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Earth at a Cute Angle

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Satellite imagery surrounds us — from Google Maps and daily weather forecasts to the graphics illustrating news stories — but almost all of it is from a map-like, top-down perspective. This view allows satellite data to be analyzed over time and compared with other sources of data. Unfortunately, it’s also a distorted perspective. Lacking many of the cues we use to interpret the world around us, top-down satellite imagery (often called nadir imagery in remote sensing jargon) appears unnaturally flat. It’s a view that is disconnected from our everyday experience.

San Francisco, Oakland, and the San Francisco Bay. Sentinel-2A January 10, 2021.

We’re used to seeing things from the side. Walking around at ground level, standing on a mountaintop, or even gazing out an airplane window, we’re never looking straight down. Our everyday perspective is more like this view of San Francisco from Maxar’s Worldview-3 satellite. Although not the most common type of imagery from orbit, these oblique views (also known as off-nadir views) connect our own lifetime of experiences with the unfamiliar view from space.

An oblique view of San Francisco from Maxar’s Worldview-3 satellite acquired on February 7, 2016. ©Maxar Technologies CC BY 4.0.

In many ways, the prevalence of nadir views—pictures taken directly beneath a satellite as it flies over the Earth’s surface—is an anomaly. The first decades of aerial photography — such as this view of San Francisco in the aftermath of the 1906 earthquake, photographed from a camera suspended by a kite — consisted entirely of pictures taken at an angle.

San Francisco in ruins, May 5th, 1906. Archived by the Library of Congress.

Even the first photos from space were oblique. This panorama was acquired from a V2 rocket launched from White Sands Proving Ground, New Mexico on July 26, 1948. It shows much of the U.S. southwest and the Earth’s limb — the thin veil of atmosphere that separates us from outer space.

White Sands Missile Range and the U.S Southwest photographed from a V2 — among the first photos ever taken from space. Applied Physics Lab of Johns Hopkins University for the Bureau of Ordnance, U.S. Navy.

Early spy satellites, such as the Corona missions, used photos of the Earth’s horizon to help analysts determine the precise location of the imagery.

The Earth’s limb, seen from a Corona satellite photographing the Tian Shan mountains in Kyrgyzstan on August 23, 1965. Archived by the USGS EROS Data Center.

Astronauts & cosmonauts continued this tradition, snapping photographs through the windows of their spacecraft from the earliest orbital flights, until today.

Australia & the Earth’s limb from Gemini XI—the highest crewed flight ever, except for the Apollo missions to the moon. Photographed on September 14, 1966. Archived at March to the Moon, Arizona State University.

This sequence of photos from the International Space Station illustrates the structure of Sarychev Volcano’s ash plume during a 2009 eruption. Look closely and you can see the ash flowing down the side of the volcano.

The eruption of Sarychev Peak photographed from the International Space Station on June 12, 2009. NASA JSC Earth Observation Lab.
Swirling clouds over the South Pacific Ocean, photographed from the International Space Station on January 8, 2021. Astronaut photograph ISS064-E-21256, NASA JSC Earth Observation Lab.

In some ways, even geostationary satellites (positioned exactly over the equator but far enough above the Earth to see almost an entire hemisphere at once) collect primarily oblique data, since their footprint extends far from directly beneath the satellite.

Image of thunderclouds rising above the Earth’s limb, collected by the Himawari satellite.

I personally became interested in working with oblique imagery from Planet’s SkySat constellation while trying to visualize a series of deadly debris flows that struck Montecito, California after devastating wildfires swept through the mountains above the town.

Debris flow, Montecito, California acquired on January 12, 2018. ©Planet Labs, Inc. CC BY 4.0.

Unfortunately, the top-down view lacks the visual cues that convey a sense of depth and obscures the relationship between terrain and landslide. Oblique imagery, however, is more like our everyday perspective and gives an immediate sense of how topography contributed to the disaster.

Thomas fire burn scar and debris flow, Montecito, California acquired on January 27, 2018. ©Planet Labs, Inc. CC BY 4.0.
Detail of the image above, debris flow, Montecito, California acquired on January 27, 2018. ©Planet Labs, Inc. CC BY 4.0.

Let’s take a look at how oblique satellite imagery is acquired with the help of this image of another California natural hazard—wildfire. The Kincade Fire ravaged Sonoma county in October and November 2019. This view, acquired while the fire was at its height, highlights the relationship between fire, smoke, and terrain. To get it, Planet’s collection planning team had to aim the satellite far off its ground track.

Smoke rising from the Kincade Fire, near Healdsburg and Mount St. Helena, Sonoma County, California. Acquired October 27, 2019. ©Planet Labs, Inc. CC BY 4.0.

The image was collected at 60˚off nadir — more than twice as far from straight down as a high-res satellite typically operates (plus or minus 30˚). Smaller angles are considered “near-nadir” meaning the imagery can be made to behave more or less like a map.

Image collected by the Moderate Resolution Imaging Spectroradiometer aboard NASA’s Terra satellite on October 27, 2019. NASA WorldView.

At 60˚ off-nadir at an altitude of 500 kilometers (310 miles), the satellite was extraordinarily far away from the target — in the case of the Kincade Fire more than 1,000 kilometers (620 miles) to the east of Healdsburg, over central Utah.

How does oblique imagery differ from nadir imagery? This view of downtown Houston was collected at only 12˚ off-nadir. It’s sharp, high-resolution, and relatively easy to line up features in the image with their true position on the ground.

Downtown Houston, TX collected 12˚ from nadir. Acquired on April 1, 2020. ©Planet Labs, Inc. CC BY 4.0.

This oblique image taken at 60˚ shows the profile of the downtown skyline at the expense of the map-like precision we take for granted in satellite imagery. It’s also lower resolution, has more interference from the atmosphere, and was harder to collect and process. And yet…there’s something compelling about this viewpoint.

Downtown Houston, TX collected 60˚ off-nadir. Acquired on March 14, 2018. ©Planet Labs, Inc. CC BY 4.0.

So why bother? There are a few concrete applications — like the ability to see under things. This off-nadir view of the North Korean port of Sinpho, reveals a submarine berthed beneath a newly constructed awning, meant to hide its presence from overhead observers.

Sinpho, North Korea, acquired on October 8, 2019. ©Planet Labs, Inc. CC BY 4.0.

Combine oblique images with nadir images and it’s possible to derive the 3D shape of a surface and create a digital elevation model. The more images collected at a wider variety of angles, the more accurate and detailed the map.

Colorized shaded relief of the Morenci Mine, Arizona, based on a digital elevation model created by Saif Aati, California Institute of Technology, based on SkySat data. ©Planet Labs, Inc. CC BY 4.0.

If you are interested in collecting video from orbiting satellites, gathering oblique imagery is almost inevitable due to their high speeds. Watch how this view of the ski trails of the Breckenridge resort evolves as the satellite swings further and further from nadir.

SkySat video ofPeak 8, Breckenridge, Colorado, collected on November 7, 2018. ©Planet Labs, Inc. CC BY 4.0.

But to me, the most interesting aspect of oblique imagery is the way it reveals the form of a landscape and acts as a bridge between our lived experience and abstract data. From the dramatic cascade of ancient, hardened lava down the sides of the Grand Canyon to the rapids of the Colorado River…

Lava Falls, near the Toroweap Overlook in the western reaches of the Grand Canyon. Acquired on October 12, 2019. ©Planet Labs, Inc. CC BY 4.0.

…to the u-shaped valleys, hanging glaciers, and towering granite cliffs of Baffin Island, Canada. See if you can spot a silhouette of the Empire State Building, an element of human scale on the landscape.

Overview of Baffin Island, Canada. Acquired on June 21, 2020. ©Planet Labs, Inc. CC BY 4.0.

Zoom in and the fractal patterns of the glaciated landscape give way to snow-covered slopes, and long shadows cast by immense vertical walls. This combination of perspective and scale is impossible to achieve in any other way.

Detail of Baffin Island, Canada. Acquired on June 21, 2020. ©Planet Labs, Inc. CC BY 4.0.

The off-nadir perspective can even help illustrate the extremes of sport, such as these images that show the length and steepness of two of the most difficult and storied climbs of the Tour de France.

Summit of the Col du Galibier (top). Spectators gather along the Col du Tourmalet, France (lower). Both acquired on July 18, 2019. ©Planet Labs, Inc. CC BY 4.0.

Cartographers have taken advantage of the oblique view for centuries, using a bird’s-eye view to illustrate everything from the whole of Italy to Maui and Haleakalā National Park.

Veduta d’Italia, published in 1853. (left) David Rumsey Historical Map Collection. Maui & Haleakalā National Park, Hawaii. National Park Service.

The connection between what we see from the familiar ground-level viewpoint and the novel, top-down perspective of a satellite view is what makes oblique imagery so powerful. Likewise, presenting new information in the context of pre-existing knowledge is an essential element in successfully communicating unfamiliar ideas. In both science communication and data visualization, it is essential to use the familiar to build a bridge to the novel.

In your own discipline, try to find the examples that connect the tangible to the intangible, the every day to the exotic, and the known to the unknown.

Lake Mead, Hoover Dam, and the Nevada/Arizona Border, acquired on December 12, 2018. ©Planet Labs, Inc. CC BY 4.0.
New York, New York, collected on August 24, 2017. ©Planet Labs, Inc. CC BY 4.0.
Paris, France, collected on September 24, 2016. ©Planet Labs, Inc. CC BY 4.0.
Frankfurt am Main, Germany. Collected on March 25, 2018. ©Planet Labs, Inc. CC BY 4.0.
Brukkaros Mountain, Namibia, acquired on March 9, 2020. ©Planet Labs, Inc. CC BY 4.0.
Mount Kenya, acquired on March 16, 2020. ©Planet Labs, Inc. CC BY 4.0.
Mount Lico, home to some of the last remaining pristine forest in Mozambique. Acquired on March 15, 2020. ©Planet Labs, Inc. CC BY 4.0.
Trango Tower, Pakistan. Acquired on January 23, 2020. ©Planet Labs, Inc. CC BY 4.0.
Half Dome, Yosemite National Park, California. Acquired on June 18, 2019. ©Planet Labs, Inc. CC BY 4.0.

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Robert Simmon

Robert Simmon

Data Visualization, Planet Labs. Ex-NASA. Blue Marble, Earth at Night, color.

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