ng 
nd 
er, 
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