Proceedings International Workshop on Mobile Mapping Technology

li, rongxing

Bibliographic data

Monograph

Persistent identifier:: 856671290

Author:: Li, Rongxing

Title:: Proceedings International Workshop on Mobile Mapping Technology

Sub title:: April 21 - 23, 1999, Bangkok, Thailand

Scope:: 1 Online-Ressource (Getr. Zählung [ca. 400 Seiten])

Year of publication:: 1999

Place of publication:: London

Publisher of the original:: RICS Books

Identifier (digital):: 856671290

Illustration:: Illustrationen, Diagramme, Karten

Language:: English

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2016

Document type:: Monograph

Collection:: Earth sciences

Chapter

Title:: [Session 4: Sensor Integration and Calibration]

Document type:: Monograph

Structure type:: Chapter

Chapter

Title:: The Calibration of Imaging Sensors Integrated into a Rapid Route Mapping System. C. S. Fraser, A. M. Judd.

Document type:: Monograph

Structure type:: Chapter

4-1-6 • the interior orientation and distortion parameters for each camera, • the relative orientation between the cameras, and • the EO of the dual-camera unit with respect to the GPS/inertial sensors which define aircraft position and orientation. It is well known that in order to recover camera interior and exterior orientation via self-calibration, in a situation where the object point field is not well distributed in three dimensions, two fundamental requirements must be met. The first is provision of a convergent imaging configuration, whereas the second involves inclusion of orthogonal camera roll angles in the photogram- metric network design. With the airborne RCAMS, convergent imaging geometry can be attained quite simply by flying over a target array from different directions since the Pulnix CCD cameras are both mildly convergent with respect to one another and have a downward pointing angle of 35° from the horizontal. At the low-level operating flying height of 50m the ground coverage of each image was approximately 40m x 35m. A practical self-calibration scenario could therefore be formulated using a calibration range of only about 50m x 50m on the ground. By flying over this range from the four cardinal directions, as well as from the NW, NE, SE and SW, the convergent imaging geometry illustrated in Figure 3 could be realized. Given that the cameras are mounted on the wings, the prospect of providing a diversity of camera roll angles presented some practical difficulties. These were overcome by first mounting the cameras upside down and recording a set of images, after which the cameras were re-mounted in their correct, upright orientation. The final calibrated EO related to the second camera configuration. Although the provision of a 180° kappa-angle diversity did facilitate an adequate recovery of camera interior orientation parameters, the absence of a 90° roll angle hindered the recovery of any differential scaling (affinity) introduced into the imagery by the video image acquisition system. Nevertheless, initial analysis of the calibration data suggested that a sufficiently robust recovery of camera system parameters and EO could be achieved with the adopted imaging arrangement. A considerable number of individual synchronized image pairs were available for the multi-sensor self-calibration of the airborne RCAMS stereo imaging system as a consequence of the recording of video sequences. The final photogrammetric network adopted comprised 12 image pairs, eight as indicated in Figure 3 (about half of which had a 180° roll angle), and four from the cardinal directions at the higher flying height of 70m. Thus, the final self calibrating bundle adjustment comprised two cameras and 24 images. 3.2 Control Point Array The object array itself comprised a grid of 15 x 15 targets at approximately 3.5m spacing. Under normal circumstances governing camera self-calibration with strong network geometry, ground control information would not be required if the aim were restricted to dual-camera self-calibration and relative orientation. In this instance, however, it was imperative that camera EO be determined in the same coordinate reference system as the GPS/inertial system used to position the aircraft. All ground points were surveyed with a total station to about 30mm relative precision and to about 0.1m absolute accuracy via GPS surveys of a number of the points. 3.3 Image Recording and Mensuration Under the normal operating scenario for airborne RCAMS, time tags corresponding to an aircraft interval of 20m, or about every 15 th frame, are written to the video tape. In subsequent extraction of the desired image pairs from the SVHS video tape, these tagged images are converted to digital form, compressed via JPEG with a compression ratio of approximately 17:1 (i.e. to a file size of around 20 kb) and written to CD-ROM as geo- referenced images. Within the self-calibration, account had to be taken of the possible perturbations to the original video images arising from this analog-to-digital conversion, which also involved an effective resampling of the imagery to a resolution 736H x 560V. To allow for any differential scaling within the analog-to-digital conversion, an affinity term was carried in the self-calibration model (Eq. 1). Also, the principal distance derived within the bundle adsjustment would compensate for any common scale error in assumed pixel array dimensions arising from the image conversion process. An important component of the airborne RCAMS is the DVS (for Digital Video System) which allows an operator to access an image pair from CD-ROM by time, geographic location, or powerline attributes (e.g. pole number). Once the stereo images are displayed side by side on the PC monitor, the spatial position of any point of interest in the scene, be it a power pole or vegetation, can be determined through simple intersection once the interior and exterior orientation of the cameras is known. Figure 4 illustrates the display presented to the operator of the DVS. A zooming facility is incorporated for more precise pointing to image points, as was warranted in the measurement of the target points of the calibration range. Although the camera calibration range comprised 225 targets, the number measured in each of the 24 images averaged 90, with the range being from 40 to 170. Only targets that could be reliably observed to a precision of 1 pixel or better were recorded. As an immediate check on the validity of the image measurement process, spatial resections were carried out using all available control points and the nominal calibration data for the two Pulnix CCD cameras. In spite of possible adverse influences on camera calibration parameters, such as the fact that each camera was housed behind a glass viewing port, all resections yielded RMS image coordinate residuals of between 5.8 pm and 8.6 pm, or just under 1 pixel. This was an encouraging sign for it offered early verification that the data was free of gross errors and that the desired relative positioning accuracy of the stereo video system of 0.15m, corresponding to 1 pixel precision, could be attained.

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