We recently came across this story about scour marks on the floor of the Caspian Sea. It is believed the scour marks are caused by ice that forms in the winter months and is then broken up and blown around scratching the bottom of the shallow sea.
The article states that the featured imagery is from Landsat 8. Landsat 8 imagery is freely available and we have featured it a number of times in the past. We thought it would be interesting to download the images and have a look for ourselves. However, when we used our usual source, the USGS’s Earth Explorer to download a quick preview image, the scratch marks are not visible at all.
Low resolution Landsat 8 image.
So, we decided to download the high quality Landsat imagery, 670 MB download. This includes a number of separate images representing different wavelengths which need to be processed to get a high quality full colour image. Previously, we have used an excellent tool from GeoSage, which at the time was free for non-commercial use. However, that is no-longer the case. So, we decided to see whether or not we could process the imagery with GIMP, an open-source image editing program. We are still learning the best way to do it, so we will not give details at this time, but we were successful and the result is seen below:
High resolution Landsat image processed with GIMP.
We then had a look at ASTER imagery for the same location. ASTER imagery has recently been made freely available and can be easily obtained here. The ASTER imagery looks about the same resolution as the high resolution Landsat imagery, but the ASTER website provides it through a handy KML file and it requires no pre-processing at all. The only disadvantage of ASTER imagery is that there appear to be fewer images of any given location than for Landsat imagery.
The Landsat 8 image is from March 21st, 2016, while the ASTER image is from September 28th, 2015. If you look carefully you can see that the Landsat 8 image has a lot of new scratch marks. There is also an ASTER image available from April 23rd, 2016 which shows almost identical scratch-marks to the Landsat 8 image. So it would appear that the scratch-marks last over multiple seasons.
To see the different images in Google Earth download this KML file. Note that we have cropped the Landsat imagery to make the file size smaller. The ASTER imagery will automatically download from the ASTER site when you view it.