Geocoding Sentinel-1 GRD Products using GDAL Utilities

Transcription

1 Geocoding Sentinel-1 GRD Products using GDAL Utilities Adapted from coursework developed by Franz J Meyer, Ph.D., Alaska Satellite Facility. GDAL installation adapted from UCLA s Technology Sandbox website In this Document you will Find: A. Background B. Materials List C. How to install GDAL D. Geocoding Steps A) Background This data recipe will allow users to work with Sentinel-1 data in GIS. Sentinel-1 Ground-Range Detected (GRD) products are georeferenced, but they are not geocoded and some pre-processing is needed before they can be used in a GIS system. Users will learn how to geocode GRD data using the GDAL raster utilities. GDAL is open-source, free, broadly supported, updated often, and runs on almost any operating system. GDAL runs on the command line, and installation is only marginally more difficult than the typical commercial app. B) Materials List Making remote-sensing data accessible since 1991 Windows 10, Mac OS X, or Linux GDAL - Geospatial Data Abstraction Library (version 2.2) Python 2.7 or higher (Download is only necessary for Windows) Download Sentinel-1 GRD data using Vertex o You may use this Sample Granule 14 March 2018 v.6.4 1

2 Note: You will be prompted to enter your Earthdata Login username and password, or you must already be logged in to Earthdata before the download will begin. C) How to Install GDAL The GDAL project does not produce regular downloadable binaries (executables) for each release the available version of GDAL will vary between operating systems. If you already have GDAL installed, skip to Section D. Installing on Mac OS X The easiest approach to download GDAL for Mac is to download the GDAL 2.2 Complete disk image from the KyngChaos website. Download the disk image Open the downloaded file (GDAL_Complete-2.2.dmg) Double-click on the Install GDAL Complete.pkg file Note: You will be warned that the file is from an unknown developer. Open System Preferences, click on Security & Privacy, then click Open Anyway. Note: When you are installing GDAL 2.2 with Python 2.7, a message will be displayed stating that Python 3.6 is required. For this tutorial, you can ignore the message. Follow the prompts to install Copy and paste the following line into a Terminal window. This will allow you to run the GDAL commands without specifying a path: echo PATH=/Library/Frameworks/GDAL.framework/Programs:$PATH >>~/.bash_profile Then copy and paste source ~/.bash_profile To test your installation: Type or paste the following command in the Terminal: gdalinfo - version A correct installation will output the version number followed by a release date (e.g., GDAL 2.2.3, released 2017/11/20) 14 March 2018 v.6.4 2

3 Now that you have GDAL installed, you can skip forward to Section D. Installing on Linux The method of installation will depend on your flavor of Linux. On Ubuntu, simply install the GDAL package via the Command Terminal. To get the latest GDAL version, add the PPA to your sources, and then install the gdalbin package, at the Terminal prompt type or past these commands: sudo add-apt-repository ppa:ubuntugis/ppa sudo apt-get update sudo apt-get install gdal-bin To test your installation: Type or paste the following command in the Terminal: gdalinfo - version A correct installation will output the version number followed by a release date (e.g., GDAL 2.1.3, released 2017/20/01) Now that you have GDAL installed, you may skip forward to Section D. Installing on Windows 10 Install Python Python is necessary for GDAL, and if you already have an installation of Python then you may skip to Step 4 below. 1. Download the latest 2.7.x version of Python (rather than the 3.x Python version) 2. Install Python with the default options and directories. 3. After installation, go to Python > IDLE (Python GUI) to find out what version of Python you are using. Note: If you do not know where this is, simply use the search function by searching the word python in the search bar. 4. Make a note of the number that shows the version of your Python in the top right, as highlighted below: 14 March 2018 v.6.4 3

4 Figure 1: Python Shell displaying version number Note: MSC v.1500 may differ if you are using a different Python installation, if it does then please make a note of that number. Note, if you installed the 64-bit version of Python, for the rest of the tutorial please remove the (x86) from the paths. Install GDAL 1. Go to the GISInternals website, and download the appropriate GDAL binary For this tutorial, we are using the MSC v.1500 on a 32-bit system; the picture below illustrates how to match the version with your own Python version. The release number should match the number (See Figure 1). 2. Clicking the link will take you to the list of binaries (installers) to download Continued on next page 14 March 2018 v.6.4 4

5 3. Locate the core installer, which has most of the components for GDAL 4. After downloading your version, install GDAL with standard settings 5. Next, return to the list of GDAL binaries and install the Python bindings for Python version 2.7 (the same version as we installed earlier) 6. Download the Python bindings and install them 14 March 2018 v.6.4 5

6 Add Path Variables We need to tell the Windows system where the GDAL installations are located, so we need to add some system variables. 1. Go to the File Explorer and right-click on This PC (or My Computer ) and go to Properties 2. Click on Advance System Properties (Figure 2) Figure 2: Advanced system settings > System Properties dialog 3. Select Environment Variables 4. Under the System Variables pane, find the Path variable and then click on Edit 5. Go to the end of the box to copy and paste the following in the dialog: ;C:\Program Files (x86)\gdal Note: Be sure to include the semicolon at the beginning of the path name Note: For 64-bit GDAL installation you must remove the (x86) after Program files 14 March 2018 v.6.4 6

7 6. In the same System Variables pane, click on New and then add the following in the dialogue box: Variable name: GDAL_DATA Variable value: C:\Program Files (x86)\gdal\gdal-data 7. Click OK 8. Add one more new variable by clicking New 9. Add the following in the dialogue box (Figure 3): Variable name: GDAL_DRIVER_PATH Variable value: C:\Program Files (x86)\gdal\gdalplugins 10. Click OK 11. Click OK to close the System Properties window Figure 3: Environment Variables > New User Variable dialog To test your installation: Type or paste the following command in the Command Terminal: gdalinfo - version A correct installation will output the version number followed by a release date (e.g., GDAL 2.2.3, released 2017/11/20) 14 March 2018 v.6.4 7

8 D) Geocoding Steps For this tutorial, you will use the GDAL raster utilities gdal_warp and optionally gdal_translate. Visit for more information. 1. Download Sentinel-1 data Using Vertex, download a Sentinel-1 GRD granule. 2. Extract the.tif file that you are interested in In the downloaded zip file there is a base directory with the name of the granule followed by.safe and then a number of lower level directories. One of these folders is named measurement and inside are the georeferenced.tiff files (Figure 4). Figure 4: Contents of downloaded GRD zip file Unzipping with OS X, Linux Using the Unix unzip utility (works with OS X) in the command terminal, you can extract just the file you want with a command like this: unzip <filename.zip> */measurement/*vv*.tiff This will extract just the VV polarized image from this zip package. If you know which file you want this is usually much faster than extracting the whole zip file. Unzipping with Windows 10 Alternatively, if you are a windows user you may download the 7-Zip File Manager 14 March 2018 v.6.4 8

9 or use a zip utility to open your product and simply copy the file to a directory of your choosing via the GUI (Figure 4). 2. Project Your Product using gdalwarp (Geocoding Step) Using the GDAL command line utilities you can project and export files. To begin, we want to project the files. The Sentinel-1 s ground control points (GCP) are provided in geographic coordinate system (GCS) coordinates (latitude/longitude), but can easily be changed into another projection. The Louisiana sample data provided is projected to UTM zone 15 (EPSG 32615). If you know the EPSG code for your projection, you simply enter the code as your output spatial reference. 1. Open a Command Terminal 2. Browse to the directory of your GRD product Windows commands: o cd <directory> to change directory, cd.. navigates to parent directory o dir to list directory contents, cd alone to display path of current directory Linux, OS X commands: o cd <directory> to change directory, cd.. navigates to parent directory o ls to list directory contents, pwd to display path of current directory Hint: use the <Tab> key as a shortcut to typing in the entire file or directory. You may also use the <Up> or <Down> key to recall previous commands. 3. Use this gdalwarp command: gdalwarp -tps -r bilinear -tr t_srs EPSG:32615 s1a-iw-grd-vv t t e2e8-001.tiff s1a-iw-grd-vv t t e2e8-001-utm.tiff -tps specifies to use the thin plate spline technique when interpolating the control points -t_srs specifies the output projection -r specifies the resampling method (default is nearest neighbor) -tr specifies the output pixel size Note: You must provide the full path to the input file and output file unless you run the command in its current directory. 14 March 2018 v.6.4 9

10 How to Find Your Projection For a list of spatial references, please navigate to the Spatial Reference database. You may search or browse to the spatial reference you are interested in (Figure 5). Figure 5: Spatial Reference website How to navigate Spatial Reference database website: Search for the projection you are interested in Once you have found your spatial reference, you will see a list Click on the Proj4 link (Figure 5) to get the full definition. Copy and paste the syntax into your gdalwarp command (see below) 14 March 2018 v

11 Using the gdalwarp command without EPSG code: gdalwarp -tps -r bilinear -tr t_srs +proj=utm +zone=15+datum=wgs84 +units=m <inputfile.tif> <outputfile-utm.tif> Feel free to adjust the gdalwarp command according to your needs. Executing the gdalwarp command produces a map-projected geotiff file that can be read by nearly any GIS application. For more information visit 3. Scale to byte using gdal_translate (Optional) You can easily scale the data to byte if you want, which may be helpful when using large image files. Many of the S-1 data files can be rather large, so scaling will cut down on the computational time needed to process the image. gdal_translate -ot Byte -scale <infile.tif> <output.tif> In this case we convert from long int to byte and scale the range 0 to 700 to the 0 to 255 byte range. Values above 700 will be set to 255 and values below 0 (though there shouldn't be any) will be set to 0. For more information on this command, visit Ready For GIS Once these pre-processing steps are completed, the data is ready for analysis in a GIS system. 14 March 2018 v