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Resource Assessment

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Aerial Photo - Orthophoto Primer

Bill Befort, Remote Sensing Coordinator

Resource Assessment Unit, DNR Forestry

Grand Rapids MN

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Resource Assessment

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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 ()Tilt ()

Swing ()

and sometimes even (ugh)

dIsToRtIoN

!

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And they all exhibitTopographic Displacement

Oblique view

Map

Vertical photo (same nominal scale as map)

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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 . . .

it’s measurable. By measuring the “parallax difference” of conjugate objects on photos, we can determine their height.

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Resource Assessment

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Good aerial cameras have been around for some time . . .

• Metrogon-lens cameras like this CA-8 were in active use till the mid-1980s.

• 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 PhotosThis 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.

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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Resource Assessment

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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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Resource Assessment

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Modern Aerial Cameras

• 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

The films are better, and so is everything else

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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 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 -- 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 . . .

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.

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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 “hand-eye 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 double-projection 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

--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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http://spatialnews.geocomm.com/features/surdex1/

Further Reading in Orthophotography

Demystifying Advancements in Digital Orthophotography