International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
2.1 The system's hardware components
A NIKON DIH digital reflex camera with interchangeable lens
was used. The camera has an RS-232C interface serial port that
can be used to connect it to a GPS unit thereby registering the
information related to the position of the GPS antenna at the time
of shooting. A hyper-focal lens with a main auto-calibrated
distance of approx. 35mm was used. Pixel size is 11.80 um.
Two double frequency TRIMBLE 5700 GPS receivers were used
for this study. The receiver is equipped with a Zephyr antenna.
The digital compass that we used is the DMC-SX produced by
Leica Geosystem. It is a very compact sensor (33 x 3l x
13.5mm) capable of defining three angles (Azimuth A, Elevation
E and Bank B) of the object on which it is placed, corresponding
to the three standard photogrammetry angles K (drift), © (roll), ©
(pitch). Three sensors reading the terrestrial magnetic field while
the Bank and Elevation angles by two inclination sensors set the
Azimuth angle.
The nominal accuracy of the DMC is listed in table 2.
AZIMUT 0.5°
0.15° at +/- 30°
ELEVATION
0.20° at +/- 45°
0.15? at +/- 30°
BANK
0.20° at +/- 45°
them on its own hard disk. The frequency at which the compass
transmits information received by the PC is 50 Hz maximum,
Before the data is stored on the PC, it is couplet with information
regarding the time which is transmitted by the GPS at a
frequency of 10 Hz. The GPS and the compass are connected to
the PC through two RS-232 serial ports. The PC receives the
data and merges it using a proprietary synchronisation
programme written in C^*.
In brief, at the very instant that the impulse generated by the
camera releases the event-marker of the GPS, the compass
measures the three orientation angles of the camera and the GPS
allows for a determination of the position of the projection
centre. Since all the data is connected to the same time and with
knowledge of the time of the camera shot, it is therefore possible
to determine the exterior orientation parameters of every single
camera shot. The connection between the exterior orientation of
the camera and the measurements carried out by the compass and
the receiver is guaranteed by the fact that the receiver aerial, the
camera and the inclinometer are positioned at reciprocal
distances marked on the aluminium bar which was built to
censure the perfect alignment of the axis passing through the
centre of the three instruments and a connection both with the
topographic tripod and also with the system of swinging. The
flow diagram of the data is shown in figure 3, while figure 4
shows the aluminium bar design.
Table 2. Nominal accuracy of the digital compass
The compass contains a chip that can be programmed via the
interface serial port, allowing the operator to set the frequency
and format of the output data. Several options include: the DMC
either transmitting “raw” data at a maximum frequency of 150
Hz or first interpolating the data with a maximum transmission
speed of 50Hz. DMC precision is therefore influenced by
transmission frequency.The system can also correct the magnetic
Azimuth angle by giving an appropriate declination angle as well
as evaluate and compensate for the magnetic field distortions due
to the presence of external sources, i.e. tools aboard the boat or
high tension pylons. The compass contains a chip that can be
programmed via the interface serial port, allowing the operator to
set the frequency and format of the output data. Several options
include: the DMC either transmitting “raw” data at a maximum
frequency of 150 Hz or first interpolating the data with a
maximum transmission speed of 50Hz. DMC precision is
therefore influenced by transmission frequency.
The system can also correct the magnetic Azimuth angle by
giving an appropriate declination angle as well as evaluate and
compensate for the magnetic ficld distortions due to the presence
of external sources, i.e. tools aboard the boat or high tension
pylons.
3. HOW THE SYSTEM WORKS
At the moment of shooting, an impulse generated by the camera
is transmitted through an electronic circuit that receives an input
from the closure of the flash through a connector. This gives rise
to the event-marker of the GPS and thus records the exact
moment in which the image is acquired.
However, the raw GPS data is acquired within the receiver,
which, simultaneously, transmits information to the computer
regarding the time while the measurements carried out by the
inclinometer are conveyed directly to the portable that stores
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Figure 3. Flow diagram of the data and of the system
In conclusion, the POLIFEMO system supplies in output:
- a*.tiff image file, stored in the camera's memory
- a *.dat position file, stored in the memory of the GPS
- atime and angles of orientation file, stored in the PC .
