Scale Conversion Tool

Convert among common SAR scales (Power, Amplitude, dB)

Heidi Kristenson

Radiometric Terrain Corrected (RTC) products are commonly output in power scale. While this scale works fine for numeric analysis, it is not always the best scale for visualization. Because most of the pixel values are very close to zero, if there are any bright pixels at all, the grayscale stretch tends to be skewed. If you add a product in power scale to a GIS project, it may appear to be mostly or completely black.

While you can adjust the stretch settings of the image to provide a better visualization of the power scale values without actually changing the pixel values, there may be times when you would prefer to render the values in a different scale. This approach changes the actual pixel values.

Two other common scales for RTC products are Amplitude and dB (Decibel).

Scale Comparison Map. The same dataset is rendered in the three common SAR scales: Power, Amplitude, dB. Turn the various layers off and on to compare the visual appearance of the different scales. RTC products processed by ASF DAAC HyP3 2020 using GAMMA software. Contains modified Copernicus Sentinel data 2020, processed by ESA.

Amplitude values are the square root of the power values. This makes very small values larger, and very large values smaller, bringing the dynamic range of the image closer together. Amplitude scale is often the best default choice if you are most interested in an image that's easy to interpret visually.

dB values are 10 times the Log10 of the power values. This further differentiates among very small values, which can be helpful when delineating features with very low radiometric returns, such as calm surface water. Because these values are in a log scale, care should be taken when using dB values for statistical analysis, as some approaches may not be suitable for log-scale values.

Using the Scale Conversion Tool

The ASF_Tools ArcGIS Toolbox contains a tool that converts from one scale to another (power, amplitude or dB). You can  download the toolbox  from ASF's website. Extract the zip file to the directory of your choice on your computer.

Step through the slideshow below to learn how to use the tool in ArcGIS Pro. This tool can also be used in ArcMap; the process is generally the same, but the interface looks different.

Open ArcGIS Pro and Navigate to the Toolbox

In the Catalog pane, navigate to the directory containing the toolbox, and expand the toolbox until all of the tools are displayed. Double-click on the Scale Conversion tool to launch the dialog.

Select Raster to Convert

Click the browse icon next to the first parameter and navigate to the raster that you want to convert.

If the product was generated by ASF's On Demand Processing using HyP3, the input scale will be recognized automatically. Otherwise, use the drop-down menu to select the input scale.

Set the Output Scale

Use the dropdown menu to select the output scale.

The destination output folder and filename are populated by default, but you can change them if you want a different destination and/or filename.

Run the Process

Click the Run button to process the data. By default the new raster is added as a layer in the map.

When using the tool in ArcGIS Pro, hover over any of the blue information icons in the Geoprocessing dialog for more information about the parameters or the tool in general. The full tool information can be viewed by right-clicking the tool in the Catalog window and selecting View Metadata.

When using the tool in ArcMap, click the Show Help button in the tool dialog box to see the information about specific parameters or the tool in general in the Help pane. The full tool information can be viewed by right-clicking the tool in the Catalog window and selecting Item Description.