coordinates on these features, while receiving the
additional benefit of colored and geo-referenced
digital images of all features of interest. Moreover,
in the future, these images will be of enormous
value, as emergency vehicles begin to carry
computers with screens capable of displaying those
images and using the data for emergency purposes.
Both the positional data and the image data are
compatible with modern GIS.
Facilities Management
It was clear from the outset that the GPSVanTM
technology should be very effective for facilities
management (FM) purposes. This is true along
highways where culverts, signage, bridges, ramps,
guardrails, and other features are considered
important to highway engineers. It is especially true
along railroads, where the vast majority of features
of interest are within 100 feet or so of the track. The
GPSVan™ technology has proven to be efficient in
capturing digital data and mapping the right-of-way.
This paper focuses on the accuracy obtained in this
application environment.
Figure 4. GPSVan™
2. THE GPSVAN™ SYSTEM
The concept of the system is very simple: combine
GPS and dead-reckoning-based positioning systems
with electronic image acquisition (Toth, 1995). The
GPSVan™ is an ideal tool for corridor mapping,
such as surveys along transportation lines where
time and/or access is restricted. Instead of the labor-
intensive individual surveying of objects, images are
collected along the whole corridor and the actual
measurements are performed in a post-processing
center, using simple image processing tools (He et
al,1994a,b). During data acquisition, the captured
images are time-stamped by GPS time; thus, by
reconstructing the vehicle's motion, the camera
orientation parameters can be computed. The
GPSVan™ technology itself defines only the
concept; the actual hardware implementations
140
depend very much on the specifics of the individual
applications.
Figures 1 and 2 show the GPSVan™ and the
functional block diagram of the most recent systems
built and used in normal production.
Robust
Wheclcounters E
i GPSDR [CO
s — pomum
1
Stereo
Feature
Extraction
On-board 2
GPS Receiver p
Feature
Coordinates
A
Differential
GPS
p-| Computations
Base Station
A
GPS Timetagging
Image Analysis §
& Control E
Exposure
Control
Digital À
Video Cameras
Figure 2. GPSVan™ technology
Unlike traditional aerial mapping, control points in
the GPSVan™ images are rarely available, and
therefore, they cannot be used in image
measurements to establish the exterior orientation.
The camera orientation is provided by the
positioning component of the MMS, which includes
GPS with DR or inertial navigation (INS) systems.
In the simplest model, the absolute positioning error
of object space points using stereo imagery is
comprised of two factors. The first group represents
the error of the six exterior orientation parameters,
Co» basically the longitude, latitude, and height or
the three Cartesian coordinates of the camera
projection center, along with the three attitude
angles:
Crise f (Cos C5)
Cro = 4 (Oxvz> Cattitude)* (1)
The second group, the imaging system, €,
includes all the errors introduced by
photogrammetric processing, including terms for
errors in the transformation parameters, the operator
pointing error, and base/depth ratio:
Cis = Ten , Q peor ? Crist): (2)
The transformation parameters can be further
broken down into components:
Cro 5 He. > Cro > Come: Ca) (3)
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
