Upload README_DEM_SNAP.md

README_DEM_SNAP.md

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+# 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.
+
+---
+
+## 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**
+
+---
+
+## 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).
+
+---
+
+## 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
+
+---
+
+## 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">
+
+---
+
+## 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.
+
+---
+
+## 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)
+
+---