Create Digital Surface Models (DSMs) from Stereo Radargrammetry
Lesson 1 of 1
Create DSMs from Stereo Radargrammetry
In this quick guide, you will:
- •
Learn what factors contribute to the reliability of topographic height estimates. * •
Learn the data requirements of images used for stereo radargrammetry DSM generation. * •
Use the SAR Stereo-Radargrammetry DSM tool to create a DSM from two overlapping Single Look Complex (SLC) images acquired five days apart.
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_DSM_Stereogrammetry.zip
147.1 MB
DownloadArrow down with horizontal line beneath it](assets/SAREssentials_DSM_Stereogrammetry.zip)
- •
File names: Capella_EugeneOR_20220324_slc and Capella_EugeneOR_20220329_slc. ENVI header files (.hdr) and ENVI SARscape files (.sml) are also included. * •
Acquisition dates: 24 and 29 March, 2022 * •
Processing notes: The 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 Open Data(opens in a new tab), Attribution 4.0 International (CC BY 4.0)(opens in a new tab) license. The source ID is CAPELLA_C06_SP_SICD_HH_20220329150244_20220329150247.
Background
Stereo radargrammetry is analogous to photogrammetry where the absolute elevations of points on the Earth's surface are measured based on the principle of stereo vision. Here, the intersection of line-of-sight rays from two different viewing points is used to reconstruct the surface into a DSM.
With the SAR Stereo-Radargrammetry DSM capability, the viewing points correspond to the position of two SAR sensors along two different satellite paths. The separation angle between both acquisitions (also known as the geometric baseline) should be large enough---between 8 to 30 degrees---to provide the required stereo geometry. The higher the separation angle, the higher the sensitivity to the topography. The SAR Stereo-Radargrammetry DSM tool can provide good results with high-resolution sensors, and mostly in natural areas.
Terrain Implications
In SAR sensors with long wavelengths (for example, L-band and P-band), the radar signal penetrates through foliage and interacts with trunks and branches. In these areas, topographic estimation is slightly below the vegetation canopies. Thus, the output is not strictly a DSM as it would be with smaller wavelength sensors that operate in the X- or C-band.
SAR penetration capabilities by wavelenth. Image credit: NASA Earthdata (2020).
Sentinel-1 data (C-band) should not be used to create DSMs since its sensitivity to topographic variation is too low.
Data Requirements
Both images used for DSM generation must meet the following requirements:
- •
The incidence angle difference should be at least 4.5 degrees but not more than 40 degrees. Ideally, the difference should be between 8 and 30 degrees. * •
The Azimuth Line of Sight (ALOS) angle difference should be at least 9 degrees but no more than 40 degrees (30 is better). For non-squinted acquisitions in which the LOS is 90 degrees with regard to the azimuth direction, the ALOS difference is the same as the heading angle difference.
Before creating a DSM from stereo radargrammetry, use the SAR Check Data Compatibility tool to check if your images meet these requirements. See the SAR Essentials: Check Data Compatibility quick guide for more information.
Next, you will generate a DSM from two Capella SLC images.
Run the SAR Stereo-Radargrammetry DSM Tool
- 1
Start ENVI. - 2
Go to the Toolbox and expand the SAR Essentials > DSM folder. - 3
Double-click the SAR Interferometric DSM tool. The SAR Interferometric DSM dialog appears.
Set Input Options
- 1
Click the Browse button next to Reference Image. A file selection dialog appears. - 2
Go to the location where you saved the sample data for this quick guide. - 3
Select the file Capella_EugeneOR_20220324_slc.sml and click Open. - 4
For the Secondary Image, select the file Capella_EugeneOR_20220329_slc.sml. - 5
Leave the Spatial Subset field blank. This is for selecting a pre-defined shapefile or Google Earth KML/KMZ file delineating the area of interest. The Capella images have already been spatially subsetted. - 6
In the Coherence Threshold field, enter a value of 0.5. This value specifies the level of quality and precision in the output DSM. Values can range from 0 (lowest quality) to 1 (highest quality). Pixel values lower than the specified coherence will not be considered for DSM generation. - 7
Leave the Max Residual Height (m) field empty. This optional parameter refers to the maximum residual height difference with respect to the input DEM considered in the matching process.
Select a DEM
For this exercise, you will use a freely available DEM from the Shuttle Radar Topography Mission (SRTM).
- 1
Click the Optional tab. - 2
Click the DEM Option drop-down list and select Download SRTM-3 V4 DEM (90m). - 3
For the Output Coordinate System, keep the default value of WGS 1984.
Note: if you do not have internet access, we included the DEM in the SRTM DEM directory included with the sample data. Go to that directory and select the file Srtm-3_V4_dem.sml.
Set Export Options
- 1
Click the Export tab. - 2
Leave the Grid Size field empty. ENVI will automatically determine a suitable output grid resolution based on the nominal resolution of the SAR imagery and an internally applied multilooking factor. - 3
Select the DSM option in the Generate Products list. - 4
The DSM 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 minutes to complete. When it is finished, the Report panel appears and the output products are added to the Layer Manager and displayed in the Image window. - 6
Click the Finish button to close the tool.
Evaluate Output Products
The Layer Manager lists two new layers:
- •
dsm.anz: Annotation layer showing the line-of-sight (range) and heading (azimuth) directions, along with basic metadata. You must zoom out of the view to see the annotation layer, which is displayed to the upper-left of the DSM image in the Image window. * •
dsm.dat: DSM image
For more information about the other optional products you can create in this DSM workflow (Precision, Coherence, Resolution), please refer to the Create DSMs from Interferometry quick guide.
A DSM is different than a DEM or Digital Terrain Model (DTM) because elevation values include features that protrude from the surface. A DEM or DTM characterizes the topography of a bare Earth surface. A high-resolution DSM such as this one can reveal trees and buildings. Darker pixels correspond to lower elevation, while brighter pixels correspond to higher elevations. The circular white object in the center is a football stadium:
Here is a true-color image of the same area for comparison:
Tip: Viewing the DSM as a grayscale raster is not always intuitive, nor does it reveal masked pixels well. Consider using ENVI's Topographic Shading Tool to create a color shaded-relief image from the DSM. This is a good way to visualize elevation data. For more information, see the Topographic Shading(opens in a new tab) topic in ENVI Help.
Example of using the Topographic Shading Tool to view a shaded-relief image from the Eugene DSM.
Once you create a DSM, you can use it for the following analyses:
- •
Topographic analysis * •
Viewshed analysis using ENVI's Viewshed Tool (accessible from the Toolbar) * •
Mobility analysis tools, such as Helicopter Landing Zones or Topographic Breaklines, which are both under the Mobility folder in the Toolbox.
This concludes the quick guide.
Additional Resources
SAR Essentials: Create Digital Surface Models (DSMs) from Interferometry 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.
© 2025 NV5 Geospatial Solutions, Inc. This information is not subject to the controls of the International Traffic in Arms Regulations (ITAR) or the Export Administration Regulations (EAR).