Background

Sentinel-1 Ground-Range Detected (GRD) products are available for download from ASF’s Vertex data portal. These Level-1 GRD products are georeferenced to geographic coordinates using the Earth ellipsoid WGS84, but are still in SAR geometry.

By following this data recipe, users will learn how to geocode Sentinel-1 GRD products in QGIS 2.18 using the Warp (Reproject) tool. QGIS version 2.18 is the current long term release, and while the basic approach is still the same in QGIS 3.X, the interface is slightly different. For instructions specific to QGIS 3.X, refer to ASF’s Geocoding Sentinel-1 GRD Products using QGIS 3.X Data Recipe.

Why Geocode?

Once extracted from their zip file, GRD products downloaded from Vertex can be viewed directly in QGIS without any additional steps. The georeferenced TIFF files include the information necessary to allow most GIS software platforms to project the data layers on the fly to match the other layers in your GIS, so you can easily visualize the data without additional effort. If you are visualizing the images outside of a GIS platform, however, the images may appear reversed or rotated. To display the images as you would expect to see them, the image must be transformed from its SAR geometry into a map projection. When working with SAR data, this process is called “Geocoding”.

Geocoding the imagery will ensure not only that the image displays at the correct location on the Earth’s surface in any given application, but that the image will also display as expected when it is viewed outside of a spatially-enabled framework (i.e. north is up, features are not stretched or reversed in unexpected ways).

Even when working within a GIS, if you want to go beyond simply visualizing the data and perform analysis or geoprocessing functions using the GRD granule, it should be geocoded to a map projection first. In QGIS, this can be accomplished by using the Warp (Reproject) tool. If you would prefer to use GDAL or ArcGIS to geocode, refer to ASF’s Data Recipes for geocoding products within each of these platforms.

Note that using different geocoding techniques/options may result in slight differences in the output products. While one output is not necessarily “better” than another, it is a good idea to be consistent when generating geocoded products for use in the same project, especially if you are looking at imagery from one location through time. The best approach to use will vary depending on the goals and preferences of the user.

Geocoding is NOT Terrain Correction

It is important to understand that this geocoding process does not involve terrain correction. To match the imagery to actual features on the earth and correct for distortions caused by the side-looking geometry of SAR data, you must perform Radiometric Terrain Correction (RTC) instead. This will be particularly important in areas of high topographic variation. Refer to ASF’s Data Recipes on Radiometric Terrain Correction, or contact ASF to learn about other resources for RTC processing.

Geocode steps

Download Sentinel-1 Data

Using Vertex, download a Sentinel-1 GRD granule. Note that you will need to log in with your Earthdata credentials before the download will begin. The download will be served as a .zip file.

Extract the .tif File with the Data of Interest

Within the downloaded zip file is a base directory with the name of the granule followed by a .SAFE extension. This SAFE structure contains a number of directories and files, and the data images are saved as georeferenced .tiff files in the folder named “measurement.”

The image we are interested in for this tutorial is the co-polarized GRD:

s1a-iw-grd-vv-20170819t001029-20170819t001054-017985-01e2e8-001.tiff

Note: If you are using a different granule than the sample provided, the co-pol image may be labeled hh instead of vv.

Use your preferred method to extract the desired file from the zipped folder, or see below for suggestions:

Contents of the GRD SAFE file, within the downloaded .zip file

Project your Product

1. Open QGIS 2.18
2. In the Raster menu, select Projections > Warp (Reproject). 
   
   Note: If your Raster menu doesn’t have the Projections options, you may not have installed and/or activated the GdalTools plugin. See Prerequisites.
    
3. Enter the parameters into the dialog window.
   1. If you added your input raster to your QGIS canvas, select it from the dropdown list of layers; otherwise, click the Select button next to the field, browse to the image to be projected, and click the Open button. (a)
   2. Type in the output path/filename, or use the Select button to browse to the target directory, then type in the filename. Make sure to add the .tif extension to the filename. (b)
   3. If the input file has been entered, the Source SRS option should already be enabled and the European Petroleum Survey Group (EPSG) code populated with 4326 (WGS 84). If it is not, enter EPSG:4326 in the field, or click the Select button and filter for 4326. (c)
   4. Click the checkbox to enable Target SRS, and click the Select button to look up the desired projection. (d)