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

Important Details

Pair Selection for DEMs

Sentinel images to create the DEM can be downloaded using the Copernicus Browser (LINK). The ideal pair for the creation of our DEM would have a large perpendicular baseline and a small temporal baseline.

More Details for Pair Selection

Overlap in the imagery is essential. The Pair needs to share the same path number and cover the same area. Images taken with different flight directions (ascending vs descending) cannot be used as pairs for interferometry.

Coherence is crucial. You should select images with the shortest possible temporal baseline. This will minimize both coherence loss and potential surface motion. A larger time span between images will lead to higher decorrelation.

Baseline is important. The perpendicular baseline does significantly affects the sensitivity of the interferogram to height variations. While a larger baseline improves sensitivity, it also reduces coherence. If the perpendicular baseline exceeds approximately 3/4 of the critical baseline (about 5 km for Sentinel-1A), the images will likely become decorrelated.

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.
  

File:Copernicus Login

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.
  

File:Sign in Copernicus Popup.png

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).
  

File:Copernicus Search

Access Search page

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

File:Copernicus Data Sources

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

- Time range

- Search

- Select an image

- Download 2 images.

SNAP 11: Step-by-Step

Preprocessing (To be completed with both images)

open product in snap folder icon (File -> open product) - "Lock in" dropdown menu (Red outline) navigate to directory folder - .SAFE (Yellow highlight) open in file browser - Select "manifest.safe" file in folder Open - File will open in product explorer window - repeat process with second image.

S-1 TOPS Split

Apply orbit file

Backgeocoding using Sentinel-1 TOPS Coregistration -> Back-Geocoding

Enganced Spectral Diversity **Only if multiple bursts**

Interferogram formation

TOPs Deburst

Phase Filtering

Opening Data in Sentinel-1 Toolbox

• Open the Products
• View the Products
• Visualize a Band

Coregister the Data

• Coregister the Images

Interferogram Formation and Coherence Estimation

• Form the Interferogram

Visualize Interferometric Phase

• TOPS Deburst

Multi-looking and Phase Filtering

• Phase Filtering

Unwrap an Interferogram with SNAPHU

• Export to SNAPHU
• Install a Linux VM
• Unwrap an Interferogram with SNAPHU
• Move Files to PC/Mac

Open the Files in Sentinel-1 Toolbox

• Snaphu Import

Geocode the DEM

• Terrain Correction

Details

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