greenland

Recently, GEB reader Dan Jacobson, who lives in Taiwan, pointed out some large stripes in the snow on Greenland as seen below:

Stripes in the snow near the northern coast of Greenland.

Zoom out a bit and you can see the whole coastline is stripey and has square steps in it.

Stepped coastline.

We immediately recognised what this was, because we have seen it before. It is Landsat 7 imagery. If you download a Landsat 7 image captured after May 31, 2003, it looks like the image below. There is a clear strip down the centre, but the rest of the image has long spikes going out to the sides.

A Landsat 7 image.

The Landsat 7 satellite does not take a single photo of the whole scene that you see when you download a Landsat image. Instead it captures a small strip of imagery which is moved backwards and forwards across the direction of movement of the satellite in a zigzag pattern. This allows a single, low resolution camera to capture a large, high resolution image. We are not certain, but it would appear that Landsat 7 has a single sensor which captures the image one pixel at a time in a variety of wavelengths. A series of rotating mirrors are used to capture light from different directions to build up the image. Two of the mirrors, called the Scan Line Corrector (SLC), compensate for the forward motion of the satellite, thus changing what would otherwise be a zigzag into parallel lines. Landsat 7 was launched on April 15, 1999, and on May 31, 2003, the SLC failed, so what we see in Landsat 7 imagery captured after that date is the zigzag without that correction. Read more about it on the USGS website and Wikipedia.

To see a sample Landsat 7 image of Greenland in Google Earth, download this KML file. Keep in mind that it is a very low resolution version to keep the file size to a minimum.

Google could update Google Earth with better quality Landsat 8 imagery, or even Sentinel imagery, but since not many people look at the northern coast of Greenland, they don’t seem to have made it a high priority.