Photogrammetric image acquisition and image analysis of oblique imagery

PHOTOGRAMMETRIC IMAGE ACQUISITION AND IMAGEANALYSIS OF OBLIQUE IMAGERY

Gorres J. Grenzdorffer ([email protected])

Rostock University, Germany

Markus Guretzki ([email protected])

Phoenics GmbH, Hanover, Germany

Ilan Friedlander ([email protected])

Ofek MultiVision International Office, Netanya, Israel

(Extended version of a paper submitted to the ISPRS Hannover Workshopon ‘‘High-resolution earth imaging for geospatial information’’ held atLeibniz Universitat Hannover, Germany, 29th May to 1st June 2007)

Abstract

Because of the intuitive human perception of the oblique view, photogrammetrists’attention has recently returned to oblique images. The demand for oblique imageryhas been notably pushed by digital representations of the globe such as Microsoft’sVirtual Earth and by the development of special GIS viewers including thoseby Pictometry and MultiVision. In this contribution the broad spectrum ofapplications of oblique images is presented. Most commonly oblique images areacquired with flexible digital airborne camera systems, which allow for easycollection of such imagery with photogrammetric quality. The digital airborneremote sensing system PFIFF developed at Rostock University will be introducedbriefly. However, flight planning for oblique imagery differs from that for verticalaerial surveys. Two test flights with mono and stereo oblique images are presented.The data processing, display and measurement within oblique images fromdifferent perspectives requires new software such as MultiVision. Oblique imagesmay also be used to texture 3D city models.

Keywords: digital airborne imaging systems, digital photogrammetry, obliqueimages, 3D city models

Introduction

Oblique images have historically been used for visualisation and interpretation purposes,rather than for metric applications. Exceptions are the military sector and archaeology whereoblique images have long been standard for reconnaissance purposes (Welzer, 1985; Smith,1989). However, until recently oblique images were generally outside of the focus ofphotogrammetrists. They can thus be truly regarded as a new data source for photogrammetryand GIS.

The Photogrammetric Record 23(124): 372–386 (December 2008)

� 2008 The Authors. Journal Compilation � 2008 The Remote Sensing and Photogrammetry Society and Blackwell Publishing Ltd.

Blackwell Publishing Ltd. 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street Malden, MA 02148, USA.

The use of standard vertical ortho-images as a topographic background in a GIS isnowadays very common, thus generating a strong demand for current photogrammetricairborne and high-resolution satellite data. Planners, administrative users and the general publicuse the available ortho-images, for example, in Google Earth and other similar services, mainlyfor orientation and visual inspection of selected features. However, vertical ortho-images maynot be interpreted easily by everyone. Because of the intuitive nature of oblique images, whichappear similar to the common human perspective, these images are very attractive to decisionmakers, as well as to the general public. In Europe BLOM ASA (Norway) intends to acquireoblique images over the next 2 years for every urban centre in Europe with a population of50 000 inhabitants or more (BLOM, 2007; Simmons and Karbo, 2007). To exploit theinformation from the oblique perspective fully, a minimum of four images from all sides haveto be acquired and managed. Because of the variable-scale geometry of oblique images,standard GIS packages do not support them. New viewers, software packages or add-ons tostandard GIS have therefore to be developed to guide users and provide them with thenecessary functionality.

Oblique images are an indispensable tool for many different uses, which will be describedin more detail later. For tax assessment and building planning control oblique images may helpwith accurate measurement of areas and building facades and understanding of the capitalassets. The effective identification, measurement and documentation of discrepancies mayresult in an increase of the tax revenues for built estates. In urban and infrastructural planning,oblique images allow comparative measurements of buildings and structures. In landscapearchitecture and urban planning they can be used to capture and evaluate real estate. Fortelecommunication planning they can be used for line-of-sight calculations and for thedevelopment of mast networks. The management of military and security operations is one ofthe key applications of oblique images. The benefits include the immediate availability ofinformation about critical locations and accurate visualisation of these locations plus theidentification of surrounding areas and infrastructure. Additionally, the measurement ofaccesses and openings helps in the planning of access and exit routes. Oblique images are alsohelpful for the protection of critical infrastructure such as airports, ports, railway stations,shopping centres, power utilities and much more. In the field of cadastral data capture andmanagement, oblique images assist in accurate surveys and in the organisation of cadastralactivities, especially in rural areas. Oblique images also meet the increasing requirements of thedevelopment of 3D cadastral projects. Oblique images can also be used to texture 3D buildingsand 3D city models. These textured buildings can then be used as realistic objects in 3Dvisualisation projects.

The Microsoft Virtual Earth viewer (Microsoft, 2007) provides high-resolution obliqueimages for many cities around the world, but only from one viewing perspective at a time.Viewer solutions from Pictometry International Corporation (USA) (Pictometry, 2007) andOfek MultiVision (Israel) (MultiVision, 2007) allow for several perspectives of one objectsimultaneously. These specialised viewers provide additional features, such as the measure-ment of distances and the integration of supplementary GIS data. In the last part of the paperthe functionality and the workflow of the MultiVision software will be presented in greaterdetail.

The PFIFF system, which is an airborne system using a single medium-formatdigital camera, was developed at Rostock University and will be described in detail,concentrating on a demonstration of the photogrammetric potential of PFIFF. The focus of thispaper, which is an extended version of Grenzdorffer et al. (2007), will be the exploration of thepossibilities of oblique stereoscopic images, for example, for visualisation or automatedbuilding texturing, as well as for the analysis of the condition of roadside trees.

The Photogrammetric Record

� 2008 The Authors. Journal Compilation � 2008 The Remote Sensing and Photogrammetry Society and Blackwell Publishing Ltd. 373

PFIFF

PFIFF, a digital airborne remote sensing system, was developed at Rostock University bythe corresponding author to fulfil the special requirements of precision farming (Grenzdorffer,2005). The great advantage of using PFIFF compared to standard photogrammetric framecameras is the possibility of taking oblique images. The core of the system since 2005 is adigital SLR colour camera, the Rollei AIC 45 (Aerial Industrial Camera). The H25 CCD sensorfrom Phase One has a net resolution of 5436 · 4080 pixels (22 Megapixel). Table I describesthe technical details of the camera.

With the exposure interval of less than 4 s under airborne conditions, the Rollei AIC 45camera enables photogrammetric aerial surveys (60% forward overlap) with a ground resolutionof >10 cm, assuming a ground speed of 40 m s)1, achievable with a Cessna 172. The digitalback from Phase One works together with a Rollei AIC camera body and a Schneider SuperAngulon 2Æ8/50 mm lens with a minimum exposure time of 1/1000 s. The digital camera iscontrolled by a barebone PC with a storage capacity of 400 GB which stores all the image data(up to 6000 images) and is connected to the camera via an IEEE 1394 (FireWire) connection.

The camera may acquire images either in RGB or in colour infrared (CIR). To achievethis, the IR-cut filter on the top of the CCD sensor was removed. The different colourinformation of the CCD sensor is gathered via a Bayer pattern. For a CIR image a band filter ismounted on top of the lens, which filters out the blue light (<520 nm). The green, red andinfrared light up to a wavelength of 1050 nm passes the filter onto the sensor. As a result thegreen light and red light sensitive CCD elements also gather infrared light. These infrared lightcomponents have to be separated in later processing. However, the infrared light sensitivity ofthe sensor is much stronger than that for visible light. For a similarly exposed image in theRGB mode and the CIR mode, the amount of the incoming light has to be lowered by 3 f-stopsfor the RGB image.

Other important components of PFIFF are the GPS-based flight management system and anavigation unit that automatically triggers the image acquisition during a flight strip accordingto the pre-defined overlap. During the strip the optimal image exposure interval is continuouslycomputed and the camera is triggered synchronously to the PPS signal of the GPS clock toensure a perfect synchronisation with the external high accuracy L1/L2 GPS receiver. Thenavigation unit records the exposure delay of the camera. With this approach a constantforward overlap is ensured under all conditions with the best flexibility during an aerial survey.This approach is quite different from the common approach of a photogrammetric aerial surveywhere the image centres are predefined in the flight planning and subsequently flown during

Table I. Technical parameters of the digital Rollei AIC 45 camera.

Rollei AIC 45

Camera type Rollei AIC 45 withfixed digital back

Resolution (pixel) 5436 · 4080Pixel size 9 lm · 9 lmSensor size (mm) 48Æ96 · 36Æ72Colour depth per channel 16 bitColour mode RGB or CIRMinimum exposure interval (airborne) c.4 sWeight (including lens) c.1500 gConnection to computer FireWire, barebone PCSoftware Phase One 3Æ1

Grenzdorffer et al. Photogrammetric image acquisition and image analysis of oblique imagery

374 � 2008 The Authors. Journal Compilation � 2008 The Remote Sensing and Photogrammetry Society and Blackwell Publishing Ltd.

the aerial survey. For a photo flight the system is temporarily installed in a Cessna 172 with asmall hole of about 12 cm in diameter in the fuselage. See Fig. 1 for the system design.

For the use of a digital camera in aerial surveys it is not only the size of the CCD sensorwhich is important. Many other digital camera parameters such as minimum exposure interval,external storage capacity, preview options, mechanical stability of the sensor (interiororientation), temporal eccentricity (exposure delay) and also its radiometric properties, as wellas its reliability and the need for a continuous power source must be considered anddetermined. To achieve this, the system has undergone several geometric and radiometriccalibration procedures. For photogrammetric work the interior orientation of the camera wasdetermined. With the fixed digital back of the AIC 45 an in-flight calibration is not necessaryfor most applications. However, for the highest accuracies a self-calibration and the use ofadditional parameters are essential. The examination of the linearity, the spectral characteristicsof the RGB band filters and the high signal-to-noise ratio revealed that the radiometricproperties of the digital camera are far superior to an equivalent photographic film system.

In 2006 a general overhaul of the PFIFF flight navigation system and the flightmanagement system with fully automatic image triggering was undertaken. The CartaLinxsoftware from Clark Labs (USA) was formerly the basis for the flight navigation, in which theflight lines and the current position of the aircraft were displayed to the pilot on a laptopcomputer. The drawback of this solution was that the map on the display was always orientedto north meaning that the pilot had to rethink left or right on every manoeuvre. Additionally thepilot lacked important navigation information, such as current course versus planned courseand critical ground speed. For these reasons, new software was developed with dedicatedfeatures for the pilot. The software runs on a common PDA with GPS. The most importantfeatures of the software are automatic rotation of the graphics in the flight direction, automaticzoom functions within and outside the survey area, a graphical display of the current exposureinterval of the camera and the most important aerial survey navigation information. Thesoftware development was realised in VBA with GpsTools version 2Æ3Æ1 from FransonTechnology AB (Sweden).

Digital camera(Rollei AIC45)

Passive stabilisedcamera mount

Barebone PC400 GB Datastorage

Flight management(PDA with GPS)

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ema

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PPS

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Leica 1200L1/L2 GPS

Garmin 18 LVC L1 GPS

Navigation Image Acquisition

Fig. 1. Low-cost remote sensing system PFIFF 2006.



During post-processing, the recorded GPS positions (at 1 Hz) have to be interpolated tothe precise moment of the image acquisition. Due to the long exposure delay of 302 ms(±0Æ1 ms) of the AIC 45 camera, linear interpolation of the GPS position may be associatedwith significant errors due to high frequency nonlinear aircraft movements. A comparison of266 perspective centres, determined by linearly interpolated 1 Hz GPS positions and 200 HzGPS/INS measurement illustrates this well, with the most significant deviations occurring inthe main flight direction (Table II).

With the re-engineering of the GPS-based flight management system, the exposure delayof the camera is now considered during a GPS-synchronous image triggering (see Fig. 2). ThePPS (pulse per second) of the GPS signal serves as a master time signal and the exposure of theimage coincides precisely (±0Æ8 ms) with the recorded GPS position, thus reducing GPSinterpolation errors.

In order to fully automatically trigger images within the survey area, the camera triggeringis now fully integrated with the flight navigation system. The flight management softwareautomatically starts triggering the camera whenever the pilot manoeuvres the aircraft inside thesurvey area. The automatic image triggering is stopped once the aircraft leaves the survey area.Because the MS Windows XP operating system does not support real-time applications, anadditional timer card (NI-6601) from National Instruments was integrated into the computer.The programming of the timer card and the PPS-synchronous triggering was realised withLabVIEW 8Æ0.

Oblique Images

The acquisition of oblique images requires several changes in the usual workflow fornadir (vertical) images, from survey planning to image processing and image analysis.

Δ t 1

PPS-Signal

GPS-triggered image

Exposure Exposure

Triggerimpulse= GPSsec–Exposuredelay Δ t Trigger impulse = GPSsec – Exposure delay Δt

Δ t 1

Fig. 2. PPS-synchronous image acquisition considering the exposure delay time of the camera.

Table II. Deviations (m) of the perspective centres, determined by linear inter-polation of 1 Hz GPS recordings and 200 Hz GPS/INS measurements (n = 266).

X (m) Y1 (m) Z (m)

Average 0Æ004 0Æ009 0Æ015Standard deviation 0Æ077 0Æ104 0Æ051Maximum 0Æ243 0Æ581 0Æ140Minimum )0Æ175 )0Æ546 )0Æ1311East–west, main flight direction of the image strips.



Flight Planning

In the flight planning for oblique aerial survey, several issues have to be considered. Theimage scale is not constant throughout the image. The ground sampling distance (GSD) issmaller in the foreground than in the background of the image (Fig. 3). In the flight planningthe altitude above ground and the viewing angle ay across the flight direction have to bedefined. The viewing angle of the lens by defines the minimum Dmin and the maximum distanceDmax from the aircraft to the area imaged, as well as the image scale for analogue images or theGSD with digital images. The minimum mmin, average mavg and maximum mmax image scalesare calculated by the following equations:

mmin ¼hg cos by

f cosðay � byÞ

mavg ¼hg

f cos ay

mmax ¼hg cos by

f cosðay þ byÞ:

ð1Þ

The distance of the image foreground and the image background from the aircraft is basedon the following equations:

αy

βy

βy hg

12

Davg

Dmin

Dmax

αy

βy

βy hg

12

Davg

Dmin

Dmax

Fig. 3. Geometry of oblique images.



Dmin ¼ hg tanðay � byÞDavg ¼ Dmax � Dmin

Dmax ¼ hg tanðay þ byÞ:ð2Þ

Example Oblique Images for Roadside Trees

On 6th September 2003, a flight with nadir and oblique images of a 4-km section of a roadwith trees on both sides was conducted with a ground resolution of approximately 12 cm(Grenzdorffer, 2004). The purpose of the flight was to investigate the possibilities of obtaininginformation on the condition of the roadside trees, as well as the detail of the road and itssurroundings from nadir images as well as from oblique images; see Fig. 4 for the flight patternfor the oblique images.

Prior to the acquisition of oblique images a strip of nadir images was acquired. For theoblique images the camera was turned around 90 degrees and held out of the window of theaircraft manually. On small aircraft such as a Cessna 172, the wheels of the aircraft remainoutside during the flight, meaning that oblique images out of the window could not be taken atthe anticipated 45� angle. Instead, the viewing angle (lateral tilt, x) was approximately 60�; seeFig. 5 for an example. To obtain oblique stereoscopic images with a forward overlap of 60%the automatic trigger control of the flight management system had to be reset accordingly.

At first the nadir-looking image strip was processed by means of standard aerotriangu-lation. The georeferencing of the oblique images was far more complicated, because tie pointsin neighbouring images could not be found automatically in the first instance. This is related tothe fact that the starting values of x and / of the hand-held images were unknown. After a

StreetStreet

Fig. 4. Flight pattern for oblique images of linear features such as roads, streets and pipelines.



manual definition of a minimum number of tie points with a selected number of obliqueimages, a preliminary triangulation was conducted to obtain approximate values for the anglesx and /, after which the automatic tie point generation algorithm worked correctly. Due to theapparent differences in scale within the oblique images, ranging from 12 cm GSD in the imageforeground to 50 cm in the image background, the precise determination of the ground controlpoints (GCPs) was difficult. Nevertheless the results of the aerotriangulation of the obliqueimages were within 60 cm rms at the GCPs. The most interesting aspect of the interpretation ofthe trees and other features along the road is the stereoscopic view of the oblique images,because an orthorectified image does not yield the full information for the determination of thevitality of a tree.

Oblique Stereo-images

With direct georeferencing through GPS/INS, the problems of manual tie point selectionand cumbersome image rectification are overcome and the image data may be used directly fortexturing of 3D city models or other interesting purposes. However, direct georeferencingrequires the determination of the boresight angles. The boresight angles describe smalldifferences between the axes of the camera systems and the axes of the inertial measurementunit. The boresight angles are generally determined through a comparison of indirectlydetermined values of the exterior orientation by means of aerotriangulation with the directgeoreferencing values. With stripwise acquired oblique images with one perspective angle, anaerotriangulation will not yield good results for the exterior orientation. Furthermore, commonsoftware tools for the boresight determination will not work with oblique images. Therefore,two options exist:

Fig. 5. Example of nadir-looking (centre image) and oblique images of a road (left and right images).



(a) determine the boresight angles with a set of nadir images, taking into account minorerrors such as those due to differences in the lever arm and re-initialisation (Honk-avaara, 2004; Grenzdorffer, 2005); or

(b) perform a special circular flight pattern with the camera looking toward the centre.With the strong resulting overlap a bundle adjustment enables the determination of theexterior orientation (Labe and Forstner, 2006).

For the first test flight to acquire oblique stereo-images with GPS/INS, a Litton LN 200IMU from the Applanix 410 GPS/INS belonging to the German Aerospace Center (DLR)Institute for Navigation was rigidly fixed on the camera. The airborne L1/L2 antenna (SensorSystems S67-1575-76) was set on top of the aircraft, approximately above the camera. Thelever arms between the camera, the IMU and the GPS antenna were determined with a totalstation. For the determination of the boresight angles and to obtain information on the overallaccuracy, a small testfield with 12 signalised targets was established. The boresight angles weredetermined with the module ‘‘CalQC’’ of the Applanix POSPac software package. Theaccuracy of the boresight angles was 0Æ004� in x, 0Æ005� in / and 0Æ010� in j. See Fig. 6 forthe footprints of a strip of oblique images, generated at a test flight in Rostock (Grenzdorffer,2005). Due to the large overlap, the images may also be viewed and analysed in stereo (Fig. 7).

Thanks to the highly accurate georeferencing, y parallax is nearly absent (displayed in thelower right part of the screen shot) allowing perfect stereoscopic measurements in the images.

Rostock Test Flight for Visualisation and Semi-automatic Building Texturing

The first test flight with oblique images for visualisation and semi-automatic buildingtexturing was undertaken on 23rd November 2006 over the central area of the city of Rostock.The complex flight pattern is shown in Fig. 8. The average flying altitude was 400 m aboveground. The resulting ground resolution in the image centre is approximately 15 cm. A total of78 images with an overlap of approximately 60% were acquired during the flight.

Due to the low sun angle at the time of flight in late November the image quality in thedifferent view directions is quite variable. The radiometric post-processing included colourbalancing, with individual parameters for each flight direction and a 16 to 8 bit conversion. Asecond test flight with a larger extent and under more favourable illumination conditions was

Fig. 6. Footprint of strip of oblique images.



undertaken on 23rd April 2007. The data analysis of the test flights was performed using theMultiVision software (MultiVision, 2007).

MultiVision

Within MultiVision the determination of the exterior orientation of the oblique imagesmay be done by means of control points, in a similar way to the method of orientation used for

Fig. 7. Oblique stereo anaglyph image through direct georeferencing.

Metres

Fig. 8. Flight pattern for Rostock test flight of 23rd November 2006.



vertical aerial photographs undergoing rectification and orthophoto generation. The programdisplays an orthophoto and an oblique photo at the same time, so that control points can besampled on both of them simultaneously. The solution program, MV-SPECIAL ONE, isdesignated to calculate these parameters efficiently, without requiring any further flight data.The solution is based on the collinearity equations. Control points can be taken fromphotogrammetric maps, or from any other accurate source. The accuracy of the control pointswill determine the accuracy of the orientation solution. Orientation of analogue aerial photosrequires interior orientation, such as the input of fiducial marks. The determination of theexterior orientation of oblique aerial photos can also be carried out using a GPS/INS system.

The average absolute positional accuracy of the oblique images is related to severalfactors such as the available GCPs from the orthophoto, the accuracy of the underlying DEMand the accuracy of the interior orientation of the selected camera. For the Rostock project,ground control points for the oblique images were derived from the underlying orthophoto witha GSD of 50 cm. The average of the absolute positional accuracy of the images, determined byvisual inspection, is generally between 2 and 3 m, while some images are significantly worse.If the flight is conducted with a GPS/INS, the absolute positional accuracy of the obliqueimages may be below 1 m (Grenzdorffer, 2005; Simmons and Karbo, 2007). The relativeaccuracy necessary for the measurement of the height or the width of buildings is only partiallyrelated to the absolute accuracy and is generally much better.

The program operates in a DTM environment, thus enabling various calculations forlocating the user’s relevant oblique photos. MultiVision operates in conjunction with a GIS (forexample, ArcView), thus four oblique aerial photos for the specific point coordinates willappear on screen (Fig. 9).

Fig. 9. MultiVision main screen with images from the first Rostock test flight.



Texturing 3D Buildings with Aerial Imagery

Texturing buildings with the aid of aerial imagery may be done in many different ways.Wide-angle vertical images provide an oblique view at their edges, which may be used forautomated texture extraction (Zebedin et al., 2007). Alternatively, modern 3-line scanners suchas the HRSC-A or ADS 40 provide oblique views with their forward- and backward-lookingchannels, enabling an automated generation of 3D textured facades (Hirschmuller et al., 2005).

3D Building Data for Rostock

A simple 3D building model (LOD 1) of the city of Rostock is available on the basis ofofficial cadastral information (ALK) and additional elevation data. Due to the data structure ofthe ALK, building objects may represent more than one building in reality. Therefore,buildings were split up based on a true orthophoto. Additionally, several landmarks such aschurches were modelled individually. The use of such a simple 3D block model is limited, forexample, to the determination of road noise absorption of buildings according to new EUregulations (EU Directive 2002/49/EC on Environmental Noise). For city planning and otherpurposes, however, a 3D city model should have a number of different types of roofs andtextured facades. The common texturing of facades with terrestrial photographs is quitecumbersome. An aerial survey with a high degree of automated texturing is a significantlymore elegant approach.

The texturing of single buildings within the MultiVision software is done on a per-building approach whereby CAD buildings are incorporated into the software and the facadesare clipped automatically from the different views (see Fig. 10). A fully automated approach is

Fig. 10. Semi-automatic generation of building textures.



not feasible because of the geometric uncertainties of up to 1 to 3 m related to the 3D buildinggeometry provided and in the georeferenced oblique images.

Because of the lack of standardised solutions for data transfer between different 3Dsoftware packages, 3D data conversion is always a bottleneck in the generation and texturingof 3D city models. Independent standards such as the CityGML standard within OGC arecurrently under development (CityGML, 2007). Besides CityGML, which defines the classesand relations for the most relevant topographic objects in cities and regional models withrespect to their geometrical, topological, semantic and appearance properties, Google has set asimple industry standard for visualisation with the KML and KMZ formats (Fig. 11).

Outlook and Future Work

The automation of the relative and absolute orientation of oblique images is still an issueof intensive research. Particularly the differences in scale and overlap have to be considered.Labe and Forstner (2006) demonstrated promising results, also with PFIFF airborne obliqueimages. The use of a single camera provides the most flexibility but requires a complex andtime consuming flight pattern for the acquisition of oblique imagery. For the professionalacquisition of oblique images companies such as BLOM and Ofek developed multiple camerahead solutions with five cameras (Petrie and Walker, 2007). In such systems, one camera headprovides a nadir view and the other four cameras provide the fixed oblique views in differentdirections. Obstruction of neighbouring buildings in inner cities and other densely populatedplaces is common with a fixed oblique view of, for example, 45�. A possible solution is avariable multi-head camera system, enabling different view angles according to the averagewidth of the streets and the average height of the buildings. Within direct georeferencing theboresight alignment of such a multi-head camera system with overlapping images requiresspecial treatment (Kurz et al., 2007).

Fig. 11. 3D city model of Rostock within Google Earth.



references

BLOM, 2007. Homepage of BLOM Pictometry. http://www.blompictometry.com/ [Accessed: 5th October 2007].CityGML, 2007. CityGML Standard. http://www.citygml.org/ [Accessed: 3rd April 2007].Grenzdorffer, G. J., 2004. Digital low-cost remote sensing with PFIFF, the integrated digital remote sensing

system. International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences, 35(B1):235–239.

Grenzdorffer, G. J. 2005. Flexible high resolution urban remote sensing with PFIFF – a digital low cost system.International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences, 36(8/W27). 6pages (on CD-ROM).

Grenzdorffer, G. J., Guretzki, M. and Friedlander, I., 2007. Photogrammetric image acquisition and imageanalysis of oblique imagery – a new challenge for the digital airborne system PFIFF. International Archives ofPhotogrammetry, Remote Sensing and Spatial Information Sciences, 36(1/W51). 6 pages (on CD-ROM).http://www.commission1.isprs.org/hannover07/paper/grenzdoerffer_guretzki_friedlander.pdf [Accessed: 11thAugust 2008].

Hirschmuller, H., Scholten, F. and Hirzinger, G. 2005. Stereo vision based reconstruction of huge urbanareas from an airborne pushbroom camera (HRSC). Lecture Notes in Computer Science: Pattern Recognition.Springer, Berlin and Heidelberg. 3663: 58–66.

Honkavaara, E., 2004. In-flight calibration for direct georeferencing. International Archives of Photogrammetry,Remote Sensing and Spatial Information Sciences, 35(B1): 166–171.

Kurz, F., Muller, R., Stephani, M., Reinartz, P. and Schroeder, M., 2007. Calibration of a wide-angledigital camera system for near real time scenarios. International Archives of Photogrammetry, Remote Sensingand Spatial Information Sciences, 36(1/W51). 6 pages (on CD-ROM). http://www.commission1.isprs.org/hannover07/paper/Kurz_mueller_etal.pdf [Accessed: 11th August 2008].

Labe, T. and Forstner, W., 2006. Automatic relative orientation of images. Proceedings of the 5th Turkish–German Joint Geodetic Days, Berlin, Germany. 6 pages.

Microsoft, 2007. Virtual Earth Viewer. http://www.microsoft.com/virtualearth/ [Accessed: 5th October 2007].MultiVision, 2007. Homepage of MultiVision Inc. http://www.ofek-international.com/ [Accessed: 4th April

2007].Petrie, G. and Walker, A. S., 2007. Airborne digital imaging technology: a new overview. Photogrammetric

Record, 22(119): 203–225.Pictometry, 2007. Homepage of Pictometry Inc. http://www.pictometry.com/ [Accessed: 8th October 2007].Simmons, G. and Karbo, N., 2007. Aerial imagery from different angles. Professional Surveyor, 27(5):

18–23.Smith, M. J. 1989. A photogrammetric system for archaeological mapping using oblique non-metric photography.

Photogrammetric Record, 13(73): 95–105.Welzer, W. 1985. Luftbilder im Militarwesen. Militarverlag der Deutschen Demokratischen Republik, Berlin. 232

pages.Zebedin, L., Klaus, A., Gruber, B. and Karner, K., 2007. Facade reconstruction from aerial images by multi-

view plane sweeping. Photogrammetrie Fernerkundung Geoinformation, 1/2007: 17–24.

Resume

Les humains percevant naturellement les objets d’un point de vue oblique, lesphotogrammetres se sont recemment de nouveau interesse aux images obliques. Enoutre, les nouvelles cameras numeriques aeroportees flexibles permettent l’acquisi-tion aisee de telles images avec une qualite photogrammetrique. Dans cet article sontpresentes les applications possibles des images obliques, le systeme numerique deteledetection aeroporte PFIFF et deux vols d’essai avec des images obliques en monoet stereoscopie. Le traitement des donnees, leur affichage et la mesure dans lesimages obliques a partir de differentes perspectives necessitent de nouveaux logicielstel que MultiVision. Les images obliques peuvent par ailleurs etre utilisees pourtexturer des modeles 3D urbains.



Zusammenfassung

Aufgrund ihrer intuitiven Wahrnehmung erfreuen sich Schragaufnahmen immergroßerer Beliebtheit, nicht nur bei den Nutzern, sondern auch bei Photogrammetern.Insbesondere durch digitale Globen, wie z.B. Virtual Earth und der Entwicklungspezieller Viewer durch z.B. Multivision, Pictometry und anderen werden Schrag-aufnahmen schon heute fur vielfaltige Zwecke eingesetzt. In dem Beitrag wird dasbreite Anwendungsspektrum von Schragaufnahmen aufgezeigt. Die Bilderfassungvon Schragaufnahmen wird in der Regel mit Hilfe flexibler digitaler Mittelformat-kameras realisiert, die eine einfache Aufnahme der Bilder mit hoher photogrammet-rischer Qualitat moglichen. Das digitale System PFIFF, eine Entwicklung des Autorswird kurz vorgestellt. Die Besonderheiten bei der Aufnahme und Verarbeitung von(Stereo-) Schragaufnahmen wird anhand von zwei Beispielen intensiv diskutiert.Dabei wird auch auf das Boresite-Alignment bei der direkten Georeferenzierung vonSchragaufnahmen eingegangen. Anhand eines Testflugs wird die Datenverarbeitungfur Multivision, einer Software zur Georeferenzierung, Visualisierung und dersemiautomatischen Ableitung von Texturen fur digitale 3D-Stadtmodelle vorgestellt.

Resume

Debido a la familiaridad con que la persona percibe las vistas oblicuas, en losultimos anos los fotogrametristas han vuelto a prestar atencion a las imagenesoblicuas. Ademas los nuevos y flexibles sistemas de camaras digitales aereas facilitanla obtencion de imagenes con calidad fotogrametrica. En este artıculo se presenta elpotencial de las imagenes oblicuas para diversas aplicaciones, el sistema deteledeteccion digital aerotransportado PFIFF, y dos vuelos de ensayo con imagenesoblicuas monoscopicas y estereoscopicas. El procesamiento de datos, la visualiza-cion y la medida en imagenes oblicuas tomadas desde distintas perspectivas requierede un nuevo programa como es Multivision. Las imagenes oblicuas tambien sepueden utilizar para texturizar modelos tridimensionales urbanos.



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Photogrammetric image acquisition and image analysis of oblique imagery