International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004 
  
requires skilled operator and more time, terrain relief could be 
formed by accurate grid measurements. Generating digital 
terrain models not only facilitates and speeds up the 
computation of the contours, but at the same time, makes the 
applications such as volume computations, 3D representation of 
the terrain, profiling, etc. possible. 
Speed, flexibility and accuracy exposed by analytic method had 
accelerated the transition to digital photogrammetry. As it 
would be figured out from its name, digital photogrammetry 
requires digital imagery that could be either directly captured 
by digital aerial cameras or digitized form of an analog imagery 
captured by film based aerial cameras. Accuracy of the 
photogrammetric scanners, which are used to digitize analog 
imagery, has started with 25 um and improved to 7 um within 
years (Baltsavias, 1999). Until recent years, the remote sensing 
satellites were the only source for acquiring digital imagery but 
current performance of the airborne digital cameras is capable 
of fulfilling the imagery demand of the digital photogrammetry. 
Main drawback of the digital imagery was said to be the storage 
problem but today it has been overcome by high capacity 
storage devices and advanced compressing software. Digital 
workstations are suitable for high degree automation such that 
automatic interior orientation of the digital aerial images 
(Kersten &Haering, 1997), determination of exterior orientation 
parameters by bundle block adjustment and automatic aerial 
triangulation could be accomplished. Despite the fact that 
automatic aerial triangulation has become a standard procedure, 
care should be taken when working with mountainous or dense 
forest regions and automatic procedures should be supported by 
manual measurements (Jacobsen, 2002). Automatic image 
matching feature of the digital photogrammetry enables the 
generation of Digital Terrain Model (DTM). 
Exterior orientation parameters of the sensor can be determined 
directly by utilization of the Global Positioning System (GPS) 
and Inertial Measurement Unit (IMU) instead of conducting 
intensive field work to mark and survey the ground control 
points that are needed to perform adjustment. Rotation angles 
(®, ©, K ) are obtained from the attitude data of the IMU while 
the projection center coordinates (X,, Yo Z,) are acquired by 
GPS component of the system (Cramer & Stallman, 2001). 
2.2 Developments in Sensors 
Sensors could be classified in two major groups as passive and 
active sensors according to the source of the reflected ray. 
Passive systems are the electro-optic sensors that operate by 
sensing the reflected daylight. Quality of the images acquired 
by passive sensors totally depends on the weather conditions. 
On the other hand, active systems are the microwave sensors 
that record the reflected electro-magnetic waves that are emitted 
by sensor itself. Since active systems are not weather and light 
dependent, they are capable of collecting data every time of the 
day and in all weather conditions. 
When to speak about passive airborne systems, film based 
aerial cameras take the first place. Scanning the aerial films, 
which are taken by analog cameras, is the indirect way of 
acquiring digital imagery to be used at digital photogrammetric 
stations. On the other hand, airborne digital cameras give the 
opportunity of collecting digital image directly by employing 
cither linear or matrix type Charge Coupled Device (CCD) 
arrays. Among the digital cameras on the market, ADS40 of 
Leica GeoSystems operates linear CCD where DMC of Z/1 
Imaging and UCD of Vexcel operate area (matrix) type CCD. 
648 
LIDAR is an active remote sensing technique that resembles to 
radar but instead of radio waves it uses laser light. The basic 
components of a LIDAR system are a laser scanner and cooling 
system, a GPS and an Inertial Navigation System (INS). The 
laser scanner is mounted to an aircraft and emits infrared laser 
beams at a high frequency (Figure 1). The scanner records the 
difference in time between the emission of the laser pulses and 
the reception of the reflected signal. The round trip travel times 
of the laser pulses, from the aircraft to the ground, are measured 
and recorded along with the position and orientation of the 
aircraft at the time of the transmission of each pulse. Three 
dimensional X, Y, Z coordinates of each ground point are 
computed by combining the flight vectors from aircraft to 
ground and the aircraft position at each measurement instance 
(Brovelli et al, 2002). 
  
  
Figure 1. Operating principle of LIDAR system 
Another active microwave sensor SAR is a radar system that 
generates high-resolution remote sensing imagery for more than 
a decade. Major satellites that have been collecting SAR data 
are ERS-1/2, JERS, Radarsat-1 and ENVISAT. When compared 
to optical images of the same pixel size, SAR images expose 
inferior performance of object identification. In addition to this 
difficulty, a SAR-image is dependent to the view direction and 
renders geometric problems of foreshortening, layover and 
shadows in mountains. (Jacobsen, 2003). 
The main advantage of SAR system is the generation of Digital 
Elevation Model (DEM), example of which is given at Figure 2, 
by interferometric SAR (InSAR) technique that was started by 
ERS-1/2 tandem mission. Shuttle Radar Topography Mission 
(SRTM), during which the earth was imaged between 60° north 
and south latitudes, realized the single-pass InSAR technology 
for the first time ever with a ten-day mission in February 2000 
(Bamler et al, 2003). 
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