B2. Istanbul 2004 
PHOTOBUS: TOWARDS REAL-TIME MOBILE MAPPING 
H. Gontran, J. Skaloud, P.-Y. Gillieron 
Swiss Federal Institute of Technology (EPFL), Geodetic Engineering Laboratory, 1015 Lausanne, Switzerland 
(herve.gontran, jan.skaloud, pierre-yves.gillieron)@epfl.ch 
KEY WORDS: Real-time, Mapping, Mobile, Camera, Automation, CCD, Comparison, Advancement 
ABSTRACT: 
The Geodetic Engineering Laboratory at EPFL has designed a mobile mapping system to determine the geometry of the road: the 
Photobus. Currently, the navigation and imagery data are semi-automatically processed with significant input provided by the 
operator. This paper describes major steps that move the Photobus towards a real-time mapping system. The first step involves a 
crucial enhancement in the positioning component, with the trans 
port of RTCM corrections via the Internet. Such a technique allows 
the access to a nation-wide service to obtain reliable RTK solutions that are sufficient to describe the trajectory of the vehicle under 
good satellite visibility. The second step concerns a change in the imaging component to help the automation of the detection of the 
road centreline. Most algorithms of contour detection of the road centreline are deceived by shadows, since low-level filtration 
techniques may reject under-exposed pixels of shadowed areas. We are exploring means of reducing this problem directly at the 
acquisition level by adopting a logarithmic CMOS camera as the imaging module of the Photobus. This cost-effective technology 
rivals with CCD sensors and offers a unique on-chip functionality, power reduction and miniaturisation. In this paper, we will 
discuss both enhancements that will enable the Photobus to real-time map the road centreline. 
1. INTRODUCTION 
Road databases commonly use a linear referencing system 
(LRS) for a spatial description of elements of interest. A LRS is 
directly implemented on the road, with an origin and a set of 
marks painted on the pavement at each kilometre. Its use does 
not require an absolute localization of such marks, except for 
cartographic purposes. Most of the GIS applications now 
include a procedure for the dynamic segmentation of data that 
are referenced either in a LRS or in a national reference system. 
However, the road objects tend to be added using GPS-based 
positioning, which requires the description of the painted marks 
and of the centreline geometry in both systems to find the 
necessary transformation. 
The acquisition of the needed transformation parameters 
initiated the design of a mobile mapping system by the 
Geodetic Engineering Laboratory of the Swiss Federal Institute 
of Technology in Lausanne. Our system can be distinguished 
from its predecessors by its ability to georeference the road 
centreline through a vertically-oriented CCD camera (Figure 1). 
This monoscopic technique is simple and economically 
appealing for rendering the road layout with sub-decimetre 
accuracy (Gilliéron et al., 2000). 
GPS antenna 1 (front) & 2 (back) 
CS 
  
   
(Muy CI 
CCD camera E 
PHO | OBUS 
Eh RR Te i EAE 
centreline 
Figure 1. The Photobus system 
Nevertheless, the processing of the navigation data derived 
from a loosely-coupled GPS/INS remains semi-automatic and 
still requires considerable input from a skilled operator. On the 
other hand, the variation of light conditions conflicts with the 
automation of the centreline extraction from video by deceiving 
most algorithms of contour detection when a part or the whole 
image is over-saturated. 
Internet-based GPS-RTK positioning and implementation of a 
CMOS (Complementary Metal Oxide Semiconductor) image 
sensor help to deal with these issues. Both solutions are 
discussed in the paper and contribute to the goal of achieving a 
near real-time mobile mapping system. This will limit the 
operator's intervention to the data collection in the field while 
ensuring immediate quality control. 
2. ENHANCING THE POSITIONING COMPONENT 
2.1 Positioning using GPS-RTK 
A crucial element of any mobile mapping system is registering 
the frame pixels in a global coordinate system. This process, 
known as georeferencing, is partially limited by the accuracy of 
the positioning and orientation components of the image 
(Zhang et al, 2003). The decade-old RTK technology has 
gained a wide acceptance in geodetic engineering and in public 
works. This technology is an extension of relative positioning 
based on the interferometric principle of exploiting precise 
carrier-phase measurements in real-time. The attainable 
accuracy is at the centimetre level provided that the integer 
ambiguities can be resolved and fixed (Liu, 2003). 
In our context of a vehicle that is mapping the road centreline 
via a vertically-oriented camera, the positions of two GPS-RTK 
receivers and the derived azimuth provide efficient solutions 
under small banking angles (<4°). The underlying requirement