In the summer of 2018, as it became even harder for journalists to work effectively in Xinjiang, a far-western region of China, we started to look at how we could use satellite imagery to investigate the camps where Uighurs and other Muslim minorities were being detained. At the time we began, it was believed that there were around 1,200 camps in existence, while only several dozen had been found. We wanted to try to find the rest.
Our breakthrough came when we noticed that there was some sort of issue with satellite imagery tiles loading in the vicinity of one of the known camps while using the Chinese mapping platform Baidu Maps. The satellite imagery was old, but otherwise fine when zoomed out — but at a certain point, plain light gray tiles would appear over the camp location. They disappeared as you zoomed in further, while the satellite imagery was replaced by the standard gray reference tiles, which showed features such as building outlines and roads.
At that time, Baidu only had satellite imagery at medium resolution in most parts of Xinjiang, which would be replaced by their general reference map tiles when you zoomed in closer. That wasn’t what was happening here — these light gray tiles at the camp location were a different color than the reference map tiles and lacked any drawn information, such as roads. We also knew that this wasn’t a failure to load tiles, or information that was missing from the map. Usually when a map platform can’t display a tile, it serves a standard blank tile, which is watermarked. These blank tiles are also a darker color than the tiles we had noticed over the camps.
Once we found that we could replicate the blank tile phenomenon reliably, we started to look at other camps whose locations were already known to the public to see if we could observe the same thing happening there. Spoiler: We could. Of the six camps that we used in our feasibility study, five had blank tiles at their location at zoom level 18 in Baidu, appearing only at this zoom level and disappearing as you zoomed in further. One of the six camps didn’t have the blank tiles — a person who had visited the site in 2019 said it had closed, which could well have explained it. However, we later found that the blank tiles weren’t used in city centers, only toward the edge of cities and in more rural areas. (Baidu did not respond to repeated requests for comment.)
Having established that we could probably find internment camps in this way, we examined Baidu’s satellite tiles for the whole of Xinjiang, including the blank masking tiles, which formed a separate layer on the map. We analyzed the masked locations by comparing them to up-to-date imagery from Google Earth, the European Space Agency’s Sentinel Hub, and Planet Labs.
In total there were 5 million masked tiles across Xinjiang. They seemed to cover any area of even the slightest strategic importance — military bases and training grounds, prisons, power plants, but also mines and some commercial and industrial facilities. There were far too many locations for us to sort through, so we narrowed it down by focusing on the areas around cities and towns and major roads.