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PHOTOGRAMMETRIC
WORKFLOWS
Traditional, Digital and the Future
Orthophoto Generation and Mosaicking:
Digital Orthophotos are usually the primary
final product derived from the photogrammetric
workflow. There are many
different customer specifications for
orthos, including accuracy, radiometric
quality, GSD, output tile definitions, output
projection, and output file format. A
mosaicking process is usually included in
the ortho workflow to produce a smooth,
seamless, and radiometrically appealing
product for the entire project area.
Mosaicking may be performed as part of
the orthophoto process directly (orthomosaicking)
or performed as a postprocess
later. Generally, orthophoto production
follows these steps:
- Input image selection: The operator
chooses the images to be orthorectificed.
- Terrain source selection: The operator
chooses the DTM to be used for orthorectification.
This is a critical step, as the
accuracy of the orthophoto will be determined
by the accuracy of the terrain. A
terrain model with gross errors (e.g. a hill
not modeled correctly) will result in geometric
errors in the resulting orthophoto.
- Define orthophoto options: Operators
typical select a number of options for the
orthorectification process. These options
may include output GSD, the image resampling
method, projection, and output
coordinates.
Aside from defining the various parameters,
the orthorectification process is not
usually an interactive process. However,
the mosaicking process usually does
involve some operator interaction. After
images are chosen for the mosaic process,
there is usually some method of defining
seams (polygons or lines used to determine
which areas of the input images
will be used in the output mosaic). While
there are many automatic seam generation
applications, there is almost always
some element of user interaction to
either define or edit seams or at least
review the seams. Operators will typically
edit the seams so that they run along
radiometrically contiguous areas. That is,
they do not cut through well-defined features
such as buildings. This is because
the ultimate goal of seam editing is to
“hide” the seams so they are not visible in
the output mosaic. Once seams are
defined, they can usually have smoothing
or feathering operations applied to
them so that their appearance is minimized.
Another important aspect is radiometry.
While some operators will tackle
radiometry early on in the workflow (as
previously discussed in the “Image Dodging”
step), others will dodge or apply other
radiometric algorithms during this
mosaic process. The goal is to make the
output group of images radiometrically
homogeneous. This will result in a visually
appealing output mosaic that has consistent
radiometric qualities across the
group of images.A project area may be several hundred
square kilometers in size, so a single output
mosaic file is not usually an option
due to the sheer size. End customers cannot
usually handle a single large file and
would prefer to receive their digital
orthomosaic in a series of tiles defined by
their specification. Most photogrammetric
systems have a method of defining a
tiling system that can be ingested by the
orthomosaicking application to produce a
seamless tiled output product.
DIGITAL WORKFLOWS
In recent years the introduction of high
resolution satellite imagery and airborne
pushbroom sensors such as the Leica
ADS40 have added new variations to the
traditional workflow. Both types of data
are digital from the point of capture, alleviating
the need to scan film photography.
Commercially available satellite
imagery (e.g. CARTOSAT, ALOS, etc.) has
been available at increasingly high levels
of resolution (e.g. 80cm resolution for
CARTOSAT-2). While this is sufficient for
many mapping projects, some engineering
level project applications still require
the resolution available from airborne
sensors.
Pushbroom sensors such as the ADS40
can achieve a ground sample distance in
the 5-10cm range. Modern digital airborne
sensors are also usually mounted
with a GPS(Global Positioning System)/
IMU (Inertial Measurement Unit) system.
GPS technology assists mapping projects
by using a series of base stations in the
project area and a constellation of satellites
providing positional information
accessed by the GPS receiver on-board an
aircraft.
IMU’s are increasingly used to establish
precise orientation angles (pitch, yaw,
and roll) for the sensor platform in rela-
End customers cannot usually handle a single
large file and would prefer to receive their
digital orthomosaic in a series of tiles defined by their
specification tion to the ground coordinate system.
GPS and IMU information can be
extremely beneficial for mapping areas
where limited ground control information
is available (e.g. rugged terrain).
They also assist in the triangulation
process by providing highly accurate initial
orientation data, which is then further
refined by the bundle adjustment
procedure. GPS and IMU information can
also be used for “direct georeferencing,”
which bypasses the time-consuming AT
process. The caveat to direct georeferencing
is that project accuracy may suffer;
however, this may be acceptable for rapid
response mapping and other types of
projects where lower accuracies are adequate
for the end customer.
FUTURE PHOTOGRAMMETRIC WORKFLOWS
We are beginning to see some shifts in
the currents guiding photogrammetric
workflows. These shifts are being driven
by advances in computing hardware,
new sensor technology, and enterprise
solutions.
Data storage and dissemination is a
dynamic area in the industry. While
imagery was traditionally backed up on
tape systems, the cost of storage has dramatically
declined in recent years. As customer
demand for high-resolution data
increases, it is becoming less practical for
users to store data directly on their workstations.
Users are increasingly storing
imagery on servers, employing different
methods for accessing it. There is a growing
demand for tools to manage and
archive data. Users are also examining
the possibility of publishing data stored
on servers and published on web or client
applications such as Google Earth.
Sensor hardware is also rapidly changing
the photogrammetric workflow.
LIDAR has now been widely adopted and
accepted, providing extremely high-density
and high-accuracy terrain data. In
addition to LIDAR, there is a growing
trend of integrating LIDAR with digital
frame sensors, enabling rapid digital
ortho processing.
When coupled with airborne GPS and
IMU technology, terrain and georeferenced
imagery (the primary ingredients
for orthos) can be available shortly after
the data is downloaded after a flight.
Interferometric Synthetic Aperture
RADAR (IFSAR) mapping systems are also
a growing source of terrain data.
Along with the explosion of imagery is
the need to efficiently process it. One
method that various vendors have begun
exploring is distributed processing. In
this model, a processing job is divided
into portions, and then submitted to
remote “processing nodes.”
This results in a significant improvement
in overall throughput for large projects.
Most commercial efforts, such as the
Leica Ortho Accelerator (LOA), have
focused on ortho processing. However
there also several other photogrammetric
tasks that lend themselves to distributed
processing solutions (e.g. terrain correlation,
point matching, etc.).
With data increasingly stored on network
locations and the general adoption
of database management systems, enterprise
photogrammetric solutions will
likely change the face of the classical photogrammetric
workflow. With images
and GIS increasingly stored on servers,
the processing framework is likely to
change such that the operator interacts
with a client application that kicks off
photogrammetric and geospatial processing
operations. Rather than running a
“photogrammetric workstation” the
operator will be operating a client view
into the project. Also, geospatial servers
will enable organizations to store and
reuse project and other data. For example,
automatic correlation processes could
automatically identify and utilize seed
data stored in online databases, or terrain
data stored from previous jobs.
The notion of collecting data once and
using it many times will be prevalent.
Large quantities of data such as LIDARderived
point clouds should be able to be
stored, with operations such as filtering,
classification, and 3D feature extraction
applied. With a shift to enterprise solutions,
industry adoption of open standards
like The Open GIS Consortium, Inc.
will be critical.
Providing open and extensible systems
will allows users to customize workflows
to meet their specific needs, fully
enabling their investment in enterprise
technology.
CONCLUSIONS
This is an exciting time for those of us in
the photogrammetry group at Leica Geosystems. The recent trends discussed
have opened up new avenues for changing,modernizing, and empowering a
recently relatively static workflow. Our customers drive us to deliver solutions
that meet a variety of needs. While there is the constant need to pay attention to
existing workflows, it is important to pay close attention to technology trends that
will guide future workflow directions.Enterprise photogrammetric solutions will
likely change the face of the classical photogrammetric workflow
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