Going from Film to Digital: Aerial Services’ Experiences with Leica’s ADS80 Pushbroom Sensor

Going from Film to Digital: Aerial Services’ Experiences with Leica’s ADS80 Pushbroom Sensor
February 9, 2010 Jason Dolf

The aerial
photography profession is rapidly evolving from film-based cameras to digital
sensors. Aerial Services, Inc. evaluated many digital sensors and purchased in
2008 the Leica Aerial Digital Sensor (ADS80-SH82).  Here are some of the
most common questions we have been asked regarding our experience with the new
sensor.

Why did you choose the
Leica ADS80-SH82?

The ADS80 is a state-of-the-art system utilizing “pushbroom” technology instead
of traditional frame-based systems.  Pushbroom systems do not capture
rectangular image frames by “snapping” pictures over time along a flight path, but
instead capture strips of images much like a photocopier runs along a page
building an image as it goes.  The
ADS80 uses three CCD’s to simultaneously collect three image strips at three
different angles: forward, nadir (downward), and backward.  Aerial
Services is able to capture a panchromatic (black & white) band with the
forward, nadir, and backward angle and also simultaneously capture red, green,
blue, and near infrared (RGBN) on the nadir and backward views.  Further,
this allows both pan and RGBN stereo viewing. Because the camera system
integrates an inertial measurement unit (IMU) all imagery is always accurately
georeferenced.

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Have
clients been pleased with the device’s accuracy?

Yes. The ADS80 was installed in our aircraft and calibrated in November 2008. 
The results of these tests affirmed that Aerial Services made an excellent
choice with the purchase.  The sensor provides the positional accuracies needed
for large-scale mapping (and orthophotography) and provides the best
radiometric quality of any large-format airborne sensor in the industry. Our
clients have affirmed our choice by repeatedly requesting this sensor.

Isn’t acquisition different
because it is a “pushbroom” sensor?

Yes. Imagery is collected in strips where each flight line is one long image. 
Other frame-based digital sensors collect 100’s of separate images along the flight
line. With only a single image per flight line it is orders of magnitude easier
to produce seamless high-quality orthophoto mosaics of those strips.  For
example, if there are 40 flight lines across a county, then 40 images must be
mosaiced and color balanced together to produce a seamless image of the county.
But with a frame-based digital sensor, there may be 1000 or more images each
with unique color qualities that must be mosaiced. This makes it much more
difficult and time consuming to produce a seamless mosaic of the county. Easier,
cheaper, and higher quality mosaics are a major advantage of the ADS80 camera
system.

Long strips can
sometimes introduce a tonal issue that is unique to a pushbroom system. Occasionally,
the tonal characteristics from one end of a strip to the opposite end can vary
significantly with the tones in an adjacent strip because the sun angle or
intensity of the incident light changes. This then makes it more difficult to
mosaic the two strips together. Fortunately, this condition occurs infrequently
and there are simple (but time-consuming) ways to compensate.

A major technical
advantage of the Leica ADS80, and a perennial favorite of clients, is that the imagery
is not pan-sharpened. The color bands are collected at the same high-resolution
as the pan bands and all of the bands perfectly overlay (align with) each
other.  This allows for higher positional accuracies and truer color of
orthos created from the imagery.  Another important favorite with clients
is the major reduction of building lean in orthophotos. The sensor captures
strips of imagery from three different views and uses these views to create
stereo imagery. But orthophotography is generally created from only the nadir
(downward looking) view. There is no building lean at the center of this view
and increases mildly and gradually outward from the center. This is simply not
possible with frame-based digital sensors and our clients love it!

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Are
the image files large and do you need a super-computer to process them?

Strips of images are large files; often a dozen or more gigabytes in size. 
Massive storage silos and a number of powerful servers are needed to process
this imagery.  Aerial Services chose to deploy Isilon’s innovative
scale-out NAS storage solutions to meet our data storage space needs. 
This system is very reliable and allows storage expansions to be added very
easily with minimal interruption to production. The extremely large datasets
and intensive computer processing also impose the necessity of “distributed
computing.” This is not something that was common in the industry before the
advent of the digital sensors, but fortunately, inexpensive multi-core
computers are now commonplace and software vendors are writing software optimized
for distributed processing.

How
do you protect the image from loss or destruction after collection?

This goes to the core of our business. When using film cameras, if the film was
processed improperly or lost in the mail, it meant a costly reflight and then only
when the weather was favorable. Using a digital sensor there is no film. The
“raw” imagery stored in the camera system is the film-equivalent. If it is lost
or destroyed the project must be reflown. Fortunately, we have superior methods
of protecting this imagery then we had with film. First, the “raw” files are immediately archived when the
aircraft lands. A second and third copy of the data is created on different
hard drives. (Instant film duplication was not possible.) The chances of loss
and destruction are nearly eliminated and are much less likely than losing or
destroying film. Another major advantage of the Leica ADS camera system is that
the imagery can be viewed immediately upon landing. With film, the imagery
couldn’t be viewed until after the film was processed 3-4 days after
acquisition. But Leica’s new image processing software, XPro, makes it possible
to immediately “process” the “raw” imagery so it can be viewed (and output as
preliminary unbalanced orthos which we affectionately call “quick orthos”)
within a few hours of acquisition. This is not possible using other sensors.

That said, this is
not the way the system worked “out of the box.” It took much of the first year
working with Leica to fix bugs, hardware problems, and software design issues
before these benefits were realized. In fact, we were forced to lease another
non-Leica ADS image processing package to create orthos on our first projects. In
addition, several firmware components were replaced during the first year that
had major impacts on quality and reliability. The new camera mount, called the
PAV80, also did not work for much of the first year and we were forced to use
an older model that could not take advantage of advanced features of the camera
system.

Aerial Services was
one of the first users of XPro and with that we also had to overcome new
obstacles often encountered with first-generation software.  For example,
there were some issues with the color corrections applied in post-processing
that required difficult and time-consuming manual processing. These issues have
since been resolved.

What
limitations does digital acquisition have?

Storage space! These systems eat hard drives. We purchased 60,000 gigabytes of
storage with the system thinking that would hold us over for a while. We have
since tripled this. Hard drives are quite inexpensive today, but when you buy
this quantity of storage it gets to be extremely expensive. Additionally, the
storage units onboard the aircraft are very reliable solid-state drives. They
are also extremely expensive and still “too small.” We are unable to acquire imagery all day and not have to swap
out full hard drives with empty ones during an acquisition mission. Eventually,
this will be resolved but right now this is a real pain. We could swap out a
roll of film all day for a $1000 per roll. These solid state drives cost about 25
times more, so it is not so easy to have a spare in your back pocket!

After
working with the ADS80 for over a year, what is your verdict?

The decision to purchase an ADS80-SH82 has proven to be an excellent choice for
Aerial Services and in turn for our clients.  The capabilities and
reliability of the system make for faster deliveries, higher accuracies and
better quality for our clients. Knowing what we know today, if we were to make
this purchase decision again, we would go with the Leica ADS80.

To
learn more about Aerial Services, Leica’s ADS80-SH82, or how you can use such a
device on your next project, please contact Aerial Services at 319-277-0436 or
via
www.AerialServicesInc.com.