I am using the ESA open scientific data hub for Sentinel 1 SAR imagery (Interferometric Wide swath SLC VV polarization) and would like to know how to perform vegetation health analysis from this data? What tools are available for this using ESA's SNAP open source software?
[GIS] How to analyze Vegetation Index using Sentinel-1 SAR Imagery
remote sensingsentinel-1sentinel-snapvegetation-index
Related Solutions
I'm guessing you never worked with SAR data before, so I'll break your question down into parts I can answer:
1) Create high resolution DEMs in GIS
The process of creating a DEM just from SAR data is quite complex and requires a lot of processing power and memory. I don't know of a GIS software that implements DEM creation due to these constraints.
2) Investigate sites/landscapes and model their change
If you want to model/observe elevation change Differential Interferometry is the key word you should be looking for. This approach enables you to observe even small changes in elevation without the complex process of creating two DEMs.
3) How to produce high-resolution DEMs
This is a complex process requiring a lot of steps. I highly suggest you read up on SAR Interferometry in general, since Sentinel-1 data has basically the same (dis)-advantages as any other SAR data.
ESA provides a free book containing all relevant information regarding SAR interferometry, be it from a mathemathical as well as a processing viewpoint:
Steps required to create a DEM
- co-register scenes
- calculate baselines
- create interferogram and extract phase information
- correct for sensor movement, flat earth removal, athmosphere, etc.
- convert differential phase to absolute phase differences
- phase unwrapping
- phase to height conversion
- correct for absolute elevation, phase jumps, etc.
This is obviously just an outline. The detailed processes for each step are explained in the book I refered to and, in theory, can all be executed with the S1-Toolbox (soon to be SNAP) or the Next ESA SAR Toolbox (NEST).
The intensity image should be used for calibration and subsequent classification of geophysical features. To radiometrically calibrate the intensity, use the Calibrate tool in the Sentinel-1 Toolbox (SAR Processing > Radiometric > Calibrate). The S-1 Level 1 GRD product includes several Look-Up Tables (LUTs) to convert intensity values into sigma or gamma naught values. Use these LUTs as the processing parameters for the Calibration tool.
Using amplitude would produce a calibration result of a lower magnitude than using intensity because intensity equals amplitude squared. The calibration is applied using the equation
For more information about the calibration process see the ESA Sentinel-1 Technical Guide.
Intensity depends on the scattering properties of surfaces (if the spatial extent of the scattering medium is large relative to the signal wavelength) or objects (if the spatial extent of the scattering medium is smaller than the signal wavelength). The effectiveness of a scatterer is determined by the radar cross section (RCS). Many parameters influence the RCS: surface roughness, local geomorphology, scatterer density, 3-D distribution of scatterers, and the dielectric properties of the scene. These variables can help classify features in an intensity image.
Best Answer
No way to calculate NDVI from SAR imagery, like Sentinel-1 images(any polarisations). To calculate NDVI you could get 2 bands (Near Infrared and Red bands) from Sentinel-2 or Landsat-8.
So, about SAR. You just only can get some value about biomass or LAI (Leaf Area Index) and after that you can try to search some formulas to indexing vegetation for specific fields, agricultural, forests or something else.