 We used quite a few different methods. From the 1990s onwards, we used satellite imagery. And then from the 1940s and the 1970s, we used aerial photographs. And these aerial photographs were flown in the area in the time before we had GPS satellites and GPS technology. So there's no geospatial information. They're just photographs. So we had to stitch all of those photographs together to make three-dimensional models of the ice field.