Generating a Radiometrically Terrain Corrected (RTC) Image using the Sentinel Toolbox

Transcription

1 Making remote-sensing data accessible since 1991 Generating a Radiometrically Terrain Corrected (RTC) Image using the Sentinel Toolbox This data recipe is for users who wish to generate an RTC image from Sentinel-1 data using easy-to-follow instructions in a graphical user interface (GUI). In this document you will find A. Background B. Materials List C. Steps for Generating RTC Image A) Background Radiometric correction removes the misleading influence of topography on backscatter values. Terrain correction corrects geometric distortions that lead to geolocation errors. The distortions are induced by side-looking (rather than straight-down looking, or nadir) imaging and are compounded by rugged terrain. Terrain correction moves image pixels into the proper spatial relationship with each other. Radiometric terrain correction combines both corrections to produce a superior product for science applications. B) Materials List 1. Sentinel Application Platform (SNAP) Toolbox 2. Data granule (sample granule provided) C) Steps for Generating RTC Image Download Materials, and Open Data 1) Download materials a. Download the SNAP toolbox from the European Space Agency website. (Select the correct version for your operating system.) b. Install the SNAP toolbox by following the instructions provided in the installation wizard. c. Create a new folder to hold Sentinel-1 data and name it (example name: Sentinel_RTC_SNAP). d. Download a S1A GRD granule from ASF Vertex into the new directory. 9 June 2016, v. 5 1

2 Note: You may start with this sample granule: Download Sample Granule 2) Open Data in SNAP a. Initiate the SNAP interface by clicking on the SNAP desktop icon. b. In the SNAP interface, go to the File menu and select Open Product. (Fig. 1) c. Browse to the folder that contains your Sentinel-1 data, and select (highlight) the.zip file by singleclicking it. d. Click on Open Product to load the.zip file into the SNAP interface (no need to extract files from the.zip file; the program will do that for you). e. Select (highlight) the granule you have loaded into the interface by single-clicking it. Figure 1: Open Product Apply Radiometric Terrain Correction 1) Apply Orbit File a. Select Apply Orbit File from the Radar menu. (Fig. 2, 3) Figure 2: Apply Orbit File Notes: The default settings for processing options in Apply Orbit work for your purposes here. They apply precise orbit information to the input file via automatic download, which you can see is checked by looking at the Processing Parameters tab. The defaults also place the file resulting from this step into the same directory as the source-data.zip file. b. Click Run. Notes: A box pops up to show the progress of the processing. Depending on the computing power of your Figure 3: Parameters for Apply Orbit File 9 June 2016, v. 5 2

3 machine, you may be able to let it run in the background (~5 minutes, depending on system capability). 2) Calibrate Data a. The output from the previous step now appears in the Product Explorer window of the interface (filename ending in _Orb ). Single click the file name to select it. b. In the Radar menu, select Radiometric > Calibrate options. (Fig. 4) c. In the box that appears, radiometrically calibrate the image to Beta0 (the terrain flattening algorithm requires Beta0 input) by going into the Processing Parameters tab and selecting the Output beta0 band. (Fig. 5). The defaults place the output into the same directory as the input. Figure 4: Radiometric / Calibrate Figure 5: Processing Parameters d. Click Run (~20 minutes, depending on system capability). 3) Flatten Terrain a. As before, the output from the previous step appears in the Product Explorer window (filename ending in Orb_Cal ). b. Highlight that output by single clicking on the file name. c. In the Radar menu, select Radiometric > Radiometric Terrain Flattening. Note: The default settings download a digital elevation model (DEM) matching the geolocation of the scene being corrected, placing the output file into the same directory as the input. d. Click Run (~45 minutes or longer, depending on system capability). Note: To succeed, the computer must have an internet connection. 9 June 2016, v. 5 3

4 Figure 6: Terrain Flattening Figure 7: Processing Parameters 4) Apply Terrain Correction (to geometrically correct the image) a. Highlight the output from the previous step in the Product Explorer window (filename ending in Orb_Cal_TF ). b. In the Radar menu, select Geometric > Terrain Correction > Range- Doppler Terrain Correction. (Fig. 8) Figure 8: Range-Doppler Terrain Correction Notes: The processing box pops up, and the defaults for the I/O Parameters tab place the output files in the same directory as the source file. The Processing Parameters tab enables you to specify the map projection you need, pixel spacing if you wish to change it, and options for additional output files. 9 June 2016, v. 5 4

5 c. In the Processing Parameters tab, use the default options for output files, and select UTM / WGS 84 for the map projection (~30 minutes, depending on system capability). (Fig. 9) d. The output from the previous step will be the fifth file in the Product Explorer window (filename ending in Orb_Cal_TF_TC ). Double-click on it to open the folder and find the products created by the last step. e. Open the folder named Bands Figure 9: Processing Parameters / Map Projection f. Double-click on the single image file named Gamma0_VV. The image will open in the SNAP interface. 5) (Optional) View the RTC image in a GIS Program Note: These directions assume the default output settings for step 4 a. Open ArcGIS or QGIS. b. Select Add Data (ArcGIS) or Add Raster Layer (QGIS). c. Navigate to the directory that contains the output from step 4. d. Within this folder, click on the sub-folder ending in _TC.data. e. Load the.img file(s) contained within. Note: The Gamma0 file type that the SNAP RTC process produces is excellent for statistical comparisons, but can appear very dark. To view a lighter version of the image in a GIS environment, see the ASF data recipe ASF_DataRecipe_RTC_Viewing_Data_in_GIS. 9 June 2016, v. 5 5