The land in 3D: resuscitating stereoscopy

Even brightest tablet screens are a challenge for 3D viewing when sun is shining. Fortunately, most days here, that’s not a problem.

A venerable art  It used to be that every forester and field geologist was proficient in stereo-photo interpretation. The digital age has briefly distracted us, but a few cartographers are bringing it back. Evolving stereoscopy—with tools such as flicker glasses and new measuring capabilities—will be way more immersive and powerful.

Above: Old-fashioned pocket stereoscope over 7-inch tablet, which has room enough to see an entire 3D pair. While tablets are best for classroom use . . .  ●  Below: . . . I’ve recently decided my much smaller phone works fine if rotated into ‘landscape’ position. ‘Spread-zooming’ to maximum separation your eyes can deal with (~70mm ), we may no longer see the entire stereogram, but can pan around to examine any area of interest. Here, we’re viewing the 1926 stereopair for Chas’héeni (Sheep Creek) directly from JuneauNature. Stereo works well in Zoom presentations if your listeners place stereoscopes over their on-line tablets. They can even see the presenter’s cursor, pointing out features on 3D terrain.

How-to  If you properly size stereo-pairs such as these examples on your computer monitor (about 70 mm across either of those paired images), you can hold a pocket stereoscope to the screen and the landscape will ‘pop up.’ But those lenses typically magnify by 2x, and at that power, on most monitors, you’ll see the pixel-grid, giving a disappointingly grainy view. Phones and tablets, however, (and some higher-res laptop screens) have smaller pixels that don’t show under 2-power blow-up. Magnified stereo airphoto viewing is beautifully suited to the high resolution and rich color on Android and Apple phones and tablets. In fact, it’s the only use that really justifies their ridiculously high pixel density.

Free viewing  Okay, so you don’t have a pocket stereoscope at the moment. If you can see the hidden ‘pop-up’ images in those magic-eye 3D picture books, your eyes are capable of fusing these paired images into a stereoscopic model, without the aid of optical devices.. Bring one of the pairs below up on your phone. Rotate it into landscape position to fill your screen. Hold it about a foot away and focus your eyes on something distant, over the top. Slowly drifting back onto the screen, try to relax your gaze, preventing a ‘convergent focus’ on the phone itself. ‘Pull’ the 2 images together. The moment they completely overlap, a terrain model will pop up.

Field use For stereo-viewing, no printed color photos can match the resonance of a backlit aerial on a phone or tablet. In future posts, I’ll share some of the ABCs of creating and using stereograms. For now, I’ll only note that stereo-interpretation has been a key tool in my recent consulting work, thanks largely to phones and tablets. Although slightly less convenient in the field than my little 3D slide viewers (relicts from the days of film photography), I frequently stop for a stereo-orientation during forest and wetland bushwhacks.

3D at home  Stereo is handy for post-field processing and review. Back in camp or office, I drop the day’s GPS track, waypoints, and autolinked photopoints onto one of the 2 paired images. Then, habitat-mapping is enhanced by an order of magnitude more spatial information than when tracing polygons over a flat, 2D image.

Bog and scrub forest on southern Tàan, sea lion (Prince of Wales Island). Yellow line is GPS track. Dots are photopoints, auto-linked in Robogeo. Studying topography and canopy texture in 3D back at camp, we were better able to delineate patches of upland forest (red tint) from wetland matrix.

From 30,000 feet to 3 centimeters Stereo adds information to all scales of imagery, from ultra-high-elevation NASA flights at inch-to-mile scale, to creek-skimming Beaver flights that show individual alder branches, to macro-3D of red mites traversing moss-jungles. Drones can acquire high detail stereo unavailable from traditional aircraft. My first experiments were on Robin Trib, shooting from 150 feet above uplift parkland, with a Ricoh GR pocket camera strapped to a quadcopter and firing in interval mode.

Stereo and drones  I shoot almost exclusively video from my drone. But from 4K mp4s, screengrabs are usually fine for inserting into journals and reports. There are 2 interesting ways to create stereopairs from drone video. One is from a flyover with nadir (straight downward-facing) camera. The other is from an oblique camera in sideways-crabbing flight. From either of these video segments, extract your screengrabs just a second or so apart. How long to wait between extractions depends upon flight speed and distance to target.  (2023  In addition to video, I now fly preprogrammed missions collecting hundreds of overlapping nadir still images that are processed into orthophotos and terrain models. These images are also great for creating stereopairs)

Stereo pair from drone passing over Robin Trib of Eix̱’gul’héen, warm springs creek (Switzer). Trampled paths through waist-high vegetation are mostly from bear traffic; few humans leave the boardwalk here.

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