International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
Table 4-1: Comparison of area sensor and line sensor
Item of SEN
: Area sensor Line sensor
comparison
Measuring Stereo mapping is | Stereo mapping is
method possible (interpreted | difficult (mainly
by person) automatic processing)
Visibility Same as ordinary | It is difficult for human
photograph being to understand
Condition It is possible to use It is difficult. to
for system | existing mapping | develop the automatic
developmen | system processing system
t
Handling of | Only the Management is
data management of | difficult because it
stereo image pair and | depends on traveling
is easy speed
Spatial A little low | There are larger
resolution comparing with | number of pixels per
line sensor line
As a result as shown in the above, the area sensor is more
advantageous because the spatial attributes can be acquired
using existing mapping system and system development is
easier. Also, although the spatial resolution is a little inferior, it
can be supplemented by the shooting interval of stereo images.
Therefore, we will start developing employing the area sensor.
When the area sensor is used for stereo measurement, long base
line is necessary as much as possible to enhance the accuracy of
depth direction. However, since there is a restraint that the
sensor is mounted on a vehicle, 2 m is considered to be the limit
for the base line. Seeing from the experiences obtained from
applied photograph measurements, the ratio from 1 / 5 to 1 / 20
is realistic as for the ratio between base line length and photo
taking distance, and as a result, the measuring range is
necessarily limited to 10 — 40 m.
The biggest problem in measuring objects is the spatial
resolution resulting from the distance to object and focal length
of lens. The smallest ground structure on the road is eye
induction indicator of 3 cm in width, and it is necessary to
identify this line on the image. The level of 2 million pixels is
required as the area sensor that can realize this requirement.
When the vehicle is driven at the speed of 40 km per hour, the
car travels 11 m per one second (Figure 3-3). Since the range
where stereo photographing is possible is 30 m (10 — 40 m), it
will be possible to cover overlapping rate and total line based
on the work efficiency if the photograph is taken at the rate of
one picture per one second. :
N
Cn
— —+— 20km
= 20 —Bi— 30km |
|
9 —O- 40km |
B 15 50km ze
2 |
40 |
8 |
9 |
$5
É
0 |
0.2 0.4 0.6 0.8 ] 12 1.4
Shutter nterval (second)
70
Figure 3-3: Shutter interval and photo taking distance of camera
sensor
(2) Selection of position self measuring sensor
Since the required accuracy is within 25 em as stated in the
section of requirements for development, the accuracy of a few
em is required for the position coordinates of camera acquired
by the position self measuring sensor. POS / LV system made
by Applanix is appropriate to realize this requirement. This
system is able to record GPS data of 1 PPS and IMU data of
200 PPS. There are three models of POS / LV 420, 320 and 220
in the products made by this company, and the high end model
of POS / LV 420 is considered to be most proper taking into
account the reduction of accuracy due to concealment of GPS.
(3) Time synchronization of cameras
It is an important problem to synchronize the time among 2
stereo cameras and position self measuring sensor. To solve this
problem, we will adjust the timing of action of each equipment
using trigger generator. This trigger generator activate picture
shooting trigger using the GPS data of 1 PPS outputted from
POS / LV described in the above as the reference time. Each
stereo camera receives this synchronization signal and activates
the shutter of digital camera. In this way, time synchronization
is realized using the GPS time as the key.
(4) Selection of measuring vehicle
In this section, we will discuss the measuring vehicle (car) used
as the platform of this system.
* Higher position of sensor installation is preferable to reduce
the influence of obstacles (car height).
* The longer base line of stereo measurement is desirable to
secure the accuracy of depth direction (car width).
* As for position self measuring sensor, the longer base line
using 2 GPS for the correction of heading angle is preferable
(car length).
* Special large sized car is not realistic from the viewpoint of
cost and operation.
By the reasons stated in the above, one-box wagon is
considered to be most proper as the type of measuring vehicle.
(5) Installation method of each sensor
In this system, stereo camera and position self measuring sensor
(GPS / IMU) are mounted on the measuring vehicle. In order to
determine the position and inclination. of camera sensor
accurately, it is necessary to mount these sensors on one solid
body and maintain the relative position among the sensors. For
this purpose, a roof carrier commercially available is equipped
on the top of measuring vehicle, a highly solid rack is installed
on it, and all the sensors are fixed on the rack. By measuring the
relative positional relation among the sensors in advance, the
information of position and posture obtained from position self
measuring sensor can be correctly reflected on the information
of camera position.
Two-way observation will be the basis for the actual
measurement. Namely, measurement will be made on only the
ground structures at the traveling lane and those at the opposite
lane will be neglected. As a result, the ground structures to be
measured are distributed mainly at the left frontward direction,
and therefore, it would be appropriate to install the stereo
camera so as to face the left frontward direction.
4.2 Discussion on the data analysis and processing system
(1) Stereo image photographing system
Internat
Stereo ima
trigger gc
functions.
* [mage
quality (pk
* Functior
generator
photograpl
* GUI app
The probl
time to wi
install PC
frame mis
independe
of writing
plural PC’
(2) Calibr:
The intern
camera. |
accuracy i
wide angl
for the fol
*Focal len
*PPS (prit
*PPA (pri
*Radial di
Photograp
used for t
value mea
term of cz
later.
(3) Calibr:
External t
by the foll
* Establis|
The refere
site is est
spatial ba
against th
and meast
* Measure
Position s
on the vel
At the sai
vehicle is
* Calcula!
The exter
backward
coordinate
position «
value of
dimensior
will be tal
The corr
correction
measuring
Secular c
physical |
Therefore
correctior
them und
