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Resource Assessment Grand Rapids 1 Aerial Photo - Orthophoto Primer Bill Befort, Remote Sensing Coordinator Resource Assessment Unit, DNR Forestry Grand.

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Presentation on theme: "Resource Assessment Grand Rapids 1 Aerial Photo - Orthophoto Primer Bill Befort, Remote Sensing Coordinator Resource Assessment Unit, DNR Forestry Grand."— Presentation transcript:

1 Resource Assessment Grand Rapids 1 Aerial Photo - Orthophoto Primer Bill Befort, Remote Sensing Coordinator Resource Assessment Unit, DNR Forestry Grand Rapids MN

2 Resource Assessment Grand Rapids 2 Photo Projects Map Large Areas in Great Detail

3 Resource Assessment Grand Rapids 3 But Photos Arent 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.

4 Resource Assessment Grand Rapids 4... maps have uniform scale, projection, orientation, and symbology... whereas photos do not.

5 Resource Assessment Grand Rapids 5 Vertical airphotos have maplike qualities --just dont count on it

6 Resource Assessment Grand Rapids 6... but theyre seldom perfect. They exhibit: Tip ( n ) Tilt ( S ) Swing ( 6 ) and sometimes even (ugh) dIsToRtIoN !

7 Resource Assessment Grand Rapids 7 And they all exhibit Topographic Displacement Oblique view Map Vertical photo (same nominal scale as map)

8 Resource Assessment Grand Rapids 8... 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... its measurable. By measuring the parallax difference of conjugate objects on photos, we can determine their height.

9 Resource Assessment Grand Rapids 9 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.

10 Resource Assessment Grand Rapids 10 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. It was recently scanned, together with photos from 1950, 1956, 1969, 1983 and 1991, to support historical study of the park area.

11 Resource Assessment Grand Rapids 11 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. Thats 1/20,000 th. Theres a lot of redundancy in modern aerial photosthey contain more information than youre likely to use.

12 Resource Assessment Grand Rapids 12 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

13 Resource Assessment Grand Rapids 13 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, were entitled to call the result an ORTHOPHOTO

14 Resource Assessment Grand Rapids 14 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.

15 Resource Assessment Grand Rapids 15 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.

16 Resource Assessment Grand Rapids 16 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 cant deal with topographic displacement. After transformation, the image must be resampled to a regular array. The whole business is perhaps better termed Geometric Correction.

17 Resource Assessment Grand Rapids 17 To handle displacement, we must talk about--

18 Resource Assessment Grand Rapids 18 For purposes of illustration -- Heres 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.

19 Resource Assessment Grand Rapids 19 Heres 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 stereomodels features onto the mapping surface. No more topographic displacement!

20 Resource Assessment Grand Rapids 20 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.

21 Resource Assessment Grand Rapids 21 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 USGSs original T-64 Orthophotoscope.

22 Resource Assessment Grand Rapids 22 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.

23 Resource Assessment Grand Rapids 23 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.

24 Resource Assessment Grand Rapids 24 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!

25 Resource Assessment Grand Rapids 25 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! Minnesotas early DEMs showed washboard-like traces of their derivation from orthophoto scans.

26 Resource Assessment Grand Rapids 26 This obviously couldnt 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.

27 Resource Assessment Grand Rapids 27 So now its 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

28 Resource Assessment Grand Rapids 28 Were skipping over important details --e.g., project layout and control, which have a great deal to do with final cost.

29 Resource Assessment Grand Rapids 29 Expressing Film Resolving Power

30 Resource Assessment Grand Rapids 30 So what? Say you had a 1:40,000 negative at 100 lp/mm, and wanted it all in your computer -- Youd 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.

31 Resource Assessment Grand Rapids 31 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. Theres more data in most airphotos than we can easily deal with.

32 Resource Assessment Grand Rapids 32 Further Reading in Orthophotography Demystifying Advancements in Digital Orthophotography

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