Paired images for 3D
Stereoscopy is a venerable art, almost as old as photography itself. Especially for older geologists and foresters, paired photos giving a 3D perspective have been essential tools in landscape analysis. Ironically, however, the advent of GIS in all field disciplines has relegated stereoscopes to dusty boxes with other cast-offs like slide rules and film cameras. Many younger field workers have never seen a landscape in 3D.
Hopefully that will change. Beginning on page 32 of our report on Repeat photography is a 6-page history of the art, entitled Fickle fashions: stereoscopy. Although popularity of 3D images waxes and wanes, it always comes back, stronger than ever. Military-sized budgets for movies such as Avatar promise technological trickle-down to us field-geeks. After 20 years of GIS, some technicians are finally viewing true 3D on their computer monitors with liquid crystal shutter (LCS) glasses. Unfortunately, professional continuity—for example with senior foresters who used stereo every day to interpret canopy structure—has largely been broken. Those skills will have to be rebuilt, largely untutored.
In the early years of Discovery Southeast, we printed color stereograms (at considerable expense), and viewed them under pocket stereoscopes, sometimes with full classes. Margins of the scenes were lettered & numbered so that we could communicate when all heads were down, staring through the lenses. “Check out that sharp bend in the stream at E-14. What’s going on there?”
If you properly size stereo-pairs such as the examples below on your computer monitor (about 70 mm across one of the 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, you’ll see the pixel-grid, giving a disappointingly grainy view. Tablets, however, have smaller pixels that don’t show under 2-power magnification. Here’s a procedure that takes advantage of high resolution and rich color on Android and Apple tablets:
For stereo-viewing, no printed color photos can match the resonance of a backlit aerial on a good 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 tablets. Returning from the field, 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.
UAVs (unpersoned aerial vehicles) have the ability to acquire extremely low-elevation, high detail stereo, unavailable from traditional aircraft. Here’s a pair taken at about 100 feet elevation over raised former tideland recently colonized by mixed spruce and alder.