FROM ANALOGUE TO DIGITALCLOSE-RANGE PHOTOGRAMMETRY

Jörg Albertz and Albert Wiedemann

Department for Photogrammetry and Cartography, TU Berlin

The paper outlines the development in close-range photogrammetry duringone and a half century. The advancements in digital close-range photogram-metry are especially emphasized. Recent research projects of the Departmentfor Photogrammetry and Cartography at the Technical University of Berlinconcerning close-range photogrammetry are presented.

1. INTRODUCTION

Close-range photogrammetry has its origin in the activities of Albrecht Meydenbauer, whowas the first German photogrammetrist at all. Since his days close-range photogrammetryconquered the full range of scales from architectural photogrammetry down to electronmicroscope imagery. It was already in 1882 that Meydenbauer gave a course on photogram-metry and close-range photogrammetry became for the first time a matter of education andresearch at the Technical University of Berlin [Albertz 1981].

2 . DEVELOPMENT OF PHOTOGRAMMETRIC TECHNIQUES

The past development of photogrammetry can be subdivided in four phases (Fig. 1). Eachone is characterized by technological and methodological innovations which made photo-grammetry more flexible and more effective.

1850 1900 1950 2000

GraphicalPhotogrammetry

AnaloguePhotogrammetry

AnalyticalPhotogrammetry

DigitalPhotogrammetry

Figure 1: Stages in the development of photogrammetry

2.1 Graphical Photogrammetry

In the beginning photogrammetric restitution was achieved by graphical constructions on adrawing board following the principles of descriptive geometry. The camera served as aphotographic theodolite. Large image formats provided higher accuracy. This technique waswidely used in architectural photogrammetry by the »Royal Prussian PhotogrammetricInstitution« in Berlin, founded in 1885 in order to preserve cultural monuments.

2.2 Analogue Photogrammetry

In analogue photogrammetry the imaging geometry is reconstructed through optical ormechanical devices. Two images can be oriented in such a way, that a three-dimensionalmodel of the object is formed. A human operator can move a floating mark in this model andcontrol this movement under stereoscopic vision. This enables to map directly structural linesof the object as well as contour lines. Through progress in optics and mechanics analoguephotogrammetric instruments have been improved step by step in the course of many de-cades, and thus reached very high accuracy. During this stages of development photo-grammetry has been a technique to avoid calculations. The stereophotogrammetric plot of thebust of Queen Nofretete at the Egyptian Museum in Berlin is a typical example for theapplication of analogue techniques in close-range photogrammetry [Wölpert 1969].

2.3 Analytical Photogrammetry

Through the evolution of computers it became possible to develop the so-called analyticalplotters. In such photogrammetric systems the relations between image points and objectpoints are described through numerical calculations based on the collinearity equations. Thisoffers high accuracy, great flexibility and efficieny, in particular since the systems supportthe operator during the orientation and restitution processes. Furthermore the results may bedirectly transferred into CAD systems [Albertz and Wiedemann 1995]. It is evident, thatbecause of these advantages the analytical plotter replaced the analogue instruments more andmore. An example for the application of analytical plotters in close-range photogrammetry isthe restitution of the skeleton of Brachiosaurus brancai at the Museum for Natural Sciencesin Berlin in Figure 2.

Figure 2: Photogrammetric plot of the skeleton of Brachiosaurus brancai

During the last decade the point determination by bundle adjustment became a generally usedtechnique for close-range applications. This is a multi-image approach, not restricted to thestereophotogrammetric configuration for data acquisition, so that convergent images can beapplied in very flexible arrangements. Furthermore it is possible to consider additional ob-servations and also to combine calibration and restitution in an overall adjustment process.

The measurement of image coordinates is usually carried out monoscopically, either incomparators, on digitizers or even on the screen if the imagery is scanned. For simpler tasks,where relatively low accuracy is sufficient, no expensive hardware is required. This opensthe application of photogrammetry to many new users.

2.4 Digital Photogrammetry

The measurement of image coordinates in scanned images on the screen is the first steptowards digital photogrammetry. Through the application of digital image data photogram-metry becomes a special field of digital image processing, providing still more flexibility andan enormous potential for automation. Digital stereophotogrammetric systems are already onthe market or under development [Ebner et al., 1991]. Research activities at many insti-tutions are devoted to the automation of orientation and photogrammetric restitution. How-ever, the already operational stereoscopic systems are well suited for aerial photogrammetrybut not for close range applications.

2.5 The Future

The future will offer new techniques to photogrammetrists for close-range applications. Anexample is the laser scanner system under development at the University of Stuttgart [Wehr1994]. A laser beam scans the surface of an object in a regular pattern. Besides the two polarangles of the scanning system the distance to the object point as well as the intensity of thereflected light is recorded. This provides the full information of a three-dimensional object inone set of four-dimensional data. The impact of such an approach on future development ofphotogrammetry can hardly be foreseen.

3 . DIGITAL CLOSE-RANGE PHOTOGRAMMETRY

The applications of digital image processing in photogrammetry may be related to threedifferent tasks:

• The first one, and also the easiest to achieve, is the enhancement of the image quality(e.g. contrast stretching, filtering techniques) in order to provide a better image to thehuman operator. In general this means that well-known image processing tools areapplied to photogrammetric image data.

• The second task is to relieve the human operator from tedious work, for example theidentification of homologous points in two images. This requires higher level tech-niques, but great success has already been achieved.

• High-end techniques tend to automate the interpretation of digital images, i.e. to derivesemantic information by computational operations. This is subject to concentrated re-search activities.

A lot of research and development has already been done. Most photogrammetric work dealswith the standard configuration, i.e. aerial images in near-vertical configuration. But theresults of such studies can not simply be transferred to close-range applications withoutparticular adaptation. Severe problems arise from discontinuities of the object's surfaces,occluded areas, convergent orientation of images, inhomogenous image bundles, lowcontrasts, differences in the illumination and similarity of features. Therefore close rangephotogrammetry is a research field for its own [Albertz 1986 and Li 1993].

3.1 Data Acquisition and Calibration of Imaging Systems

There are two different ways to produce digital image data. The first one is to scan analoguephotographs taken with metric, semi-metric or amateur cameras. The second way is toacquire digital image data directly by means of digital CCD cameras. The problem is thattoday large CCD arrays are difficult to produce and very expensive. To reach sufficient

accuracy using low cost arrays precise calibration data of the digital cameras are required.These can be provided by bundle adjustment with self-calibration approaches. Further on, inclose-range applications the cameras have to be focused, an operation which has an impacton the parameters of the interior orientation. This is why configurations have to be developedwhich are well suited for the calibration at the object, i.e after the focus is set.

The special geometric properties of CCD cameras require expanded approaches for thedetermination of the interior orientation. Besides focal length, the position of the principlepoint and distortion parameters a factor has to be considered, that describes different scalesin the image coordinates.

3.2 Semi-automatic and Automatic Measurement of Points

For matching points in digital images two different approaches are available. The area basedmatching approach uses similarities between grey value distributions in rectangular imagematrices. With sufficient approximate values the image based matching techniques yieldexcellent results. The initial values may be provided by an approximated Digital SurfaceModel (DSM), by an image pyramid or interactively.

Figure 3: The semi-automatic measurement and identification of points with circular targets

The feature based matching technique requires the application of feature extraction algorithmsand also the identification of related features in two or more images. This identification maybe achieved based on the topology or by using the surrounding grey value information. Butboth types of information may have much larger differences in close-range imagery than inthe case of aerial imagery. Figure 3 shows the semi-automatic measurement of circular pointtargets using its shape for feature extraction and its position on the epipolar lines for theidentification process.

3.3 Extraction and Semantic Interpretation of Linear Elements

In close-range photogrammetry most object structures are defined by sharp edges in theimages. The first step for a semantic interpretation is to extract linear features from the imagedata. A great variety of operators is available for for this purpose. Figure 4 shows an imagetaken with the digital CCD camera Sony XC-77CE, and the result of an edge extractionoperation searching for the local gradient maximum. Linear segments shorter than 20 pixelshave been deleted. Additional approaches like the Hough-Transformation for straight linesare available.

Figure 4: Result of edge extraction operations from digital imagery

To define the meaning of this extracted linear elements requires higher level image analysistechniques. They are subject to concentrated research efforts in computer vision. First ex-periments related to architectural objects will be a matter of research at the TU Berlin in thenear future.

3.4 Rectification and Generation of Orthoimages

The non-parametric rectification of digital imagery using the projective transformation is oneof the mostly used techniques in digital photogrammetry. It yields excellent results in case ofplane surfaces. Depending on the surface structure of an object the approach may be modi-fied in so far, as several plane layers can be handled separately and the segments can becombined to full image. This however requires a lot of interactive work.

Figure 5: Input image with restituted data, grey value coded DSM and orthoimage

To create orthoimages of more complicated or irregulr surfaces a parametric rectificationmust be applied using the orientation parameters of the image and a Digital Surface Model(DSM). In many cases occlusions of areas will result in a lack of information in the ortho-

images. Figure 5 shows the input image with an overlay of restituted data, the grey valuecoded DSM and the orthoimage of a part of the Nicolai-Church in Jüterbog, Germany. Thewhite regions in the orthoimge result from occlusions in the input image.

4 . CLOSE-RANGE RESEARCH PROJECTS IN BERLIN

4.1 Evaluation of Electron Microscope Imagery for Microtopography Purposes

The TU Berlin has a long tradition in the photogrammetric evaluation of electron microscopeimagery. Due to the geometrical properties of such images, standard photogrammetrictechniques are not suitable. In the past the Department developed own analogue equipmentfor the restitution of electron microscope stereo imagery. Digital image processing, however,offers new approaches and the flexibility which is necessary for electron microphotogram-metry. Through computer controlled raster electron microscopes image data can be acquireddirectly in digital form for subsequent photogrammetric restitution.

The aim of the photogrammetric restitution is to determine the surface geometry of theobjects. The result of this process is in most cases a digital surface model (DSM), which cane.g. serve for the qualitative assessment of natural or technical materials. Figure 6 shows aphotorealistic visualization of a DSM of the surface of a motor catalyst.

Figure 6: Photorealistic visualization of the surface of a motor catalyst

The quantitative assessment of microelectronics and micro-mechanical structures and com-ponents, for quality control and technical developments, is the aim of a joint research projectin cooperation with the Institute for Physical High Technology (IPHT) in Jena. This researchproject is sponsored by the German Research Foundation (DFG). Photogrammetry isprimarily concerned with the development of software for calibration and restitution, takinginto consideration the particular properties of electron microscope images (approximatelyparallel projection) and of the related objects (poor texture and/or discontinuities of the sur-face).

4.2 Survey of Changes on the Length and Shape of the Human Vertebral Column

For physiological studies it is important to record the dynamic changes of the human verte-bral column due to burden or relief, e.g. during space flights. For this purpose, a system isunder development which allows the digital image acquisition by CCD cameras and the directdigital restitution in a workstation. For data acquisition, vertebrae locations are targetted onthe skin. Changes of the spinal axis in shape and size over time can be measured and docu-mented through the evaluation of image sequences. The entirely digital photogrammetricsystem contents software for the data acquisition with CCD cameras, a tool for interactive,

semi-automatic and automatic measurement of targetted points and bundle adjustment for thecalibration and restitution purposes.

The project is a joint research project with the Institute for Physiology of the Free Universityof Berlin and was sponsored by the German Research Foundation (DFG).

4.3 Documentation of Historical Sites and Monuments

Since more than 100 years the documentation of cultural monuments in photogrammetricimages is a proven technique. Since that time this purpose became more and more practicableimportance. By using modern technology new methods can be developed which are moreflexible and effective than conventional techniques. Furthermore it is possible to feed theresults of photogrammetric restitution work directly into CAD systems for further use.Studies in this context are carried out within an Interdisciplinary Research Project»Documentation of Historical Sites and Monuments« at the Technical University of Berlin.The project is managed by the data processing group in the Department for Architecture. Forthis studies, historical buildings in the state of Brandenburg are selected as test objects.Figure 7 shows a plot of the Nicolai-Church in Jüterbog, derived from restitutions usingRolleimetric MR2.

Figure 7: Plot of the Nicolai-Church in Brandenburg

Within this project effective approaches for photogrammetric data acquisition and restitutionare to be implemented and tested. One of the objectives is, to document and to archivehistorically important objects in such a way, that the actual situation can be reconstructed atany time. For this purpose, different data acquisition methods are tested and compared toeach other. An important aspect is the usefulness of the semi-calibrated camera systemRolleiflex 6006 metric for such tasks. In a similar way photogrammetric restitution is also asubject to comparative studies. The purpose is to find out the limitations of the Rolleisystem, which is in principle very flexible, for the documentation of cultural monuments.

4.4 Reconstruction of the Interior Orientation of Historical Images from the»Meydenbauer Archive« through Bundle Adjustment

As already mentioned in the introduction, the »Royal Prussian Photogrammetric Institution«in Berlin was founded in 1885 as one of the first photogrammetric institutions in the world.This was the result of the endeavours of Albrecht Meydenbauer over more than 20 years[Schwidefsky 1971]. The photogrammetric cameras have been built under the supervision ofMeydenbauer himself. Thousands of objects, mostly in Germany, have been documented insome ten thousands of images. Among them are 186 images of the Hagia Sophia in Istanbul,taken on glass plates with a format of 40 · 40 cm2 and smaller [Staatliche Bildstelle 1926].During and after the World War II not only the cameras but also most of its documentation,its inner orientation and the surveys at the objects have been lost.

Now, with the support from the Berlin government, we are going to calibrate the camerasused about one hundred years ago. For this purpose we select still existing buildings photo-graphed with the old cameras, we survey a new net of control points on it and apply bundleadjustment with self-calibration to determine the orientation parameters of the lost cameras.

4.5 Digital Image Analysis in Architectural Images

The interactive restitution of architectural photogrammetric images is a tedious task. Thehuman operator fulfills different typs of work in this process. He is partly doing interpre-tation, making use of his comprehensive experience in visual perception and also applyingspecific background knowledge. On the other hand he carries out a great deal of routinework, where his capabilities are by far not appropriately mobilized. It must be the aim offuture development, to automate the simple routine work, so that the operator can concentrateon what he does best, i.e. the difficult interpretation. In order to relief the operator from thesimple operations the images have to be analyzed. First interesting points and edges must befound. There are techniques available to achieve this with sufficient accuracy and reliability.But the next step is the automatic interpretation of this extracted elements. Even this is aneasy routine task for the operator, there are no sufficient techniques available today toachieve this by computation.

A research project, sponsored by the German Science Foundation (DFG), will start at theTU Berlin in this autumn. Our approach will be to study the criteria which the human eye-brain system uses for the image interpretation process and also which techniques are alreadyavailable in the computer vision community.

5 . CONCLUSION

Close-range applications of photogrammetry have a long tradition. In the past the evolutionof the techniques was handicapped by the need of very expensive equipment and also thenecessity of long and tedious work of well skilled personnel. The combination of low-costhardware and the digital image processing techniques supporting the operator will makephotogrammetry much more effective, so that new applications can be envisaged.

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