Full text: XVIIth ISPRS Congress (Part B4)

z all 
need 
‚the 
the 
time 
and 
f an 
n is 
e of 
xel- 
ifies 
ina 
than 
hort 
Was 
rior 
shed 
ed a 
s to 
ipai 
pre- 
ues: 
nal 
reat 
idle 
can 
|). 
n to 
k of 
ase- 
can 
lon. 
nate 
by 
the 
) is 
the 
2) 
ed 
If 
ed 
tor 
be 
(TS 
he 
Ire 
its 
so 
be 
  
  
  
  
  
  
  
Method xp [mm] yp [mm] c [mm] 
interior orientation -0.009 (20.011) -0.409 (20.011) 20.281 (40.019) 
w/radial -0.141 (40.004) -0.434 (40.004) 20.354 (40.006) 
w/radial and decentering -0.152 (20.008) -0.337 (20.009) 20.371 (40.006) 
  
  
  
Table 1: Results of the pre-calibration of the Hawkeye camera: interior orientation 
parameters (principal point xp, yp, focal length: c) were derived by different versions of the 
bundle adjustment. The values in the brackets give the estimated precision. 
  
  
  
  
  
Zu G 
| 20 Az 
| 
| A 
| e- = 
le EL peer 
IG N grid 
Xp 
XL YLo4CL 9 local grid coordinate system, 
X, Y, 2 eet image coordinate system. 
Figure 3: Calibration of the offset vector in the image 
coordinate system. 
All measurements in the airplane were done in the hangar 
with the airplane strapped to the floor in a position which 
comes close to the one it would assume during flight. A test 
grid was laid out beneath the aircraft to define a local 
coordinate system (Xy, Yr, Zı) in which the offset vector 
AO was determined. First, an image of the grid was 
captured by the Hawkeye M-3 camera in its mount. This is 
possible as the short focal length of the digital camera has a 
large field of view and thus creates a sharp image of the grid 
even at this short distance of about 1.5 m. Then, a number 
of theodolite intersections were measured to locate the GPS 
antenna's phase center G, its vertical projection onto the grid 
(G^), and a number of grid points on the floor. All 
coordinates were determined in the local grid system. 
Additionally, we marked a point G' at the bottom of the 
airplane, which is used to initialize the GPS survey of the 
mapping flight over a known target with a plumb. The 
image coordinates were measured in the grid images. 
Together with the interior orientation from the pre-calibration 
they were used to compute the perspective center (O) and the 
camera attitude in the grid system. The offset AO] is 
defined as the vector from O to G. By applying the rotation 
matrix it is transformed into the image coordinate system (3). 
AO - RL: AOL (3) 
with: AQ... offset vector in the image system, 
AO, offset vector in the local grid system, 
Ry rotation matrix of the captured image. 
During our first test flights the offset vector was 
calibrated as AO = (1.695, - 0.269, 1.434). 
3.3 Time Delay of the Shutter 
During our first test flights we found that there is a delay 
between the exposure time recorded by the computer and the 
time when the shutters actually opened. This is due to the 
fact, that the PC sends a signal to the Hawkeye and records 
the GPS time of this signal. However, the exposure is 
somewhat delayed due to the electronics transmitting the 
signal. This delay can be as large as 1 millisecond, which 
corresponds to an offset of 5 cm if the aircraft is traveling at 
a velocity of 160 km/h. 
There are two ways to calibrate this delay At: first, one 
can try to measure it with an oscilloscope and a light 
sensitive diode behind the shutter. This would tell us, how 
long the camera takes to respond, once the signal was 
transmitted from the PC. The other approach would try to 
correct for At by an additional block-invariant parameter in 
the bundle-solution. The problem is that this parameter is 
fully correlated with the principal point coordinate xp (if the 
camera's x-axis is parallel to the flight line). This would 
only be a problem, if a self-calibration is computed during 
aerotriangulation. 
4. OPERATION OF MAPCAM AND POST- 
PROCESSING 
For the test flights conducted with the first version of the 
MapCam system we used a Cessna 207 aircraft. It had a 
hole and camera mount in the bottom, so that we only had to 
install a small adapter for our Hawkeye camera. The 
installation of all equipment takes about half an hour, not 
including the calibration which was done separately. Once a 
basic operation test was performed in the hangar, the 
airplane was taxied to a known target on the runway of the 
airport. By using a plumb line the horizontal offset between 
this position and the GPS antenna is determined, the vertical 
offset is already known from the original calibration. By 
this procedure the GPS survey is being initialized. From 
this time on the pilot must try to maintain continuous satellite 
lock, which is possible if the airplane is not tilted more than 
109 during the flight. We managed to avoid cycle-slips 
during both test-flights which lasted more than one hour 
each. After the flight the known point was revisited to close 
the survey. 
Once airborne, the operation of MapCam is straight 
forward: the operator simply hits a button to start capturing 
images. This can be done at time intervals larger than one 
second, or at a constant, user-defined overlap. In the latter 
case MapCam uses on-line navigation data from GPS to 
determine the airplane's speed and relative position changes. 
After the flight, post-processing of the GPS 
measurements is completed by combining observations of 
base and rover stations. Either pseudo-ranges or phase 
measurements can be analyzed, dependent on the 
requirement. As a result we obtain the flight-lines with the 
images attached as attributes (figure 4). 
 
	        
Waiting...

Note to user

Dear user,

In response to current developments in the web technology used by the Goobi viewer, the software no longer supports your browser.

Please use one of the following browsers to display this page correctly.

Thank you.