International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
Generally we can separate softcopy photogrammetry functions into
categories:
e Film scanning;
e Aerial triangulation;
e Digital terrain elevation "scanning";
e Orthophoto generation ;
e 3-dimensional vector data stereo collection as input to a
GIS
Of course there exist various special functions, for example when
existing data need to get updated, or when pre-existing DEM-data
are used for orthophoto production. Orthophotos may be the basis
for 2D or even 3D vector collection using a mono-plotting approach,
avoiding the use of overlapping stereo images.
2.2 Practical Applications
It may be worth noting at this time that the functions of first
acquiring aerial images and then processing them into data products
are conceptually separate, with a simple transfer of film images or of
scanned image files. Therefore it is not uncommon to find that one
organization produces images, another one processes these.
Orthophoto production has been totally taken over by a softcopy
approach. In fact, no orthophotos would be produced today had
softcopy not been feasible. The function of digital terrain elevation
scanning, if done from stereo images, has to some degree been
converted to the softcopy domain. Yet, analytical or even analog
plotters still are the workhorses of stereo-photogrammetric data
processing to this day. All functions related to a human's visual
stereo impression are to a great extent not performed with digital
images. Producing a DEM may therefore be based on manual stereo
observations, since the opinion exists that an automated creation of a
DEM from scanned film requires much manual error removal so that
it is not advantageous to go digital.
2.3 Automation and Instant Gratification: Laser Scanning,
Direct Geopositioning
The inability of full automation of photogrammetric procedures has
created a need for systems that determine the exterior orientation of
each image automatically by means of GPS and IMU-
measurements. This in essence replaces the function of the aerial
triangulation (AT). And the inability of a fully automated DEM-
creation has caused a need for direct terrain elevation measurements
by means of laser scanners. Both systems have reduced the need for
traditional photogrammetric core competencies. Yet, to take full
advantage of them, associated images need to be digital. Softcopy
photogrammetry has therefore developed a branch that merges direct
geopositioning observations with pixels to project them into
orthophotos, given an independently produced DEM. And the laser
scanner observations, representing an externally available DEM, get
merged with digital images to produce an instant orthophoto.
Both the direct geopositioning and the laser scanners reduce the
need for manual labor and therefore affect costs, and they reduce the
time it takes to produce a DEM and orthophoto, thereby improving
the competitive position of a firm.
2.4 The Advent of Digital Imaging
With digital cameras, film no longer exists and the options of
processing images in an analog or analytical plotter go away.
Therefore softcopy photogrammetry takes over all photogrammetric
functions. In fact, one important new function gets added that did
not exist in the world of film-based photogrammetry:
e image archiving and cataloguing.
That function was previously in the hands of the photo lab and film
archive. With the advent of digital cameras, photo labs become
obsolete, and so do film archives.
The capabilities of softcopy photogrammetric software gets stressed:
while previously, softcopy systems needed to process what
otherwise would be processed on analytical or analog devices, now
the data quantities may increase considerably, perhaps by a factor of
5. While traditional film photogrammetry may have each object
point on only 2 images, a digital system can increase the at no extra
cost for the images to 10. The traditional forward-/side-lap
percentages at 60/20 can and will be increased to 80/60. This affects
of course the data quantities and requires the acceptance and
implementation of advances in computing. Moore's law is a
welcome factor that promises that over the next 10 years, computing
will improve its price-performance ratio by a factor 100. Image
storage, retrieval, transmission, processing and information
extraction are currently feasible with such increased data rates, and
this feasibility will improve dramatically over the coming years.
3. DIGITAL VERSUS FILM CAMERAS
Superior Economy
No consumables such as film, processing
No scanning
No cost color and color IR
No cost of duplication
Reduced manual labor
Superior Image Quality
Note: 12 bits/pixel; No grain noise
Better matching accuracy (2.5 x film)
More flying days in marginal weather
Better interpretability
Better success in automated procedures
Increased Redundancy in All Measurements
Note: Higher forward overlaps at no extra costs
Better success in automated procedures
Superior automated DEM generation
Fewer occluded areas in urban projects
Superior geometric accuracy
No image shrinking or other deformations
Inner orientation is system-inherently available
Identical duplicates
Absence of grain results in higher accuracy
Radiometric range improves accuracy
Superior Work Flow
In-flight quality control
Automation with less manual intervention
Improved manual interpretability (RGB&IR)
No more film management
Intra-/Internet-enabled archiving and cataloging
Camera maintenance via Internet
Figure 1: List of advantages of digital sensing over film cameras in
photogrammetry
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