Aerial Photo - Orthophoto
Primer
Bill Befort, Remote Sensing Coordinator
Resource Assessment Unit, DNR Forestry
Grand Rapids MN
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Photo Projects Map Large Areas in
Great Detail
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But Photos Aren’t Maps -A map is an orthographic view, and shows every object as if from
directly above it . . .
whereas
even a
perfectly
vertical
airphoto is a
perspective
view from a
central
point.
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. . . maps have uniform scale, projection,
orientation, and symbology
. . . whereas photos do not.
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Vertical airphotos have maplike qualities
--just don’t count on it
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. . . but they’re seldom perfect. They exhibit:
!
Tip (n)
Tilt (S)
Swing (6)
and sometimes
even (ugh)
dIsToRtIoN
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And they all exhibit
Topographic Displacement
Map
Vertical photo (same
nominal scale as map)
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Oblique view
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. . . which is bad, and good.
Topographic displacement (the effect of perspective) is the least
tractable obstacle in turning a photo into a map, BUT . . .
it lets us view
overlapping
airphotos
stereoscopically
(i.e. in three
dimensions), and
better still . . .
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it’s measurable. By measuring the “parallax
difference” of conjugate objects on photos, we
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can determine their height.
Good aerial cameras have been around for
some time . . .
• Metrogon-lens cameras
like this CA-8 were in
active use till the mid1980s.
• Their optics matched
the Kelsh Plotters much
of the U.S. was mapped
with: such distortions as
the Metrogon lenses
introduced, the Kelsh
could remove in plotting.
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They Made Great Photos
This 1939 photo of a
portion of St. Croix State
Park typifies the 1:20,000
scale coverage of rural
Minnesota taken in the first
generation of ASCS
photography.
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It was recently scanned,
together with photos from
1950, 1956, 1969, 1983
and 1991, to support
historical study of the park
area.
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Cameras and Films Are Better Now
This 80kb JPEG of a DNR
color infrared airphoto in the
St. Croix Park area contains
only .000005 of the total
possible information content
of the original 9x9” film it was
taken on. That’s 1/20,000th.
There’s a lot of redundancy in
modern aerial photos—they
contain more information than
you’re likely to use.
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Modern Aerial Cameras
The films are better, and so is everything else
• Resolving power
greater than 100 lp/mm
• Forward motion
compensation
• Distortion-free
• GPS-controlled shutter
• Gyro mount can be
stabilized within a degree
of vertical
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If we want to turn an aerial photo into a map:
We must deal with --
• Camera orientation (tip, tilt, swing)
• Optical distortion, if significant
• Topographic displacement, if significant
If their effects can be reduced to within the relevant map
accuracy standards, we’re entitled to call the result an
ORTHOPHOTO
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If tip, tilt and swing are the main problems . . .
They can be dealt with by an
expensive photogrammetric
projector with a table and lens
that can be tilted and turned to
reproduce and remove the effects
of these camera misorientations
and bring the print to the desired
over-all scale. This is called
PLANE RECTIFICATION.
The projector lens may also
compensate for camera lens
distortions. But topographic
displacement remains.
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For example -Back in the 1950s some
MN photo projects were
flown with cameras
pointed obliquely fore
and aft. The negatives
were then printed on a
rectifying projector to
make sections come out
square at the desired
scale. The frames
became trapezoids.
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In the digital image world, “rectification”
is a term often loosely applied to a
process analogous to plane
rectification, in which a mathematical
transformation is applied to rotate,
warp, stretch or “rubber-sheet” a
digital image to match a set of known
ground control points. As with plane
rectification, this process (if the
transformation is properly chosen)
can compensate for systematic effects
like orientation and scale, but can’t
deal with topographic displacement.
After transformation, the image must be “resampled” to
a regular array. The whole business is perhaps better
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termed Geometric Correction.
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To handle displacement, we must talk about--
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For purposes of
illustration -Here’s an old Kelsh
double-projection
stereoplotter. Most
U.S. topographic maps
were created on this
type of instrument.
Stereoplotters were
invented to deal with
topographic
displacement.
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Here’s how it works -When the projectors are properly adjusted in:
• Interior orientation (same inside geometry as
the cameras)
• Relative orientation (same tip, tilt, swing the
cameras had)
• Absolute orientation (leveled with the
mapping surface)
Then as long as the tracing table is
kept in contact with the surface of the
stereomodel, the pen orthographically
traces the stereomodel’s features onto
the mapping surface. No more
topographic displacement!
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Well, then -we can record a copy
of the original photo
bit by bit, with all its
topographic
displacement removed.
The process is called
DIFFERENTIAL
RECTIFICATION,
because each bit of the
photo gets its own
special treatment.
If we can contrive to put a piece of film on the
tracing table, and keep it in constant contact
with the oriented & projected stereomodel . . .
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All sorts of really wild instruments . . .
were invented for
doing this on the
fly. Making
orthophotos this
way brought a new
world of meaning
to the term “handeye coordination.”
This is the USGS’s
original T-64
Orthophotoscope.
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A French approach
. . . to the problem of
orthophotoplotting is
seen in this Engins
Matra model. Its
kinship to the Kelsh
type of doubleprojection stereoplotter
is obvious.
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Film table of Matra plotter
. . . shows the track in which the aperture moves to expose the film.
Telescoping
table legs keep
the aperture in
contact with the
stereomodel.
The entire
aperture track
steps across the
width of the
film to record
successive
swaths of the
orthophoto
image.
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Moving right along here . . .
-- because film was being continuously exposed! As the aperture
was mechanically cycled in X and Y directions across the film bed,
the operator was on
his mettle to keep it
continuously in
contact with the
surface of the
stereomodel by
raising and
lowering the entire
film table in the Z
direction!
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Orthophoto Byproduct
And as successive rasters of an
orthophoto were scanned, the
combined XYZ movements of the
aperture traced the topography of
the stereomodel. Once methods
for recording these movements
were perfected, they could be
turned into contours or -- hey! --
Digital Elevation Models!
Minnesota’s early DEMs showed
washboard-like traces of their
derivation from orthophoto scans.
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This obviously couldn’t go on . . .
it wore out operators too fast. About this time computers
came to the rescue -• First, with “off-line” ortho scanning, in which the operator
scanned the model at his own speed and “played back” the
recorded XYZ movements to film the orthophoto.
• Then later with digital orthophotography, which turned
everything on its head. Once the technology became available to
move pixels around to their correct positions electronically rather
than photographically, the DEM became, for most users, the
driver of the process rather than its by-product. Orthophotos are
now typically made by matching a photo with a pre-existing DEM
in a computer. Of course the DEM still ultimately derives from
some form of stereoplotting.
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So now it’s all different -And thanks to fast computers, all sorts of people who never heard of
double-projection stereoplotters are busy creating orthophotos.
These days the necessary inputs (besides scanned photos) are:
• Ground control points
• Camera calibration
parameters
• An adequate DEM for
the area covered
• Photogrammetric
software
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We’re skipping over important details
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--e.g., project layout and control, which
have a great deal to do with final cost.
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Expressing Film Resolving Power
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So what?
Say you had a 1:40,000 negative at 100 lp/mm,
and wanted it all in your computer -You’d need a scan
aperture of 1/200 mm,
or 5 microns. The
pixels would measure
8” on the ground . . .
And the file, for a color image, would be about 6.3 gigabytes.
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How about orthophoto resolution?
How much can you afford? A 50 lp/mm negative contains 1.5gb of
potential information. Even an 800ppi scan (right, below) contains
only 1/10 of the total. There’s more data in
most airphotos than we can easily deal with.
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Further Reading in Orthophotography
Demystifying Advancements in Digital
Orthophotography
http://spatialnews.geocomm.com/features/surdex1/
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