One of the world’s largest reservoirs lies hidden in the mountainous center of Borneo. Bakun Lake filled in 2010 and 2011, when Malaysia completed construction of the Bakun dam — a hydroelectric project capable of generating 2,400 megawatts. Despite being more than five years old, the lake is missing from Google Maps, the Bird’s Eye View in Bing, and Apple Maps. (Interestingly, the lake does show up in Google Maps satellite view, but only at a coarse zoom level, and with some artifacts resulting from a partial failure of Landsat 7 in 2003.) Even the community-sourced Open Street Map shows only a jagged, low-resolution outline of the complex shoreline.
In recent imagery from Planet Lab’s Dove and RapidEye satellites, however, the fractal shoreline of the lake is clearly visible.
How can this 700-square-kilometer (270-square-mile) lake be missing from the world’s leading map services? The answer is simple.
Clouds.
Located directly on the Equator and covered with mountains and rainforest, Borneo is one of the cloudiest places on Earth.
Even the sunniest places in Borneo are cloudy about half the time. Some areas of the island—particularly along mountain ridges—are cloud-free only once or twice a year. The map below shows cloud frequency measurements derived from satellite images. (The lighter the color, the cloudier the skies: dark blue areas are cloudy 50 percent of the time or less, and the palest blue areas are cloudy over 99 percent of the time.)
This persistent cloud cover makes Borneo (and other forested tropical areas) very hard to observe from space—especially with satellites that need to be tasked (tasking a satellite is the process of scheduling it to take a picture of a specific place), or satellites with a limited field of view (there’s usually a tradeoff between resolution and area covered by an image). For example, the most recent mostly cloud-free Landsat image we could find of the area doesn’t even have Bakun Lake in it. Landsat collected the data in 2001, 10 years before the reservoir filled.
Landsat collects data of Borneo every 16 days, so it would take about 5 years to be guaranteed a mostly cloudless map of the island—and even that’s not enough to compile a pristine map. Over those five years, a lot can change. Cities grow, forest is converted to plantations, fires create burn scars, trees begin to grow on abandoned fields, and massive reservoirs will fill valleys behind tall dams.
It’s in places like cloudy and rapidly-changing Borneo that the advantages of a daily monitoring capability become clear. Clouds have always been public enemy number one for visible light Earth observation from space. Planet’s ability to capture frequent imagery—without tasking—allows us to peer between the clouds, and helps ensure a picture is taken during the rare moments when the clouds clear. It even encourages serendipitous discovery—notice how Bakun Dam’s spillway was inactive on January 15.
As Planet Labs’ collection capacity increases (we released 8 more doves from the International Space Station this week), so will our ability to create seamless, cloud-free mosaics—and keep them up to date with the most recent changes on our ever-evolving home planet.
