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 3.X using the Warp (Reproject) tool. QGIS version 3.4.10 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 2.18, refer to ASF’s Geocoding Sentinel-1 GRD Products using QGIS 2.18 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.

Geocoding Steps

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 refer to the suggestions below.

1. Select one of the recent folders (which will start the extraction immediately), or use the Choose Location option at the bottom of the dialog box to navigate to the desired extraction location and click the Copy button to extract the file.
2. The single .tiff file will be extracted from the zipped folder structure.

You may also wish to download the 7-Zip File Manager for a more flexible zip utility than the standard Windows zip utility, especially if you have a Windows operating system earlier than Windows 10.

Project your Product

   3.  Open QGIS 3.X
   4.  In the QGIS Raster menu, select Projections — Warp (Reproject)
   5.  Enter the parameters into the dialog window.

Notes: If you chose to download a granule other than the sample granule, you will need to determine an appropriate projection system to use for that location. If you have other imagery for that same area of interest, you may want to set the Target coordinate reference system (CRS) of your warped granule to match the projection system of your other imagery.

If you add an image that is already in the desired projection to the QGIS Layers Panel first, the projection of that image will be registered as the Project CRS and be included in the dropdown list of recently used projections.

1. If you’ve 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. Select the coordinate system of the input raster. Note that the layers listed in the Input layer dropdown box all have the European Petroleum Survey Group (EPSG) code of their coordinate system listed, so if your input layer is in your canvas, you can see that the source CRS is EPSG 4326. If it is not listed in the Source CRS dropdown menu, click the CRS button next to the field to open the Coordinate Reference System Selector. WGS 84 should be listed in the Recently used coordinate reference systems list. Highlight it and click OK. (b)
3.  To select the output coordinate system, click the CRS button next to the field to find and select your desired projection. (c)

The sample granule provided is located in Louisiana, which is UTM Zone 15N (EPSG 32615).

• If you know the EPSG number of the desired projection, you can type that number into the Filter field and press Enter to locate the projection. Highlight the desired CRS and click the OK button.
• If you would like your image to be output in the same projection as another image that is not currently in your project canvas, navigate to that image in the QGIS Browser panel, right-click on the layer, and select Properties. Scroll down to the CRS and copy the EPSG code to paste into the CRS filter in the CRS Selector.

1. Select the Bilinear option from the dropdown for a Resampling method to use. (d)
2. Set the Nodata value for output bands to 0. (e)
   • This option sets any padding pixels added around the edges during the warp process to No Data instead of 0 values.
   • This option also sets any of the incoming 0 values from the original GRD to Nodata. See No Data Pixels section below for more detail on Nodata Settings.