Creating A Digital Elevation Model using Sentinel-1 Data

Guide to Creating a Digital Elevation Model Using Sentinel-1 Data in the Sentinel Application Platform (SNAP)

Introduction

Digital Elevation Models (DEMs) are and essential tool in geospatial analysis. They provide detailed representations of the earth's surface elevation and are widely used in application like, hydrological modeling, land cover classification, infrastructure planning, and environmental monitoring. Synthetic Aperture Radar (SAR) data, such as that provided by Sentinel-1, offers a method to create DEMs.

This tutorial will guide you through the steps to create a Digital Elevation Model using Sentinel-1 data in the European Space Agency's (ESA) Sentinel Application Platform (SNAP). SNAP, the official open-source software developed by the ESA, provides an extensive range of tools for processing Sentinel data, including SAR interferometry (InSAR). Sentinel-1, a part of ESA's Copernicus Program, provides high-resolution C-band SAR imagery that can be processed using interferometric techniques to extract elevation data.

This guide will allow users to:

• Acquire and prepare Sentinel-1 SLC (Single Look Complex) data.
• Perform coregistration of SAR images.
• Generate an interferogram and apply phase filtering.
• Unwrap the phase and convert it into elevation values
• Export and visualize the final DEM

No experience with SAR data is required for this tutorial, but a basic understanding of remote sensing will be helpful. This tutorial caters to students, researchers, or other GIS professional in need of Digital Elevation Models for areas they are studying, and will equip you with the base skills needed to create and analyze these models using radar imagery in Snap

Data Consideration

Interferometric SAR or InSAR is a form of geospatial analysis that uses the measurement of phase from two or more synthetic aperture radar (SAR) images. SAR imagery is a form of active remote sensing where the sensor emits microwaves towards the target and receives the reflected waves. Several factors can influence the accuracy of InSAR analysis, and the table below summarizes these factors, their effects, and some best practices to create the best DEM possible.

Factor	Characteristics	Effect on Analysis	Recommendation
Land Cover	Urban landscapes, forests, built structures, agriculture, water	Forests, agriculture, and rough water cause incoherent scatter which negatively impacts the accuracy of analysis. Urban landscapes, built structures and calm water cause coherent scatter which is desirable for InSAR analysis	For best results, choose a study area with land cover that causes coherent scatter
Image Selection	Orbit of satellite, viewing geometry, path of sensor	Any differences in the extent of the selected images, the viewing geometry, the path of the sensor (ascending or descending) will have significant repercussions on the coherence between the images and the accuracy of the DEM	Ensure that the selected images cover the same extent and have the same path. If not, the DEM will not be usable. For best results, select images with as similar viewing geometry as possible to reduce incoherence
Time	Period between the capture of images	The longer the time between image captures, the greater the incoherence between the images and the lower the accuracy of your DEM	For best results, choose images with the lowest possible time between captures. For Sentinel-1 data in Canada, this is usually 12 days
Wavelength	L-band (~20 cm), C-band (~5 cm), X-band (~3 cm)	Shorter wavelengths get reflected by smaller features, meaning that C- and X-band get reflected by tree canopies and other vegetation, while L-band passes through	For best results, use the longest wavelength available to you. If you are using C- or X-band SAR, recognize that your output is a hybrid DEM and DSM depending on the land cover in your study area
Date	Date at which images were captured	If a significant event that impacts the landscape of the study area occurred around the time the images were captured (ie. earthquake, volcanic eruption, bombing, flood) the DEM will not be accurate.	For best results, check if any major geological or anthropogenic occurred around the dates of image capture. If they did, change your selection

Prerequisites

Materials List

1. Sentinel-1 Toolbox (SNAP 11)

2. Statistical-cost, Network-flow, Algorithm for Phase Unwrapping (SNAPHU V2.0.7)

3. Copernicus Browser (Link to browser)

Copernicus

Downloading Sentinel-1 Data

Access the Copernicus Browser

• Select "Login" on the pop up, as you are required to sign in or create a free account to download data from their platform.
  

Login Popup - Login to download data

• On the sign in page either sign in with your credentials or click the “Register” button and follow the instructions to create your account.
  

Copernicus Sign in page

• After logging in, navigate to your AOI (Area Of Interest) on the global map. Remember DEMs created from SAR data work best in urban or non-vegetated areas (Add a little bit of info)
• Once at your AOI, select the "SEARCH" tab on the upper left side of the screen (Red Outline Below).
  

Access Search page

• Once this is done, select both the Sentinel-1 and Level-1 SLC Boxes (Red Outline Below).
  

Select Needed Data Sources (Sentinel-1, C-SAR, Level-1 SLC)

• Next, Scroll down and set the time range using the "From" and "Until" drop down calendars (Red Outline Below). An appropriate range should be within a few years of your analysis (e.g. this tutorial made in early 2025 uses images from 2022). If significant changes in the landscape of you AOI have happened (e.g. earthquake, volcano eruption, flooding), ensure that the time range is set after this event. (Minimum time range...)
  

Select time range

• Click the "Search" button at the bottom of the menu. This will generate results that satisfy your criteria. The menu will display a list of images captured in the time range and the map will show outlines of the image extents in blue.
  

Selectable Images within time range and other criteria

• You can now select an image by clicking on a blue extent that covers your AOI. A pop up window should appear listing the images with that coverage within your time range (See image below). For InSAR, we require two images. Remember that a low temporal baseline (Time between images) will increase the accuracy of you DEM, so selecting two images with the shortest period between captures is suggested.
  

Selectable Images within AOI pop up. Downloading Images

• To download the images click the icon on the far right of each row (Red Outline seen above).
• Once the zip folders containing the images have downloaded, extract the files to the folder you will be using as your directory in the next steps.

SNAP 11

Preprocessing (To be completed with both images)

Opening Data in SNAP

• After launching SNAP, click the folder icon in the top left of the window (Red Outline) or navigate to File -> Open Product. This will open a pop up window.
  

How to access products

• Use the "Look in" dropdown menu (Outlined in red below) to navigate to your directory folder. Once this is done double click the .SAFE folders (highlighted in yellow below) to open them in the file browser.
  

Select your .SAFE folders

• Once in the folder select the "manifest.safe" file and click open.
  

Select and open the manifest.safe file

• The file should open in the product explorer window in the top right
  

S-1 TOPS Split

The S-1 TOPS Split tool allows the user to select which part of the image to use in the analysis. Sentinel-1 images are captured in subswaths and burts. The subswaths divide the extent horizontally in three (3) zones. These subswaths are then divided into vertical bursts. While we are limited to selecting one (1) subswath, we can select multiple bursts, as long as they are adjacent.

• In the I/O parameters tab, make sure the correct image is selected as source, and verify that you are saving to the proper directory.

Make sure the correct source and directory are selected

• In the Processing Parameters tab, select the subswath and burst(s) that cover you AOI. The subswath is selected from the dropdown subswath menu, and burst(s) are selected by dragging the arrows (outlined in red) along the bar. The outline of the area of coverage will be shown on the map below in a white outline. At this stage you also select the polarization of the image. Sentinel 1 SAR data comes in HH, VH, and VH polarization, each with its own uses.
  

Select Polarization

Apply Orbit file

This tool applies information about the orbit of the sensor to the image, correcting for drift in the satellite's path and ensuring that the image is accurate.

• Click Radar -> Apply Orbit File

Radar Drop Down Menu

• In the I/O Parameters, make sure the file ending in "_split" is the source

File:Orbit Page

Apply Orbit Files Processing Parameters

Back Geocoding

This tool combines the input images into a single image.

• Click Radar -> Coregistration -> S-1 TOPS Coregistration -> Back-Geocoding

How to access Back Geocoding through drop down menu

• Add all open files to the ProductSet-Reader by clicking the Add All button (marked in red). Remove the files not ending in “_split_Orb” by selecting them using ctrl+click and clicking the remove button (marked in blue)

Adding files to ProductSet-Reader

• In the Back-Geocoding tab, select SRTM 1Sec HGT (Auto Download) from the Digital Elevation Model dropdown (highlighted in yellow), then click run at the bottom of the window

SRTM 1Sec HGT (Auto Download)

Enhanced Spectral Diversity (Only required if using multiple bursts)

This tool corrects for the Doppler effect which occurs across bursts due to the motion of the satellite during data collection. This process increases accuracy and creates continuity across bursts.

• Click Radar -> Coregistration -> S1 TOPS Coregistration -> S-1 Enhanced Spectral Diversity

How to access Enhanced Spectral Diversity through drop down menu

• I/O Parameters: make sure the "_split_Orb_Stack" ending file is the Source Product and run.

Enhanced Spectral Diversity Processing Parameters

Interferogram Formation

Interferograms allow for the visualization of phase difference between the two input images.

• Click Radar -> Interferometric -> Products -> Interferogram Formation

How to access Interferogram Formation through drop down menu

• In the I/O Parameters tab, make sure the file ending with "_Orb_stack_esd" is the Source Product
• In the Processing Parameters tab, check Subtract Flat-Earth Phase (marked in red) and Substract Topographic Phase (marked in blue) boxes. The Subtract Flat-Earth Phase function accounts for curvature of the earth, and the Subtract Topographic Phase function uses DEM to reduce impact of topographic variation on accuracy.
• Select SRTM 1Sec HGT (Auto Download) as te Digital Elevation Model (highlighted in yellow)

Interferogram Formation Processing Parameters

TOPs Deburst (Only required if using multiple bursts)

This tool removes the line between bursts, merging the bursts into a continuous image

• Click Radar -> Sentinel-1 TOPS -> S-1 TOPS Deburst

How to access S-1 TOPS Deburst through drop down menu

• In the I/O Parameters tab, ensure that the file ending with "_Stack_esd_ifg" is the Source Product
• Run the tool

Phase Filtering

Phase Filtering reduces phase noise, this improves the signal noise ratio of the image resulting in a clearer image.

• Click Radar -> Interferometric -> Filtering -> Goldstein Phase Filtering

How to access Goldstein Phase Filtering through drop down menu

• In the I/O Parameters tab, ensure the file ending with “esd_ifg_deb” is set as the Source Product and run the tool.

Repeat these steps with the second image

SNAPHU

Snaphu Export

We need to export the image to Snaphu

• Click Radar -> Interferometric -> Unwrapping -> Snaphu Export

How to export using Snaphu through the dropdown

• In the "SnaphuExport" tab be sure to change the target folder, and change the Row Overlap and Column Overlap values to 50

Snaphu Export Parameters

• Run the tool

Running Snaphu

Now we need to run Snaphu, navigate to the folder containing the exported file.

• Open the snaphu.conf file in notepad and copy the "command to call snaphu" that starts with snaphu -f snaphu.conf

Snaphu Command

• Open command prompt and navigate to the same folder
• Run the copied command (This process may take a while)

Importing Snaphu back into SNAP

We need to import the unwrapped phase back into SNAP for some final processing and the DEM creation

• Click Radar -> Interferometric -> Unwrapping -> Snaphu Import

How to import using Snaphu through the dropdown

• Navigate to the UnwPhase file (Select the one with the .hdr extension)

File to Import

• Click Radar -> Interferometric -> Products -> Phase to Elevation

How to navigate to Phase to Elevation

• Leave all the options as their default

Post Processing

One final step, terrain Correction

• Click Radar -> Geometric -> Terrain Correction -> Range Doppler Terrain Correction

How to navigate to Terrain Correction

In the Processing Parameters change:

• The digital elevation model to: SRTM 1Sec HGT (Auto Download)
• Map Projection to the desired projection, in this case NAD27(CGQ77) / UTM zone 18N

Changes made to Terrain Correction Parameters

Finally you can export to a GeoTIFF

• Click Fle -> Export -> GeoTIFF / BigTIFF

How to Export

References

Braun, A. (2021). Retrieval of digital elevation models from Sentinel-1 radar data – open applications, techniques, and limitations. Open Geosciences, 13(1), 532–569. https://doi.org/10.1515/geo-2020-0246
Copernicus browser. (n.d.). Copernicus Browser. https://browser.dataspace.copernicus.eu/
Create a DEM using Sentinel-1 Data. (2024, December 3). NASA Earthdata. https://www.earthdata.nasa.gov/learn/data-recipes/create-dem-using-sentinel-1-data#toc-the-dem-product
Geophysical Software Training. (2023, July 22). 8 INSAR - unwrapping - exporting and unwrapping [Video]. YouTube. https://www.youtube.com/watch?v=By6wo0a6xQk

Details

• Last Updated: 2025-04-07
• Published on: 2025-04-07
• Created By: Alex Fortin, Alex Hoferek, Leo Keenan