Medium-Format Digital Cameras Come of Age
Frank Artés
Introduction
Recent advances in technology and wider commercial
applications have prompted the aerial survey and remote
sensing community to take a closer look at digital imaging
and its advantages over conventional film. Film cameras,
by their very nature, are comprised of mechanical moving
parts that require precise calibration and careful
maintenance, to ensure they remain within the necessary
mapping accuracy standards as required by various
governing bodies. By setting precise shutter speeds,
adjusting the aperture, and selecting the lens filters,
users of aerial film cameras can produce high quality
images. The end product, however, is still at the mercy of
the media. While it’s typically fairly uniform, film is
subject to deformation and tonal differences, which are
difficult to correct.
Digital sensors offer more flexibility in image
quality. An operator can view a digital image on-the-fly
immediately after exposure and adjust the radiometry
(balancing image illumination for brightness, darkness,
vignetting [variation of brightness across the image],
etc.) to fit changing atmospheric conditions. The time
required to process digital imagery is significantly
shorter than that needed to process standard film,
reducing the turnaround time from weeks or days, to a
matter of hours. Digital cameras produce a sharper image
than scanned film, and the increase in image correlation
allows automatic measurements to be undertaken with
extreme accuracy. An all-digital approach to providing
geospatial information is the way of the future.
Digital aerial cameras have been available to the
industry for several years now and have met with much
success. However, the expense associated with moving to an
all digital operation can be prohibitive, and in some
cases outweigh the advantages gained through operating in
a completely digital environment. Small, medium and
large-format digital cameras (Table 1) have been used to
generate aerial imagery for a number of applications, some
of which are more suited to the task than others.
The major distinction between the three formats is
the size of the CCD array (a light-sensitive integrated
circuit referred to as a Charge-Coupled Device, designed
to store and display image data) which establishes the
area of ground coverage produced by the camera. There are
a number of other factors that come into play also, such
as flying height, camera lens focal length, and speed of
the aircraft, to determine best Ground Sample Distance (GSD,
the distance on the ground represented by each pixel in
the x and y components) etc. for each, but that is beyond
the scope of this article.
Small-format Digital Systems
The use of small-format digital cameras as an
aerial imaging system for commercial use has been fairly
limited. Most examples are based on the Kodak DCS and
MegaPlus series of equipment, often custom configured by
independent organizations, such as Geotechnologies in the
UK, and SenSyTech in the US, for their own particular
requirements. The array size has been a restrictive factor
for standard mapping applications, generally producing
images in the region of 1000 x 1000 pixels up to 2000 x
3000 pixels. Therefore, imagery users with small, focused
areas of interest including foresters, geoscientists in
the field, and organizations involved with environmental
or shoreline monitoring, find small-format systems
attractive. Users with larger areas of interest, including
topographic mapping needs, may be better served by
larger-format options. Infrared imagery generated by
small-format systems, has been a popular tool among
cultivation specialists and other individuals involved
with field-crop production and soil management, where
careful image analysis can identify growth patterns and
diseased vegetation. These systems are primarily used for
small areas, which even then can sometimes require a lot
of images to cover the location being studied.
Large-format Systems
The large-format digital imaging systems, currently
on the market, have been developed by companies with solid
reputations for providing state-of-the-art technology to
an industry with diverse and ever-changing needs. At the
moment there are three major players producing
large-format digital aerial cameras: Z/I Imaging, Leica
Geosystems, and Vexcel Corporation. Both Z/I Imaging and
Leica have extensive experience in the design and
development of aerial camera systems for mapping purposes.
Their standard aerial film cameras are recognized
worldwide as being among the best in the business.
Likewise their digital products are achieving a similar
reputation. As a relative newcomer to the arena, Vexcel, a
company with a lot of expertise in remote sensing
applications, has developed its own large-format imaging
system, which has established itself as a serious
contender within the industry.
Why are some users of large-format film cameras
reluctant to make the jump to digital imaging? The answer
is primarily cost. All three of these systems are
expensive to purchase and are a serious investment for a
company contemplating a move to total digital production.
Cost has fostered growth in the design and development of
a medium-format alternative for a market that is eager to
take advantage of digital imaging. Format size, once
thought to be the limiting factor for digital cameras is
now less of a consideration with the availability of
larger CCD arrays of 4000 x 4000 pixels and up. Although
ground coverage produced is still less than that of
large-format examples, medium-format cameras are becoming
a viable alternative for specific applications. The
correct positioning of these systems in the market and
their wide use by data collection organizations will be
key to their success.
The Medium-format
Alternative
Within the last few years there has been a steady
increase in the number of medium-format, digital aerial
cameras coming onto the commercial market. Companies in
Europe and North America are at the forefront of this
developing technology, and in their own way are
spearheading a campaign to bring medium-format imaging
systems to center stage. Up until now the aerial survey
and remote sensing industry has been somewhat slow to
realize the impact this technology can make as an
entry-level alternative to large-format systems.
Most medium-format cameras have been used as an
imaging component to enhance LiDAR generated data. With
increased direct georeferencing accuracies, in which the
camera’s integrated GPS/IMU inertial navigation system
is used to generate exact position and orientation values
for each image frame, the industry is now looking at
medium-format technology for its photogrammetric mapping
potential. Companies wishing to make that all-important
step into the total digital workplace are looking closely
at the medium-format option, but with an eye to specific
niche market utilization.
The producers of medium-format, digital imaging
systems are confined to just a handful of companies. These
include US-based Spectrum Mapping developer of the NexVue
camera system (Figure 1), which uses a camera body with a
digital back together with optional 50mm and 90mm lenses.
The DiMAC system (Digital Modular Aerial Camera, Figure 2)
produced by Aerophoto in Bergem, Luxembourg, uses single
and multiple camera units, and is similar in concept to
the large-format Z/I Imaging DMC, with its modular
approach. The DiMAC product can be configured as a small,
medium and large-format system, which gives it a wider
market appeal. Rollei of Germany, a company with a long
history of producing medium-format photographic technology
for the commercial market, manufactures the db44/45 metric
cameras for terrestrial and aerial surveying.
Canada-based Applanix produces the DSS Digital
Sensor System (Figure 3). The DSS is an example of a
completely integrated turnkey system that includes an
embedded Flight Management System (FMS), direct
georeferencing from integrated GPS/Inertial technology,
and a stabilized azimuth mount for automatic yaw control
of the camera. These features are also available as
options on some of the other medium-format systems
mentioned, providing additional flexibility. In
particular, the use of third-party stabilized platforms
and inertial navigation systems with GPS/IMU integration,
can increase a system’s potential and give it an
operational edge. Generally speaking, medium-format
digital camera developers produce modular systems that use
off-the-shelf-components specifically configured for the
aerial survey industry. They are reasonably similar in
what they offer: color and/or CIR capability, integrated
FMS, and small aircraft/single pilot operation.
The Right Tool for the Job
The principal market for medium-format systems is
the small to medium sized service provider looking to move
to a digital system, but still relying on its film-based
cameras to undertake major mapping projects. Key to the
technology’s success has been application-specific
targeting, identifying the medium-format position within
the marketplace, and focusing the systems attributes
accordingly. From an application standpoint, it is safe to
say that medium-format digital cameras are not really
designed as a direct competitor to their large-format
cousins, but rather as a niche market solution for
specific project types.
Topographic mapping projects requiring image scales
from 1:2500 to 1:30,000 cover the whole gamut of mapping
applications, and large-format digital cameras
successfully undertake them all. Engineering-standard
mapping, covering large urban areas requiring full ground
control, aerotriangulation block adjustment, digital
terrain model (DTM) extraction and orthophoto generation,
normally require horizontal and vertical accuracies of
better than 9 cm, and a GSD of 5 cm. For this type of
project a medium-format digital imaging system would not
be the answer.
For companies regularly undertaking projects of
this type, large-format imagery makes the most sense,
where precise flight planning for maximum ground coverage
with the least number of flight lines, is the most
economical approach. However, the scenario changes for
smaller, linear-based, location-specific and emergency
response projects, where image coverage, ground control
availability, aerotriangulation components, and time to
data delivery can shift in priority and have an enormous
effect on project costs.
Positioning the Medium-Format
Digital Camera in the Marketplace
Small, irregular shaped areas, strip mapping,
transmission line corridors or pipeline contracts, do not
always require the ground coverage produced with a
large-format camera. Projects such as disaster response
analysis require quick data turnaround. Available ground
control can be an issue, particularly in a hazardous
environment, such as localized forest fires or oil spills,
or in locations where survey crews would be working around
heavy construction vehicles, as in an open pit mining
operation. A direct georeferencing capability in these
instances is a tremendous advantage. For location-specific
strip map coverage, single line imagery can be utilized
without the need for the second tie-line, and small block
areas can be easily georeferenced to produce orthophoto
mosaics. Taking a large-format digital camera off-line to
undertake these smaller projects can be costly, even
though the all digital environment means such projects can
be completed promptly.
For smaller aerial survey/remote sensing
organizations, the medium-format alternative is changing
the face of the industry, as an affordable technology that
can deliver increased performance, a marked reduction in
operating costs, and most importantly a digital product
when it is most needed. A number of recent global events
have placed an emphasis on timely spatial data
acquisition. Stretches of the San Andreas Fault previously
hidden with dense vegetation, and exposed as a result of
the California fires, were captured quickly and
efficiently using a medium-format imaging system (Figure
4). In the wake of Hurricane Isabel a medium-format aerial
camera system was successfully used to generate linear
digital imagery of the North Carolina shoreline. Soon
after Isabel had passed through the area, the National
Oceanic and Atmospheric Administration (NOAA) had an
aircraft in the air over the North Carolina coast (Figures
5 and 6). As an example of how effective directly
georeferenced digital imagery can be, two hours after the
aircraft had landed, high-resolution digital images were
in the hands of the NOAA staff.
Identifying applications for which the technology
is most suited, can open up a host of new business
opportunities for those organizations capable of producing
rapid, georeferenced digital imaging. As a modular,
compact, and price-conscious alternative, medium-format
technology is no longer just a novelty, it is now a viable
option.
About the Author
Frank Artés is a technical communications writer
at Applanix Corporation, where he specializes in aerial
and terrestrial photogrammetric applications. He may be
reached at [email protected].
Back
