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