International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
precision on pixel size and B/D ratio versus the dependency of
the across-track photogrammetric precision on pixel size only.
50 3 a
pixel size (mm)
45 stereo overlap (m)
B/D ratio x 100
along-track dev. (cm)
40 across-track dev.. (cm)
35
30
25
20
15
10 -
5 i
0 5 10 15 20, 25 30 35 40
Distance cameras to object (m)
Figure 3: Relationship between distance (units in m) and
photogrammetric precision (units in cm) across and along-track
(along-track precision strongly depends on B/D ratio), stereo
overlap (units in m), pixel size (units in mm) and B/D ratio
(B/D ratio times 100).
2.4 Image acquisition subsystem
The image acquisition subsystem . selects photo parameters,
generates the trigger pulse and handles data. In order to freeze
the stereo scene, both cameras are synchronized at the time of
image capture. The photographs are taken by the image
acquisition subsystem, which generates a pulse (rain (trigger) at
a frequency depending on the traveled distance or at a given
constant frequency.
If the acquisition frequency is configured spatially, the trigger
period depends on the distance covered by the van and partially
on road turns. This required information is obtained from
vehicle speed and heading, continuously provided by the
orientation subsystem. A typical spatial period would be 10
meters or a turn higher than 60 degrees, which corresponds to
the camera field of view.
The hardware components of the image acquisition subsystem
are two Frame Grabbers, one Counter/Timer and two removable
disks, all of which are managed by a Control PC. A Frame
Grabber, required to control the digital cameras, is the interface
between the cameras and the acquisition software. The
Counter/Timer is a device for generating pulse trains used to
trigger the camera and to synchronize the timeboard. The
software components of the image acquisition subsystem are
integrated in the general GEOMOBIL software application that
is in charge of the hardware equipment configuration,
acquisition control, GPS time synchronization process and
system status displaying.
The data storage capacity of the system has been evaluated to
be higher than 100 Gbytes. Considering that a GEOMOBIL
survey session can last seven hours at 1 Mbyte image size,
driving at a 72 Km/h vehicle speed and with a spatial
acquisition frequency of 10 meters/image, a minimum storage
capacity of 101 Gbytes is needed per session. Hence, the system
storage capacity is composed of two removable 73.4 Gbytes
disks. If necessary, the disks can be exchanged to increase the
Storage capacity. According to the current hardware
configuration and lo the data recording rate of disks, a
maximum of four pairs of images per second can be taken by
the system. This number is enough to cover the requirements of
the system, and can be easily enhanced by using larger and
faster disks as soon as they arc available.
2.5 Synchronization subsystem
The synchronization subsystem aims to synchronize in a
common temporal reference. (GPS time) all the sensors
integrated in the GEOMOBIL (GPS/IMU/Image sensors/laser).
This subsystem integrates a timeboard and handles different
synchronism signals: PPS, Trigger and Resync.
The timeboard is a device that allows timetagging of received
TTL signals with 20 ns resolution. Thus, all the received signals
are precisely referenced to the temporal reference system
defined by the timeboard. However, the requirement is to
synchronize the sensors in a global temporal reference (GPS
time). Therefore, the synchronization subsystem process is
divided into two steps, namely initialization and data
synchronization.
The goal of the initialization process is to establish the
difference between GPS time and timeboard start time, which is
defined as the instant when the timeboard resets its internal time
to zero and starts working. In this initialization step, Ty is
defined as the result of the subtraction between synchronism or
the GPS-timetagged Resync pulse and the same pulse but
timetagged by the timeboard. During subsystem operation, the
drift of the timeboard internal clock is also monitored and
corrected using the I PPS signal provided by the GPS.
2.6 Sensor Calibration Procedure
A sensor calibration protocol must be defined for each sensor
on board of the GEOMOBIL.
In general, a sensor calibration protocol is divided into two sets
of calibration procedures: the sensor inner parameter calibration
(if necded), and the relationship between all the sensors on the
platform, in particular the relative orientation of the two
cameras and the boresight calibration of the sensor in relation to
the orientation subsystem.
In the case of digital CCD cameras, calibration comprises the
calibration of optical parameters —focal length, principal point
and lens distortion— and the relative orientation of the cameras
and boresight calibration; that is, determination of the
eccentricity vector and the misalignment matrix between the
image reference system (defined by each camera) and the
inertial reference system (defined by the GEOMOBIL
orientation subsystem).
In the case of the laser scanner, only boresight calibration is
performed.
2.7 GEOMOBIL: data extraction software
The data extraction software assists the interactive digitization
of features for creating, updating or revising georeferenced
data. The system allows the point measurement on the images
obtained with the GEOMOBIL, and the classification of the
feature attributes according their semantic contents.
The original idea, to use a stereo environment that allowed
superimposition of vector data, based in the photogrammetric
model obtained from the orientation data of two images, was
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