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Demystifying Advancements in Digital Orthophotography Brian MayfieldProposal Coordinator / GIS Mapping
Scientist
Surdex Corporation520 Spirit of St. Louis
Blvd. St. Louis, MO 63005 Phone: 636-532-3427 Fax: 636-537-9638
Abstract: Recent market analyses indicate that more and more GIS professionals are utilizing digital orthophotography for their base mapping needs. This paper examines the latest technological advancements in digital orthophotography production. These advancements are sometimes misinterpreted, misunderstood, and sometimes alienated by the GIS community when procuring orthophotos. Ultimately these advancements provide GIS professionals with a cost-efficient orthophoto basemap solution without jeopardizing accuracy, image resolution, or overall quality. The goal of this paper is to provide the GIS data purchaser with descriptions of recent production enhancements and how they can reduce project costs while yielding an improved product quality. Introduction All technology-based industries are in a continuous state of evolution; in photogrammetry, emergent technologies provide photogrammetrists with an innovative means of providing equivalent or improved mapping data at a significant cost or scheduling savings. The object of the reputable photogrammetrist is to evaluate these new technologies, testing them against the accuracy of known and trusted methodologies, and to ensure that any cost and scheduling savings do not come at the cost of product accuracy or precision. This paper will discuss the viability of five recently emerged technologies that significantly will impact the course of digital orthophotography Prior to incorporating any new technology into its production process, Surdex subjects new technologies to extensive and thorough testing; to reduce errors in findings owing to small test samples, testing is repeated until findings can be conclusively demonstrated. With over 40 years of research and development experience, Surdex has established a 1700-Acre Test Site in St. Louis County, Missouri. This site, known as the “Ballwin” Test Site, has over 150 known vertical points located on photo-identifiable black iron manhole covers. This test site is controlled horizontally by eleven (11) known surveyed points around the perimeter of the project. These control points were established to First Order accuracy. Surdex has used the Ballwin Test Site to test the five evolving technologies that will later be discussed Surdex utilizes Statistical Process Control (SPC) to validate and test
emerging technologies within the photogrammetric industry. Overall, product
accuracy is the statistical summation of the errors associated with each of the
parts of the production processes. The equation below allows us
The
accuracy or quality of the final mapping product can be assured by limiting the
errors at each of the production steps.
Surdex eliminates the causes of production problems systematically, and
verifies the quality of the products statistically. When testing new technologies, we monitor each project based upon
the stipulated specifications and statistically verify that our resultant
mapping products remain well within known accuracy tolerances. The remaining sections of this paper will evaluate five recent technological advancements, demonstrating their viability as trustworthy tools for the photogrammetric industry. Airborne
GPS Control Photogrammetric
control provides the foundation for spatial orientation or referencing of
aerial imagery to the Earth’s surface, thereby allow the imagery to be used in
the photogrammetric compilation of topographic and planimetric mapping
features. The accuracy of the photo
control solution is dependent upon the desired accuracy specifications of the
resultant mapping products. Since the ground control solution provides the
framework and accuracy of the entire process, this initial step is the most
critical. Traditionally,
the photogrammetric control solution was accomplished through the establishment
of a basic network of “photo-identifiable” ground control points with known
horizontal and vertical values within a specific ground coordinate system.
These points have known coordinates (x, y, and z) on the Earth’s surface and
can easily be identified in the aerial imagery. For some large scale mapping, it was often necessary to have a
ground control point in at least every third or fourth stereo model. In those cases, a countywide project would
often require as many as 1000 control points for the photogrammetric control
solution. Surdex was a pioneer of Airborne GPS collection for
photogrammetric control, having undertaken experimental research with Iowa
State University and with our first project in 1991 for the USGS. Since that time, Surdex has routinely used this
technology to support, and often to supplant conventional ground control for
the Corps of Engineers, USGS, USAF, FAA, and County/Municipal photogrammetric
survey and mapping projects. Although
Airborne GPS is not a new technology, it is often misunderstood and its
viability in the photogrammetric process is often questioned by many
non-photogrammetrists. Digital
Orthophotography is a perfect application for Airborne GPS. The airborne collection of photo-center
control data allows the user to collect an extremely dense data set of
photogrammetric control in a very short period of time. Essentially, a photo-center control point is
established for each frame of photography flown during the mission, as opposed
to establishing control points spanning three to four models utilizing
conventional ground control. During
the photo mission, a minimum of three GPS receivers are simultaneously
collecting GPS observation data at 1-second intervals. The airborne unit collects both at
one-second and at an event epoch, which is the center-point of the exposure
cycle for each frame. Two receivers, or
“base stations,” are generally co-located with previously established, or to be
established, high accuracy ground control points. A third receiver, the “rover” is located in the aircraft and
utilizes an L1/L2 antennae which has been surveyed into the aircraft’s coordinate
system, all referencing the nodal point of the aerial camera’s lens and known
focal length. Surdex
has independently executed specific experiments to determine the accuracy of
Airborne GPS as solitary control for a project as well as evaluating the
affects of minimal perimeter conventional ground control on the vertical
accuracy's. As a result of these tests, Surdex has determined that Airborne GPS is adequate for horizontal control. Even without additional ground control, Airborne GPS data does not reflect any serious effects of deformation in the triangulation. The expected problem would result in tortional deformation longitudinally along the flight line, since airborne control is collected along a straight line through the air. However, there is enough excursion variation, such as drift and altitude change, along the flight line to suppress any adverse effects. Additionally, the geometric strength of normal strip triangulation minimizes the tortional deformation as well. The combination of triangulation and flight variation guarantees elimination of the problems expected from linear collection in the air. Scanning from Aerial
Negatives
In
order to be used for digital orthophotography, any aerial photograph exposed in
an analog camera must be translated into digital format. This is accomplished by scanning the aerial
images with a radiometrically and geometrically precise digital aerial film
scanner. When scanned, each pixel of
the aerial image consists of a radiometric value plus an XY coordinate set
unique to that image. Until recently the most accepted methodology to do this
was to scan from film diapositives; a diapositive is a second-generation
positive reproduction of the original film negative with positive tone. Once
scanned, the photogrammetrist develops a histogram to adjust radiometric
contrast to insure that the raw data has a more pleasing overall image tone. The
problems associated with this process are three-fold. First, scanning from a second-generation product creates a third-generation
scan. It should be noted that each
replication of an image is destructive to the overall accuracy, precision, and
quality of the original aerial image. Imagine taking a photocopy of a photocopy
of a photocopy … it’s easy to visualize the rapid loss of resolution. When
photogrammetrists scan from a film diapositive, they are using a “copy of a
copy” as their source material, the source material upon which the entire
basemap is based. Secondly, the process of scanning from a film diapositive is
extremely laborious; the photogrammetrist must load each individual diapositive
into a single frame flatbed scanner.
Conversely, technicians can scan continuous aerial photography film
directly from the spool in an automated fashion. Consequently the costs for
scanning from hand-placed diapositives, in comparison to scanning direct from
film, is significantly higher. Finally, the chance of image degradation is
increased significantly due to processing blemishes or excessive handling of
film diapositives. To
combat the problems associated with scanning from second-generation film
diapositives, Surdex scans directly from the archive-quality aerial negatives
with our Vexcel VX4000 Digital Imaging Systems, radiometrically and
geometrically precise digital aerial film scanners. The VX4000 is capable of scanning an entire roll of aerial film
at one time, significantly automating the digital imaging process. The VX4000 will accept rolls of film up to
500’ in length. It has a Geometric
Accuracy of 1/3 pixel RMS. For example,
the geometric accuracy of a 7.5-micron scan will be 2.5 microns. The Radiometric Accuracy is 2 gray values
RMS, and the pixel depth is 8 bits monochrome and 24 bits color (8 bits per
color). Our
testing universally affirmed our premise that scanning directly from film
improved the quality of the scans, plus all subsequent mapping products, as
well as significantly improved schedules. Additionally, the Vexcel scanners
allow us to preprogram scanning requirements for a full 500’ roll of aerial
film. Histogram and scanning parameters
are applied to the entire roll insuring that the project imagery is
radiometrically precise. In some cases,
it may be necessary to scan individual frames as opposed to the entire roll.
This process too can be programmed, thereby yielding the same time and cost
savings. Direct Camera Orientation
Measurements
The purpose of aerial triangulation in the photogrammetric production process is to establish precise and accurate relationships between the individual photographic film coordinate systems and a defined datum and projection. This relationship is used to link the ground surveyed control points via photographic measurements. This activity is often termed “geo-referencing” of the aerial imagery. Traditionally, the photogrammetrist takes photographic measurements with precise photogrammetric instrumentation to be processed through computer algorithms; this process is designed to eliminate blunders and systematic errors from the data while minimizing the residual random errors within the entire triangulation network. The result of the triangulation is a densified set of ground control points that are used to control the remainder of the mapping process. The triangulation can be thought of as an interpolation process where a dense pattern of ground control points, known as pass points, are interpolated from a sparse pattern of surveyed ground control points and Airborne GPS (ABGPS) control points. Surdex
employs the POS/AV 510-DG Applanix device for direct geo-referencing of aerial
photography. When used in conjunction
with Airborne GPS exposure station control and aerial camera systems, the
inertial measurement unit (IMU) of the device measures the camera’s six
exterior orientation parameters to an accuracy that enhances and in some cases
even eliminates the fully analytical aerial triangulation process. The POS/AV
510-DG provides highly accurate position and orientation data specifically for
airborne photogrammetric applications.
It uses strapdown inertial sensors, Kalman filtering and Airborne GPS,
to measure the aerial camera’s position (x, y, and z) and orientation (roll,
pitch, and yaw) to an accuracy at the level of 0.328 feet RMSEposition and
20 arc seconds RMSEorienation or better on the ground from an altitude
of 3280’ without aerial triangulation. Some
of the benefits of using an Applanix device in conjunction with Airborne GPS
exposure station control for direct geo-referencing of aerial photography
include: · Minimum number of ground control points required · No required analytical aerial triangulation in some cases depending on final product accuracy requirements · Higher level of horizontal and vertical accuracy and precision for photogrammetric mapping · Expedites mapping process by as much as three months for large, multi-faceted projects This is of great benefit to our clients by
significantly reducing project time and cost.
Softcopy
Photogrammetry
Photogrammetry is the art and science of determining qualitative and quantitative characteristics of features from spatial images recorded on photographic emulsions. Many industries, including the GIS community, rely heavily on photogrammetrists to provide accurate and precise mapping data in a fixed or transitory state. As mentioned before, photogrammetry, like other information-based technologies, is in a constant state of evolution. New instrumentation and techniques are introduced into the photogrammetric community on almost a daily basis. Traditional photogrammetric procedures were performed on analog or analytical stereoscopic instrumentation. These procedures include the point measurement activities described earlier as well as the stereoscopic compilation of topographic and planimetric mapping features. The remainder of this section will focus on the aerial triangulation and stereo-compilation processes and the new technologies used for each. SoftCopy Aerial
Triangulation In
a conventional hardcopy triangulation process a set of contact prints and film
diapositives are printed from the original negative film. These diapositives
are placed into a precise stereoplotter, C-120, for point measurement. The control planning phase follows
printing. In this phase the contact
prints are laid out and all the required tie, pass and control points are visually
located, marked with the appropriate symbols and numbered. Once completed on the contact prints, the
point markings are transferred to the film diapositives. The
next phase of the triangulation process is the “pugging” process. During this process the film diapositives
are placed in a Wild PUG instrument for stereo viewing. After alignment of the
diapositives is complete, the pugging device accurately drills the pug mark
into each of the diapositives at the desired location when activated. This process is repeated for all pass and
tie points in the block of photography. The
final step in the hardcopy aerial triangulation process is the measurement of
the required points on the film diapositives.
This process is called mensuration.
During mensuration, the film diapositives are placed onto the film
stages of a first order analytical stereoplotter. Once loaded into the
instrument the operator goes through an interior orientation process. This process involves reading the fiducial
marks on each frame of the film in a monoscopic mode. Once measured, software within the plotter performs the interior
orientation. This interior orientation
correlates the precise film coordinate system defined by the fiducial marks
with the precise analytical plotter measurement system defined by the stages
and measurement encoders within the plotting equipment. With interior orientation complete, the operator can move to the required tie, pass and control points and make measurements. This process is similar to process that was performed during the pugging process. The output of this mensuration process are precise film x and y values for each required point that will be used as input to the fully analytical aerial triangulation. The
softcopy aerial triangulation process
begins with precise scanning of the negative aerial film (described
earlier). This process produces the
digital imagery that will be manipulated during the softcopy mensuration
process. The aerial film is scanned
with a precision photogrammetric scanner.
Surdex utilizes a Vexcel VX4000 roll film scanner. The nominal scanning resolution for
photogrammetric mapping purposes within Surdex is 15 microns. This scanning resolution was derived after
extensive experimentation at Surdex.
Measurement precision tests were performed with variable scan
resolutions on variable scales of aerial photography. The goal of the testing was to determine an appropriate scan
resolution that did not reduce the measurement precision. The scanned imagery is converted from a
negative to a positive (similar to the film diapositive) and digitally dodged
to balance the exposure. The
next phase of the softcopy triangulation process is the mensuration
activity. Surdex uses the SoftPlotter
Version 1.8 digital photogrammetry suite of software from Autometric,
Incorporated to perform this mensuration.
The first function performed by the operator is to set up the project
parameters. This involves selecting the
appropriate camera calibration files, project coordinates and digital image
file locations. Once setup, the
interior orientation process is performed.
Due the unique characteristic of the fiducial marks on the digital
imagery, this process is automatic. All
eight image fiducials area measured and the interior orientation performed
automatically by the software. The software is controlled by processing
parameters selected by the operator to flag any fiducial measurements and/or
orientations that exceed the defined limits. The outcome of this process is a
defined mathematical relationship between the digital image pixel coordinates
system and the calibrated aerial camera film coordinate system. This process also accounts for any geometric
errors introduced by the scanning process.
The
next phase of the softcopy mensuration process is to perform the tie points
measurements. This process is also
initially performed automatically.
Based upon the operator’s input parameters for point distribution, the
SoftPlotter B-TIE software digital correlation to derive the required point
locations. Once completed, the operator
is presented with a graphical representation of the digital images with the
measured points overlaid. The operator
can then interactively measure any additional points to obtain the appropriate
density. At this time the ground
control target locations and strip to strip pass point locations can be
interactively measured by the operator.
The outcome of this process is a set of image coordinates, x and y, for
each required ground point on each frame in which the point appears. The
final activity performed by the operator in the softcopy mensuration process is
to run an analytical block adjustment on the measurement data. This process is performed to evaluate the
quality of the measurement data and locate any measurement errors. The SoftPlotter BlockTool aerial
triangulation software is utilized for this process. BlockTool has a data snooping function that flags any point measurement
that is identified in the block adjustment to be more than a set number of
standard errors. These flagged points
are revisited by the operator and remeasured or deleted from the measurement
data. The final output of this process
is a set of image x and y values that will be used as input to the final processing
of the fully analytical aerial triangulation. Review
of the description presented in the previous section on hardcopy mensuration
clearly displays the amount of detail and repetitive operations that are
performed during the measurement process in aerial triangulation. Recent advancements that have been made in
computer hardware and software have enabled the hardcopy mensuration process to
be replaced with softcopy mensuration.
softcopy mensuration involves similar processes to those performed in
hardcopy mensuration except that they are performed on digital imagery of the
aerial film, and most of the redundant operations are performed by the
computer. Softcopy Compilation The first step involved with photogrammetric compilation, whether it is on an analytical stereoscopic instrument or a digital softcopy workstation, is the stereo model setup. A stereo model is a three-dimensional projection of the Earth’s surface in a particular geographic area at a known scale. This projection allows the photogrammetric technicians to reconstruct the scene from a particular geographic area at the time of exposure in a three-dimensional environment. The sequential steps involved with setting up the stereo model are the following: ·
Acquisition
of Film Diapositives for stereo compilation on analytical instrumentation or
Digital Scans of aerial imagery for stereo compilation on digital softcopy
workstations. ·
Interior Orientation – The interior orientation
process is the mathematical introduction of any known systematic errors induced
by equipment limitations, the Earth’s curvature, and atmospheric
refraction. Items such as the camera’s
focal length, fiducial coordinates, as well as the radial and tangential
distortion are introduced into the stereo model during this process. ·
·
Absolute Orientation – Absolute Orientation can
be thought of as the mathematical process of introducing Ground Control
Coordinates into the stereomodel. This
process constrains the stereo projection to a particular position on the
Earth’s Surface at a fixed scale. Two
sub-processes are involved in the absolute orientation of a stereo model known
as scaling and leveling, both of which introduce coordinate information to
mathematically solve for ground scale and tilt of the stereo model. Through feature extraction tests and analysis at our Ballwin Test Site, Surdex has concluded that the softcopy compilation environment can only expand our production capabilities and should not yet replace the traditional analytical stereoscopic instruments. Our research found the softcopy environment to be extremely beneficial to data update projects. The softcopy environment allows for simultaneous superimposition of old vector mapping data over newly acquired aerial imagery, providing the operator with a visual interpretation of physiological or cultural changes in a particular area. Additionally, due to the epipolar projection in the softcopy environment, the operator can see the sterographic image with more ease than on the analytical instrumentation. Unfortunately, only perfectly acquired stereoscopic imagery is cleanly viewable with common off-the-shelf stereoscopic viewing systems. Thus, softcopy photogrammetric exploitation systems require the generation of stereoscopic imagery in an “epipolar-resampled” form. The epipolar geometric condition stipulates that the nodal point of the lenses of the two frames and the ground point must all lie within a plane. Using this condition and the exterior and interior orientation information for each frame, the source images of a stereopair can be resampled (“warped”) into an epipolar presentation for perfect viewing. In an epipolar-resampled stereopair, corresponding lines on each image lie on an epipolar plane. Thus, differences in conjugate point pixel coordinates within a line directly correspond to a change in depth in the stereomodel. If the stereopair is vertical in nature, then the depth nearly directly corresponds to a point’s elevation on the ground. Generation of stereopairs, though a necessary step in the softcopy compilation process, only take a few minutes to process and result in roughly 50% increase in the workstation disk storage. Given the fast generation time, these can be created and deleted as needed. LiDAR
Traditionally, the terrain surface used in the production of a digital orthophoto has been photogrammetrically compiled with either an analytical or a softcopy stereoscopic instrument. Now, in some cases, the terrain surface may be created using an airborne system known as LiDAR (Light Detection and Ranging). LiDAR systems can be extremely useful for a number of limited applications by providing rapid, accurate ground surveys at an economical price. Airborne LiDAR acquisition systems utilize a laser
ranging device to collect distances to points below a moving aircraft as a
basis for quick and accurate collection of ground elevation data. The laser
ranging principle is not unlike the technology used in surveying
distance-measuring systems and in determination of automobile speeds. At
closely-spaced intervals, the LiDAR system measures range and intensity
information as the beam is scanned perpendicular to the aircraft flight
direction forming an array. For each pixel in the array, the system thus returns
an extremely precise distance to each feature “visible” to the LiDAR device.
When this range information is combined with precise position (x, y, and z) and
attitude (roll, pitch, and yaw) data collected by the GPS and IMU package, a
digital surface elevation model can be created. The GPS information provides
the LiDAR position at each instant in time and the IMU information determines
the systems roll, pitch, and yaw. Though the LiDAR returns may be absorbed or
scattered by some types of ground cover and objects, the accuracy of the range
information is typically on the order of centimeters. Post-processing of the
LiDAR data includes reducing range information to elevations, removal of
erroneous returns, and filling of areas with no returns. Given the extremely
small angular field of view of the array, the data is very closely equivalent
to an orthogonal (vertical) view of the surface at a nearly uniform grid
pattern on the ground. Since the LiDAR elevations are of a surface nature,
as opposed to “bare earth elevations”, and because a very dense set of points
result, precise digital orthophotos can be generated. Additional processing of
the LiDAR elevations can be done to remove man-made and vegetative features
from the data to achieve a digital elevation model suitable for contour
generation. One problem associated with a LiDAR system is the
overall fluctuation of positional accuracy, depending on land type and
vegetation. Another problem associated
with LiDAR is that the data blocks produced from the system create an enormous
strain on a computer’s resources. We
have found data blocks for relatively small geographic areas to be as large as
8 gigabytes. The file size also creates
a problem when developing a project-wide surface model to create the orthoimages. It is nearly impossible to perform matching
and to combine the data blocks due to the extremely large file sizes. SUMMARY The photogrammetrist’s ultimate goal is to achieve the highest degree of precision and accuracy possible, providing the end user with the most reliable data possible. Balancing this objective is the end user’s time constrains and budgetary limitations – both extremely real factors in the formula. The photogrammetric advancements described herein, we conclude after exhaustive testing and scrutiny, provide end users with significant cost and scheduling rewards for most types of projects while providing equivalent or even improved mapping accuracy. These innovative technologies are in the process of becoming the new “traditional” technologies, stepping into their role as cornerstone technologies in the production of dependable and accurate mapping products. Works Consulted Hutton,
Joseph J., Lithopoulos, Erik, 1998. Airborne Photogrammetry Using Direct Camera
Orientation Measurements.
Photogrammetrie, Fernerkundung, Geoinformation 6, 363-370. Li,
Xiapeng., Baker, Bruce A., 2000. Analytical and Digital Photogrammetric
Technologies In A Mapping Production Environment. American Society of Photogrammetry and Remote Sensing (ASPRS)
Conference Proceedings. Moffitt,
Francis H., Mikhail, Edward M., 1980. Photogrammetry. Third Edition.
Harper & Row Publishers, Inc. New
York. US
Army Corps of Engineers. 1993. Engineering and Design Photogrammetric Mapping. Engineer
Manual No. 1110-1000. |
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