Full text: Proceedings International Workshop on Mobile Mapping Technology

Y.D. Huang and D. Chen 
School of Surveying 
University of East London 
United Kingdom 
Y.Huang@uel.ac.uk D.Chen@uel.ac.uk 
KEY WORDS: Camera Calibration, Zoom Lens, Videotheodolite, Terrestrial Image Based Survey. 
The paper presents the technical detail of calibrating a zoom lens CCD imaging unit in a motorized videotheodolite system. The camera- 
on-theodolite calibration method was used. Both the camera interior parameters including lens distortion parameters and the exterior 
parameters with respect to the theodolite were determined at various focal l.ength settings. The accuracy of the calibration was assessed 
objectively. The repeatability of the calibrated parameters against zooming action was evaluated. The paper gives conclusions and 
recommendations for future work. 
A terrestrial image based survey system is being developed for 
3D city modelling at University of East London. The core part of 
the system is a zoom lens CCD camera mounted on the telescope 
.of a motorised total station. Although such a CCD-theodolite 
combined unit has many working modes and applications (Huang 
1992), in the current project the unit is to work mainly in the 
manner that the camera captures images for photogrammetric 
measurement and photo rendering with the theodolite providing 
precise orientation parameter values for all the images (Huang 
1997). The main reason for choosing a zoom lens is to enable the 
unit to achieve relatively uniform object pixel size for objects at 
varying distances and to consequently gain more efficiency and 
flexibility for the system. 
One of the fundamental tasks for the project is the calibration for 
the interior camera parameters and the camera-to-telescope 
parameters at various focal settings within the range of zoom. 
Another is assessing the repeatability of those parameters against 
zooming action and time. 
Although zoom lenses have many advantages, previous research 
in photogrammetric community has not offered extensive 
experience in calibrating these lenses and the existing 
applications of zoom lens is in fact rather limited in 
photogrammetry. Nevertheless, Wong and Wiley reported very 
encouraging experiment results on their chosen cameras and 
lenses (Wong & Wiley, 1990, 1995). That has proved the 
existence of quite calibratable zoom lenses for photogrammetric 
purposes. As photogrammetry meets machine vision and the use 
of CCD/video cameras increases for metrology, the desire for 
exploiting the on-site flexibility zoom lenses offer is increasing 
and more experience in using them for photogrammetry is quite 
This paper reports on the calibration of an off-the-shelf zoom lens 
CCD camera. The terrestrial image based survey system in which 
the camera is used is briefly described. The relatively new 
camera-on-theodolite calibration method (Huang and Harley 
1989) used for this calibration is explained. The accuracy for 
image location and the camera warm-up effect are assessed. The 
calibration results and repeatability test results are presented. 
Conclusions are drown and future work recommended. 
The videotheodolite is as shown in Fig. 1. During a survey, this 
videotheodolite is set at a number of stations. At each station, the 
theodolite rotates at angular steps for the camera to capture 
images to cover the panorama. The theodolite angular readings 
are recorded on capture of each image for calculating the 
orientation parameters of the image. The zoom is adjusted 
according to the average object distance within each frame so that 
the average image scale is uniform among all the frames of 
The zoom CCD camera used is the Sony EVI-371, which is 
designed for use in camcorders. It is a self-contained block which 
encloses the CCD chip, the circuitiy and an internal zoom lens all 
built in as shown in Fig.l. The CCD chip is a 1/3 inch interline 
transfer chip with 752 x 582 imaging cells and the cell size is 
about 6.50(H) x6.25(V) microns. The lens can zoom from 5.4 to 
64.2 mm focal length in 12 electronic settings. Focusing is 
switchable between auto and manual modes. For this calibration, 
the focus was fixed at a certain distance. The focus tracking 
feature enables image to remain in focus during zooming. The

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