ground. Dense forests have a negative influence on the results because they are not penetrated by the short wavelengths of the X-band.

Contribution of Normalized DSM to Automatic Building Extraction from HR Mono Optical Satellite Imagery

Abstract Building extraction from high resolution (HR) satellite imagery is one of the most significant issue for remote sensing community. Manual extraction process is onerous and time consuming thats why the improvement of the best automation is a crucial topic for the researchers. In this study, we aimed to expose the significant contribution of normalized digital surface model (nDSM) to the automatic building extraction from mono HR satellite imagery performing two-step application in an appropriate study area which includes various terrain formations. In first step, the buildings were manually and object-based automatically extracted from ortho-rectified pan-sharpened IKONOS and Quickbird HR imagery that have 1 m and 0.6 m ground sampling distances (GSD), respectively. Next, the nDSM was created using available aerial photos to represent the height of individual non-terrain objects and used as an additional channel for segmentation. All of the results were compared with the reference data, produced from aerial photos that have 5 cm GSD. With the contribution of nDSM, the number of extracted buildings was increased and more importantly, the number of falsely extracted buildings occurred by automatic extraction errors was sharply decreased, both are the main components of precision, completeness and overall quality.

Area-based quality control of airborne laser scanning 3D models for different land classes using terrestrial laser scanning: sample survey in Houston, USA

Airborne laser scanning (ALS) is a remote-sensing technique that provides scale-accurate 3D models consisting of dense point clouds with x, y planimetric coordinates and altitude z. Using ALS, very high-resolution (VHR) digital surface models (DSMs) have been widely used for commercial and scientific applications since the early 1990s. Although there is widespread usage, there has been little comprehensive investigation of quality control for ALS DSMs in the literature, as most studies have been limited to assessing point-based vertical accuracy. This article is dedicated to investigating the quality of ALS DSMs for different land classes using statistical and visual approaches based on absolute and relative vertical accuracy metrics. Rather than a limited number of ground control points (GCP), the model-to-model-based approach is applied and DSMs derived from terrestrial laser scanning (TLS) point clouds that have around 5 mm absolute and 3 mm relative geolocation accuracy were used as the reference data for comparison. The results demonstrate that in open, grass, and building land classes, the ALS DSMs reached both standard deviation (σ) and normalized median absolute deviation (NMAD) of 3–5 cm after the elimination of any systematic biases. This result sufficiently satisfies the vertical accuracy requirements for 1/1000-scale topographic maps determined by National Digital Elevation Program (NDEP) specifications. In tall vegetation, a higher number of discrepancies larger than 0.5 m exist, reversing the relation between σ and NMAD. These vegetation errors also do not appear to be normally distributed. As an additional investigation, the performance of ALS DEMs under dense high-vegetation areas was assessed. These under-canopy ALS DEMs, created using only classified ground returns, offer both σ and NMAD of 12–14 cm, a performance level that is difficult to achieve under-canopy using photogrammetric techniques.

Quantitative assessment of remotely sensed global surface models using various land classes produced from Landsat data in Istanbul

Digital elevation model (DEM) is the most popular product for three-dimensional (3D) digital representation of bare Earth surface and can be produced by many techniques with different characteristics and ground sampling distances (GSD). Space-borne optical and synthetic aperture radar (SAR) imaging are two of the most preferred and modern techniques for DEM generation. Using them, global DEMs that cover almost entire Earth are produced with low cost and time saving processing. In this study, we aimed to assess the Satellite pour l’observation de la Terre-5 (SPOT-5), High Resolution Stereoscopic (HRS), the Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER), and the Shuttle Radar Topography Mission (SRTM) C-band global DEMs, produced with space-borne optical and SAR imaging. For the assessment, a reference DEM derived from 1:1000 scaled digital photogrammetric maps was used. The study is performed in 100 km2 study area in Istanbul including various land classes such as open land, forest, built-up land, scrub and rough terrain obtained from Landsat data. The analyses were realized considering three vertical accuracy types as fundamental, supplemental, and consolidated, defined by national digital elevation program (NDEP) of USA. The results showed that, vertical accuracy of SRTM C-band DEM is better than optical models in all three accuracy types despite having the largest grid spacing. The result of SPOT-5 HRS DEM is very close by SRTM and superior in comparison with ASTER models.

Point-based and model-based geolocation analysis of airborne laser scanning data

Abstract. Airborne laser scanning (ALS) is one of the most effective remote sensing technologies providing precise three-dimensional (3-D) dense point clouds. A large-size ALS digital surface model (DSM) covering the whole Istanbul province was analyzed by point-based and model-based comprehensive statistical approaches. Point-based analysis was performed using checkpoints on flat areas. Model-based approaches were implemented in two steps as strip to strip comparing overlapping ALS DSMs individually in three subareas and comparing the merged ALS DSMs with terrestrial laser scanning (TLS) DSMs in four other subareas. In the model-based approach, the standard deviation of height and normalized median absolute deviation were used as the accuracy indicators combined with the dependency of terrain inclination. The results demonstrate that terrain roughness has a strong impact on the vertical accuracy of ALS DSMs. From the relative horizontal shifts determined and partially improved by merging the overlapping strips and comparison of the ALS, and the TLS, data were found not to be negligible. The analysis of ALS DSM in relation to TLS DSM allowed us to determine the characteristics of the DSM in detail.

Geometric accuracy and information content of WorldView-1 images

Abstract. Advancements in the geometric resolution of space images have improved the conditions for generations of large-scale topographic maps. Using WorldView-1, WorldView-2, and GeoEye-1, images can now be captured from space with a 0.5 m ground sampling distance (GSD). Geometric accuracy and information content are the most significant components of mapping from space images. Depending on the resolution, image quality, and shadows, the identification and classification of ground objects may prove challenging. In this research, the geometric accuracy and information content, of panchromatic WorldView-1 images, were analyzed by covering parts of Istanbul and Zonguldak in Turkey. Each of these locations has various topographic characteristics. For the orientation and investigation of the geometric accuracies of images, a number of ground control points (GCPs) were developed as independent checkpoints. Based on bias-corrected rational polynomial coefficients with one GCP, a standard deviation of independent checkpoints on the range of one GSD was obtained. The information content of images was analyzed by mapping all buildings, in both test areas, and comparing the results with reference 1/5000 scaled topographic maps. The results verified that the WorldView-1 images can be utilized for generating and updating 1/5000 scaled topographic maps of urban areas.