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ON THE INTEGRATED CALIBRATION OF
A DIGITAL STEREO-VISION SYSTEM
Guangping He, Kurt Novak, Wenhao Feng
Department of Geodetic Science and Surveying, Center for Mapping
The Ohio State University
Commission V
ABSTRACT
A fixed base digital stereo-vision system is a powerful tool for positioning objects in
3-dimensions without control in object space. It can be integrated in a vehicle together
with GPS and inertial systems to collect spatial information while driving at highway
speeds.
In this paper we discuss the integrated calibration of the stereo-vision system using a
simultaneous, constrained adjustment of multiple image-pairs. It is based on the well-
known bundle technique which is extended by the following constraints and unknown
parameters: the base between the two cameras is measured externally and fixed in the
adjustment; as the relative orientation does not change it must be the same for all image-
pairs. Furthermore, the interior orientation is left open and additional parameters
compensate for lens distortions. A test-field of control points is photographed with the
digital stereo-vision system. It serves both as control for the bundle solution and as a
reference for an independent evaluation of the accuracy of spatial positioning.
Keywords: Calibration, Close-range, Integrated System, Machine Vision, Stereoscopic.
1. BACKGROUND
The creation of geographic information systems
requires enormous amounts of digital information. To date
most land-related databases still rely on existing line maps
which are manually digitized. In order to collect digital
data faster and more accurately the combination of new
mapping sensors is necessary. Such an integrated system
can produce digital maps on-line on a moving platform;
therefore, we talk about "Real Time Mapping".
At the Center for Mapping of the Ohio State University
a number of mobile mapping systems have been designed,
assembled, and demonstrated. The most successful system
to date is the so-called GPS-Van (Bossler, et. al. 1991). Its
development was initiated by the US Federal Highway
Administration, 38 state transportation agencies, and
private companies. In principal it consists of three
components: an absolute positioning system, a relative
positioning device, and tools for gathering attribute data
(figure 1).
Figure 1: The GPS-Van integrates a digital stereo-vision
with absolute positioning sensors.
The absolute positioning sensor is a combined
GPS/inertial surveying unit. Using differential GPS the
road-alignment can be mapped with an accuracy of 1-3
meters in a global coordinate frame. The inertial system,
which consists of two gyros and a wheel counter, takes over
when satellite-lock is lost. For relative positioning a
stereo-vision system was mounted on the GPS-Van. It
yields 3-dimensional coordinates relative to the van; they
can be transformed into a global system by the absolute
positioning sensors. Finally, an analog video-camera and a
touchscreen are available to collect attributes. All data
captured by the GPS-Van are immediately stored in a
relational data-base that was enhanced by an image
management and analysis system.
The most important pre-requisite for accurate point
positioning with the vision system is the calibration of the
cameras, and the determination of their relationship to all
other sensors. In this paper we describe the mathematical
models used to calibrate the camera geometry, their relative
orientation on top of the GPS-Van, as well as their offsets
from the GPS antenna and the gyros. Using practical test-
data we show how accurately points can be located by the
cameras, and how they are transfered into the global
coordinate frame. As a short introduction the hardware
components of the GPS-Van are discussed. In the
conclusions we point out various modifications of the GPS-
Van, and its potential for revolutionizing digital mapping.
2. HARWARE COMPONENTS
Absolute positioning of the GPS-Van is achieved by
two surveying type GPS-recievers (Trimble 4000ST). One
serves as a base-station at a known location, the other one
is a rover station mounted on the van. When satellite
signals are blocked a dead-reckoning system takes over. It
consists of a directional and a vertical gyro, and a wheel
counter. The directional gyro measures horizontal angular
changes (directions), the vertical gyro determines two
angles (pitch and roll) that measure the van's tilts relative to
the vertical. Additionally, a magnetic proximity sensor
counts wheel-rotations at the disk brakes of the two front