Create an Amplitude Difference Image

来源: https://vis-webcontent.s3.amazonaws.com/quickguides/SAR+Essentials/Create+Amplitude+Difference+Image/index.html#/

Lesson 1 of 1

Create an Amplitude Difference Image

In this quick guide, you will:

•

Learn how amplitude difference is a simple, fast, and visual method for change detection. * •

Open and display two overlapping Single Look Complex (SLC) images of a port in the Bay of Gibraltar from different dates. * •

Create an amplitude difference image that reveals changes in vessel positions during an eight-day period.

Sample Data

The exercises in this quick guide use two Capella SLC images for demonstration. Download the ZIP file below and extract its contents to a directory on your computer.

[SAREssentials_AmplitudeDifference.zip

1.1 GB

DownloadArrow down with horizontal line beneath it](assets/SAREssentials_AmplitudeDifference.zip)

•

File names: Capella_Gibraltar_2021-07-30_slc and Capella_Gibraltar_2021-08-07_slc. ENVI header files (.hdr) and ENVI SARScape files (.sml) are also included. * •

Acquisition dates: 30 July and 7 August, 2021 * •

Processing notes: TThe source datasets were Sensor Independent Complex Data (SICD) files in National Transmission Imagery Format (NITF), which require the ENVI NITF/NSIF Module to read. Since not all users have the ENVI NITF/NSIF Module, we used the "Imported data" option in the SAR Basic Data Processing tool to create SLC images that are in SARscape format instead of SICD. * •

Source: Capella Space Open Data(opens in a new tab), Attribution 4.0 International (CC BY 4.0)(opens in a new tab) license. The source IDs are CAPELLA_C03_SP_SLC_HH_20210730095929_20210730095932 and CAPELLA_C03_SP_SLC_HH_20210807095836_20210807095839.

Background

For this quick guide, you will use the SAR Amplitude Difference Image tool to detect changes that occur between images from two different dates. It works as follows:

•

It applies pseudo (approximate) geocoding to the "before" and "after" SLC images. The pseudo geocoding is coarse, which means that stable objects like buildings may appear to shift between acquisitions. * •

It creates a red/green/blue (RGB) image where the red channel corresponds to the "before" image, while the green and blue channels correspond to the "after" image.

Tip: To check if your images are suitable for the SAR Amplitude Difference tool, run the SAR Compatibility Check tool in the SAR Essentials folder of the Toolbox. See the SAR Essentials: Check Data Compatibility quick guide for more information.

Open and Display Capella SLC Images

1

Start ENVI. - 2

Select File > Open from the Menu bar. An Open dialog appears. - 3

Use the Ctrl key to multi-select the following files, then click Open:

Capella_Gibraltar_2021-07-30_slc
Capella_Gibraltar_2021-08-07_slc - 4

Wait for the raster pyramids to build in the Status bar, then click the Zoom to Full Extent button in the Toolbar. Both images are displayed at their full extent. In the following screenshot, the July 30 image is displayed first:

Both images cover part of the Bay of Gibraltar, which is east of Algeciras, Spain. The images are flipped along the North-to-South direction because they were acquired from descending (South-to-North) orbital passes.

Google Earth imagery shows that this is a commercial port with cargo ships and shipping containers.

Run the SAR Amplitude Difference Tool

1

Go to the Toolbox and expand the SAR Essentials > Change Detection folder. - 2

Double-click SAR Amplitude Difference. The SAR Amplitude Difference tool appears. The Input tab is active.

Select Input Files

1

Click the Browse button next to Before Image. A file selection dialog appears. - 2

Select the file Capella_Gibraltar_2021-07-30_slc.sml and click Open. - 3

Click the Browse button next to After Image. - 4

Select the file Capella_Gibraltar_2021-08-07_slc.sml and click Open. - 5

Leave the Spatial Subset field blank. This is for selecting a pre-defined shapefile of the area of interest. You will process the entire images instead.

6

Click the Optional tab. - 7

Click the DEM Option drop-down list and select Sea Level. - 8

For the Output Coordinate System, leave the default value of WGS 1984.

Set Export Options

1

In the Grid Size field, enter a value of 3 meters. Although the Capella image resolution is about 0.5 meters, increasing the Grid Size to 3 meters will speed up processing and reduce noise in the final result. - 2

Keep the default selections of No for Precise Geolocation and Use Virtual Raster. Precise geocolocation requires a high-resolution DEM of the area of interest, and it can significantly increase processing time. - 3

The amplitude difference image will be written to the directory specified in the ENVI Output Directory preference. To specify a different output folder, click the Browse button next to Output Folder and choose a different folder.

5

Click the Next button. Processing takes several seconds to complete. When it is finished, the amplitude difference image is added to the Layer Manager and displayed in the Image window. - 6

In the Layer Manager, uncheck both Capella images to hide them. Only the amplitude difference image is displayed.

Red pixels indicate objects that moved from their original position after July 30. Light-blue pixels correspond to new objects that appeared on August 7.

6

Click the Finish button to close the SAR Amplitude Difference tool.

This concludes the quick guide.

Your input is important to us, please take a few moments to fill out ourQuick Guide Feedback(opens in a new tab)form.