README_DEM_SNAP.md

# Interferometric DEM Generation using Sentinel-1A SLC and SNAP

This guide walks you through the process of generating a **Digital Elevation Model (DEM)** using **Sentinel-1A IW SLC products** with the **Sentinel Application Platform (SNAP)**.

## Overview

**Sentinel-1** satellites provide SAR data in SLC format, which can be used to generate interferometric DEMs using InSAR techniques. This involves processing a pair of SLC scenes over the same area, computing their phase difference (interferogram), and unwrapping the phase to generate elevation data.

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## Requirements

- **ESA SNAP Toolbox** with **SAR/InSAR plugins**
- **Sentinel-1 IW SLC product (2 scenes, same orbit, different acquisition dates)**
- **SRTM 1sec HGT DEM** (automatically downloaded by SNAP during terrain correction)
- At least **8 GB RAM**

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## Input Data

- Download **two Sentinel-1 IW SLC scenes** for the same area with temporal baseline of ~6–24 days using ASF Alaska-1 Vertex by uploading the kml file for the targeted event co-ordinates (lat,long).
- Make sure both scenes are from the **same orbit direction** (Ascending/Descending).

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## Processing Steps in SNAP GUI

### 1. **Split (Optional if specific sub-swath needed)**

- Menu: `Radar > Sentinel-1 TOPS > Split`
- Select IW1, IW2, or IW3 and the burst as needed

### 2. **Apply Orbit File**

- Menu: `Radar > Apply Orbit File`
- Input: Both SLC images
- Output: Orbit-corrected products

### 3. **Subset (Optional)**

- Use if you want to focus on a smaller area (can speed up processing)

### 4. **Co-registration: S-1 TOPS Coregistration**

- Menu: `Radar > Interferometric > InSAR Stack Preparation > S-1 TOPS Coregistration`
- Input: Master and Slave images
- Output: Stack product

### 5. **Enhanced Spectral Diversity (ESD)**

- Menu: `Radar > Interferometric > InSAR Stack Preparation > Enhanced Spectral Diversity`
- Input: Co-registered stack
- Output: ESD-corrected stack

### 6. **Interferogram Formation**

- Menu: `Radar > Interferometric > Products > Interferogram Formation`
- Input: ESD-corrected stack
- Output: Wrapped interferogram

### 7. **Deburst**

- Menu: `Radar > Sentinel-1 TOPS > Deburst`
- Input: Interferogram
- Output: Continuous interferogram

### 8. **Goldstein Phase Filtering**

- Menu: `Radar > Interferometric > Filtering > Goldstein Phase Filtering`
- Output: Filtered interferogram

### 9. **Phase Unwrapping (SNAPHU Export/Import)**

- Export data for SNAPHU:
  - Menu: `Radar > Interferometric > Unwrapping > SNAPHU Export`
  - Run SNAPHU (external tool)
- Import unwrapped phase:
  - Menu: `Radar > Interferometric > Unwrapping > SNAPHU Import`

### 10. **Phase to Height Conversion**

- Menu: `Radar > Interferometric > Products > Phase to Elevation`
- Input: Unwrapped phase

### 11. **Range-Doppler Terrain Correction**

- Menu: `Radar > Geometric > Terrain Correction > Range-Doppler Terrain Correction`
- DEM: Select SRTM 1Sec HGT (auto-download)
- Output: Final geocoded DEM

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## Output

- A **GeoTIFF** or **BEAM-DIMAP** file containing your **interferometrically derived DEM**.
- You can view, export, or analyze the DEM using QGIS, SNAP, or any GIS tool.

## Sample Output:

<img src="assets/dem.png" alt="DEM" width="400" height="500">

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## Additional Notes

- **Temporal Baseline**: Try pairs that are not too far apart in time to maintain coherence. Recommendation [ within 24days and 150 - 250m ]
- **Coherence Map**: Check before unwrapping. Poor coherence leads to unreliable DEMs.
- **SNAPHU**: Needed for phase unwrapping. Install separately and configure its path in SNAP.

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## References

- [SNAP Documentation](https://step.esa.int/main/doc/)
- [Sentinel-1 User Guide](https://sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar)
- [InSAR Principles: ESA Training](https://earth.esa.int/documents/10174/12102/InSAR_Principles_Part1)

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