International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004
the Rollei db23 camera enables photogrammetric aerial surveys
(60% end lap) with a ground resolution of > 12 cm. The digital
back works together with a Rolleiflex 6008 Integral camera
body and a Zeiss distagon 4/50-lens with a min exposure time
of 1/500 s. The digital camera is controlled by a laptop, which
also stores all the image data via a firewire connection.
Table 1: Technical parameters of the digital Rollei db23 camera
Rollei db23
Camera type Rolleiflex 6008 with fixed digital
back
Resolution 3.036 * 2.032
Pixel size 12 um * 12 um
36.67 * 24.38
Colour depth per channel 12 Bit
Sensor size [mm]
€
min. exposure interval ca. 1.8' se
Weight (incl. camera) ca. 1,500 g
Connection to computer — Firewire, MS-Windows notebook
Software Phase One 2.7
i 7 5 = i
value for two consecutive images, for secure image recording
of a strip add 50%.
Other important components of PFIFF are the GPS-based flight
management system and a navigation unit that automatically
triggers the images during a flight strip according to the pre
defined end lap. The navigation unit records the exposure delay
of the camera as well as the approximate parameters of the
exterior orientation with an attitude heading reference system
(AHRF). The AHRS consists of a digital compass and a two
axis inclinometer. The exposure control is coupled with the
PPS-signal of the GPS-clock to ensure a perfect synchronisation
with the external high accuracy L1/L2-GPS receiver.
For a photo flight the system is temporarily installed in a Cessna
172 with a small ground hole of ca. 12 cm in diameter. See
Figure 1 for the system design.
Navigation Digital Image Acquisition System
Position (X, Y,Z) AHRF (0.9.x)
passes L1/L2-GPS GPS 2 axe inclination-
GPS i :
(Garmin II Plus) (Leica SR 399) — (Garmin 25) sensor (ap) Compass(x)
|
|
Survey / Navigation ees ,. Li
>»
(Laptop with Cartalinx) Nav. PC A/D
ss m = (Pent. 90)
-
D
o
“osindwi 196611 —
esinduij use|4
Digital Camera
—
(Rollei db23) Firewire |
Passive stabilisized Laptop
camera platform (24 GB hard disc)
Figure 1: Low-cost remote sensing system PFIFF
For the use of a digital camera in aerial surveys not only the size
of the CCD-sensor is of importance, but also many other criteria
of the digital camera such as the minimum exposure interval,
the external storage capacity, a continuous power source,
preview options, the mechanical stability of the sensor (interior
orientation), the temporal eccentricity (exposure delay), the
reliability and also the radiometric properties have to be
considered and determined. Therefore the system has undergone
thorough geometric and radiometric calibration procedures. For
photogrammetric work the interior orientation of the camera
was determined. With the fixed digital back of the db23 an on-
flight calibration is not necessary. The examination of the
linearity, the spectral characteristics of the RGB band filters and
the high signal to noise ratio revealed that the radiometric
properties of the digital camera are superior to an equivalent
photographic system.
2.2 Practical experiences with PFIFF
Between 1999 and 2004 single components and PFIFF as a
whole was well tested in over 70 aerial surveys with more than
8.000 images recorded. The sensor system was tested in the lab
as well as under airborne conditions with an additional geodetic
GPS receiver and a multi antenna GPS on board of the
aeroplane to explore the accuracy potential for direct
georeferencing of the system.
The first photogrammetric test-flight in Nov. 2000 revealed the
full geometric potential of the system, allowing sub pixel
accuracy without ground control points through aerotriangu-
lation and high accuracy GPS-receivers. Through the
comparison of the exterior orientation parameters of both
systems the absolute positional and angular accuracy of the
GPS/AHRS-system could be determined. The accuracy
assessment is complete because the residuals of the orientation
parameters as well as the calibration errors of the optical system
and errors of the sensor orientation are incorporated in the
difference vectors to the well known ground control point co-
ordinates.
Due to large number of images (^ 2.000 per vear) the
management and archival of the image data and the meta data is
quite a challenge. The meta data collection has been automated
with several routines and special avenue scripts. For a
convenient search and ordering of data via internet a JAVA
based inquiry tool was developed.
2.3 Flight planning for high ground resolution aerial
surveys
In the flight planning for urban surveys several special issues
have to be considered. The surveys are generally done at low
altitudes and thereby take place in a turbulent flight zone,
especially during the summer when the urban heat dome
introduces additional thermal turbulences. For surveys with a
high ground resolution (12 — 15 cm) the minimum exposure
interval at normal ground speed of the aircraft (40 — 55 m/s)
becomes critical. A flight with reduced speed (30 — 40 m/s)
however decreases the stability of the airplane furthermore. This
causes higher deviations in the roll, pitch and heading angle.
Due to the lack of an active stabilized mount a high side lap of
more than 30% has to be considered in the flight planning.
A further source of inaccuracies may be caused be image
motion due to the ground speed of the aircraft. Additionally
image motions due to rotations and vibrations of the aircraft
during image acquisition also occur. For digital images the
resolving power of the lenses is generally better than the pixel
resolution and therefore the image movement shall be no larger
than one pixel to maintain a sharp image. The image motion
may become a limiting factor for PFIFF at high ground
resolutions, because the minimum exposure time of the lens is
1/500 s.
3. EXAMPLES
To demonstrate the potential for urban and agricultural
applications of PFIFF two recent projects will be described in
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