Dijital ortofoto üretimi ve kullanımı

Generation and use of digital orthophotos

Tez No: 75407
Yazar: OZAN DİVAN
Danışmanlar: DOÇ. DR. SITKI KÜLÜR
Tez Türü: Yüksek Lisans
Konular: Jeodezi ve Fotogrametri, Geodesy and Photogrammetry
Anahtar Kelimeler: Belirtilmemiş.
Yıl: 1998
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Fen Bilimleri Enstitüsü
Ana Bilim Dalı: Jeodezi ve Fotogrametri Ana Bilim Dalı
Bilim Dalı: Belirtilmemiş.
Sayfa Sayısı: 81

Özet

ÖZET Coğrafi Bilgi Sistemlerinde (CBS) günümüzde bilgisayar teknolojisinde sağlanan önemli gelişmeler sonucunda veriyi üretenlerin öncekinden farklı bir kullanıcı tabanıyla karşılaşmalarına neden olmuştur. Birçok kullanıcı projenin gereksinimleri hakkında yeterince bilgili olmasına rağmen bu gereksinimlerini yeni gelişmeler çerçevesinde tamamıyla karşılayacak veriler üretilebileceğinden habersizdir. Dijital ortofotolar, CBS veri kaynaklarının en karışık özelliklere sahip olanıdır. Sayısal olarak ortofoto üretiminde yaşanan bu sorunların anlatımı ve aydınlatılması önemli ölçüde bu çalışmada ele alınmıştır. Birinci bölümde bu çalışmanın amacı belirtilmiş ve ortofoto kullanımının gerekliliği üzerinde durulmuştur. İkinci bölümde, ortofoto üretiminin genel aşamaları girdi, işleme ve çıktı kısımları altında detaylarıyla açıklanmış, ortofoto üretiminin nedenleri, karşılaşılan problemler ve çözümleri ele alınmıştır. Üçüncü bölümde, dijital ortofotoların kullanım avantajları hakkında bilgi verilmiştir. Son bölümde ise farklı yöntemlerle ve sistemlerle ortofoto üretim aşamaları anlatılmış ve bir örnek ortofoto oluşturma uygulaması verilmiştir. Ayrıca elde edilen karşılaştırma sonuçları önerilerle birlikte sunulmuştur. Vlll

Özet (Çeviri)

SUMMARY GENERATION AND USE OF DIGITAL ORTHOPHOTOS Digital orthophotos are becoming more and more important as a basic data source for a variety of applications. The first orthophotography was produced by computer driven optical methods and equipment. Today, these pieces of equipment have been replaced by the computer workstation and sophisticated computer software algorithms. The product of this technology is a digital image that can stand alone on its own visual merits or be integrated and employed within a geographic information system. Assessing the spatial accuracy and quality of the final product should be of significant interest to the GIS manager who will use this imagery in daily operations. In this thesis, general information about orthophotos and the production of digital orthophotos, as well as some applications will be presented. The usefulness and importance of digital orthophotos, principle of orthophoto generation, possible problems in using orthophotos and solutions as well as application examples will be emphasised. In order to be able to exactly overlay images and maps, their geometry has to be same: that is, an orthogonal projection of all points of the ground to a reference surface. This process is called rectification and the resulting image is the orthophoto. Different methods can be applied to generate digital orthophotos. There are commonly three approaches. They can be applied to rectify both digitised aerial photographs and satellite scenes. These methods are polynomial, projective and differential rectifications. The first two are defined by analytical transformations between image and orthophoto without considering the geometry and orientation of the camera. They are approximate solutions. The last one models the physical reality of the imaging process by means of the colinearity equations and corrects for relief displacements. Many image processing systems designed for digital mapping and remote sensing provide routines that perform this rectification procedure; it is sometimes called geo-referencing or geo-coding. These mathematical models applied range from simple affine transformation, utilising higher order polynomial and IXprojective transformations, to differential rectification with relief displacement correction. Differential rectification is of particular importance if a surface model of the landscape has to be created. Because digital images and digital elevation data are both related to a planimetric grid, they are basically stored in the same way: as matrices of gray or elevation values. Digital rectification assigns a gray value to each grid-element of the digital elevation model (DEM), so that both elevation and density of the surface are stored at the same planimetric location. Relief displacement is a distortion that affects the spatial accuracy of the image. Simply stated, points that are higher than the nadir are displaced outward from the center of the photograph and points that are lower in elevation are displace inward from their true position. Relief displacement of the terrain is removed during the orthophoto development, thus the hills and valleys will appear in their true location. Relief displacement of flagpoles, buildings, trees and other similar features will remain on the image since the top and bottom of the objects occupy the same X & Y coordinate on the ground. This artifact of relief displacement can cause some image distortions or illusions, particularly along the joint between orthophoto sheets. While sometimes disturbing, most are not defects in the image product. This thesis investigates mostly orthophoto production with differential rectification method. Three ways of orthorectification were implemented here on Silicon Graphics workstations. To determine the planimetric accuracy of the results, the coordinates of checkpoints were measured in three digital orthophotos and compared to known map coordinates. The following information/data must be available before starting the orthorectification process:. One or more aerial photos;. Information allowing points on the aerial photograph to be tied to a geocoded position on the ground;. Digital elevation data for the area of interest. Vertical aerial photos are best for orthorectification. Oblique (side view) photos may yield very poor results. It is critical that two opposite fiducial marks be available. It is also important to know the focal length or the Field of View (FOV) for the camera.Part of the correction process requires points on the photo to be tied to known ground coordinates (including elevation). This requires either a list of ground coordinates or corresponding marks on the photo or a high-resolution topological map of the area (eg, 1:25000 scale). The Digital Elevation Data (DEM) must be available for the area covered by the photos and it must be already geocoded and georeferenced (usually in UTM or LONG/LAT coordinates). This digital elevation model will form the basis for the correction of scale differences across the aerial imagery due to elevation changes. It will also be used to remove the relief displacement in the terrain. The elevation data is collected using an analytical stereo plotter to view the photography in 3-D and collect a representative sample of elevation points that will describe the relief of the area. The amount and type of information collected in the DEM will vary. Obtaining a DEM can be difficult. Bellow is some suggested sources:. General Command of Mapping has 1 -second (approx. 20m) resolution data for most of Turkey.. Elevation data can be extracted from satellite stereo pairs (usually Spot Panchromatic Scenes).. If vector contours are available then they can be converted into a digital elevation model. With the DEM collection complete, the photography is ready to be scanned and converted into a digital image. In the ideal situation, one aerial photograph will be used to create an entire orthophoto. The area should come from the central portion of the original photography to minimize any lens distortion near the edges of the photo. Photographs are scanned at a very high resolution to assure a high image quality in the final product. It is during the scanning process that defects of dirt and lint pieces can be added to the digital orthophoto image detracting from the quality of the final product. In addition, any scratches in the original photography that would normally be minimized during optical enlargements, will be clearly be captured and preserved in the scanning process making them clearly visible in the final product. To transform a digital image by an analytical function, one can pursue either a direct or an indirect approach. By using direct method the image pixels of a regular grid are transformed into the orthophoto. In doing so it is impossible to relate them xidirectly cause of the unknown height information used to determine the corresponding orthophoto position. For this reason the calculation has to be done iterative using the terrain height of the at last computed orthophoto position, until there is no more change registrated in the result. Therefore this method is most time-consuming; moreover the result is an irregular grid of output pixels in the orthophoto file with possible unfilled and multiple filled orthophoto pixels, unfavourable for following processes. By using the indirect method, the image position for each grid point of a regular orthophoto grid is calculated. With the help of the known terrain height of each orthophoto grid point the corresponding image position can be found out directly; therefore this method is appropriated to decrease the computing time to obtain a homogeneous orthophoto matrix without any gap and multiple information. For this reason the digital rectification process is usually executed indirectly. The actual orthophoto creation is a computer-based process that marries's the rasterized aerial photograph with the DEM. This process allows for the software to reposition the pixels of the scanned aerial photo to remove the effects of relief displacement and terrain elevation differences. With pixels properly positioned and associated X & Y coordinate values assigned the orthophoto is ready for viewing. The resulting image is now at constant scale across the entire image. Only correction for tonal differences between images remains to complete the process. Radiometric corrections may be necessary to smooth tone or color differences between orthoimages to improve the overall image quality. Pixel brightness values range between 0 and 255 and in some cases a localized adjustment may be required. If the photography is flown just prior to leaves returning to the trees, matching images between flight lines, recorded on different dates may be challenging. Differences in the time of day the photography is acquired can also hinder the radiometric balancing between digital images. Different Orthophotos were as application from PCI and ZEISS Phodis programs produced. In PCI program, there are several options: 1) To orthorectify an image without using a digital elevation model, (ie. assume the whole image has zero elevation). xu2) To orthorectify an image without using a digital elevation model, (ie. use an average elevation). In this case, the user does not have the elevation model but would like to use an average elevation value for the entire image during orthorectification. 3) To orthorectify an image using a digital elevation model. In this case, the user has the elevation for each pixel on the image. PHODIS OP software processes any type of grids-shaped elevation models, which are either taken from an existing DEM data base or obtained by photogrammetric methods. PHODIS TS software package permits the automatic generation and interactive processing of digital elevation models. The feature- based matching technique implemented in the TopoSURF software module uses a pair of digitized stereophotos to generate a dense point cluster from which a close-meshed raster DEM of high quality is derived. The digital stereoplotter is used for the visualization and interactive processing of the DEM. An orthoimage without using a digital elevation model, (ie. assume the whole image has zero elevation) in PCI software, an orthoimage using a digital elevation model from General Command of Mapping in PCI software and an orthoimage from the feature-based matching technique implemented in the TopoSURF software by using a pair of digitized stereophotos are derived. These three orthophotos were compared by assuming of the 1:25000 topographic base map of the area without error. The result show that it is important to consider DEM's cell size carefully, precision and homogeneity of control points. Xlll

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