Monday, May 30, 2016

Radar Preprocessing - Axel Heiberg terrain corrections

Maria, one of Catherine's undergraduate interns, has been working diligently on the VNIR data for the Axel Heiberg project.  Catherine has decided it is time to start comparing her VNIR data with the radar imagery.  Since the radar is my part of the project, today I had the opportunity to apply my new knowledge of the Sentinel-1 Toolbox.  To compare the radar data with the ArcGIS package, it needs to be georeferenced.  We can do just that by using the Terrain Correction discussed in the previous post.

This sounds easy enough.  We open the Axel Heiberg product.xml files in SNAP the same way as we opened the Vancouver files, and should be able to run the Range Doppler Terrain Correction in the same way.

However, we encountered an error:

Error!  No DEM for this area!

This is because the SRTM DEM that SNAP uses as default does not extend into the Canadian Arctic.  Never to fear, this is an easy fix.  Instead of using SRTM 3Sec DEM, we can implement our own.  Last week, I had used some of the data previously available on the CPSX server to export a .tiff DEM for Axel Heiberg, which is perfect for this purpose.

But there is another problem:

Well, that projection doesn't look like it will fit our needs.

This is certainly an odd projection, that most definitely will not fit with our ArcGIS files.  The reason this occurred is because we did not specify that SNAP georectifies the image with the proper projection.
Once this is solved, then we can clearly see the differences between the original radar data images, and the terrain corrected images ready for incorporation with ArcGIS:

Original radar data
Terrain Corrected/Georectified radar image

There are a few notable differences between these two images.  First, the georectified image has been rotated somewhat.  This will help it overlap properly with our VNIR files.  Second, the mountain ranges on the left are foreshortened.  You can see that the "bright" strips on the right side of the mountains is thinner than the left, dark side of the mountain ranges.  This is an error from the incidence angle of the radar beam.  After applying the DEM during terrain correction, Sentinel-1 realizes, "Oh, that is actually a mountain!" and fixes the images accordingly.

Soon, we will start seeing how these images compare to Maria's work!  Cheers!

RADARSAT-2 Data and Products (c) MacDonald, Dettwiler and Associates, Ltd. (2009) - All Rights Reserved. RADARSAT is an official trademark of the Canadian Space Agency.

1 comment:

  1. A few comments:

    1. When you're showing RADARSAT-2 data, please acknowledge the CSA and MDA: "RADARSAT-2 Data and Products (c) MacDonald, Dettwiler and Associates, Ltd. (year of acquisition) - All Rights Reserved. RADARSAT is an official trademark of the Canadian Space Agency.”

    2. Foreshortening isn't an error, it is the natural consequence of turning the time delay that the radar measures into a geographic location. Returns from a mountain top will arrive earlier than the returns from a flat surface. Without topography information, the resulting pixel is placed closer to the radar than it really is.