Calculating biomass from Sentinel-1 SAR data 

Mar-17: The tutorial has been written for F-TEP portal version 0.4.3.

Radar signals can penetrate into forest canopy and scatter back from plant elements of various sizes. The greater biomass in a forest the higher the backscatter.  L-band and longer wavelength radar data have been used successfully to map forest biomass for large areas.  Even though the C-band radar of Sentinel-1 may saturate at moderate forest biomass levels, biomass maps can be produced.  The Sentinel-1 biomass service of F-TEP uses multi-temporal VH-polarized data from dual-polarized IW (Interferometric Wide swath) products.  The biomass mapping is based on multi-temporal averaging combined with a fixed model relating radar backscatter to forest stem volume or biomass. The model is based on the water cloud model introduced by Attema and Ulaby 1978 (Attema, E. and Ulaby, F. 1978. Vegetation modeled as a water cloud, Radio Science, vol. 13, no. 2, p. 357-364).

The Sentinel-1 biomass service within F-TEP requires 3 data inputs:

  • A geotiff file to be used as a Digital Elevation Model (DEM)
  • A Forest mask
  • A collection of Sentinel-1 images collected into a Databasket 

The DEM as a geotiff file is required so that the service is available globally, not just in the zone between latitudes -60 degrees to 60 degrees, where SRTM DEM is available in SNAP.  The DEM must be in plate carrée projection (regular grid in latitude and longitude).  Its unit must be 1 meter (i.e the pixel value is the elevation of the pixel in metres). The name of the DEM file must begin with “DEM_” and it must end in “.tif”.  The DEM file must be loaded to some web server so that it can be accessed via its URL (Universal Resource Locator) or web address. An example of a valid DEM address is:

http://enviobserver.vtt.fi/ftepref2/DEM_ChiapasSRTM.tif

The Forest mask must be a geotiff file, whose name begins with “ForestMask_” and ends in “.tif”.  The pixel value must be 1 for forest and 0 for areas outside forest. The forest mask must also be loaded to a web server. The forest mask can be made using the Sentinel-2 land cover service, but usually this requires that all forest classes are combined to a single forest class with a class code of 1 and all non-forest classes to a single non-forest class with a class code of 0. An example of a valid forest mask address is:

http://enviobserver.vtt.fi/ftepref2/ForestMask_chiap.tif

The input Sentinel-1 images must be grouped into a Databasket.  Adding input scenes one by one without data baskets may be possible but not feasible.  Use the search tool and specify VH as the polarisation.  All images must cover exactly the same area, so either ascending orbit images (from south-east to north-west) or descending-orbit images (from north-east to south-west) should be used.  Only images in GRDH format can be accepted.

Running the Biomass service:

The Sentinel-1 biomass service requires defining the following 6 parameters:

  1. Input data: The input Sentinel-1 data files which have been collected in the input data basket. To select them, drag the input Data Basket (using the horizontal lines to the left of the name, marked 1a below to Input Field 1) 
  2. Reference data archive containing digital elevation model: Expected is a single .tif with filename matching 'DEM_*.tif' (or a .zip containing same). Write the URL or web address of the DEM file to Input Field 2. 
  3. Reference data containing forest mask: Expected is a single .tif with filename matching 'ForestMask_*.tif' (or a .zip containing same). Write the URL of the Forest Mask file to Input Field 3.  
  4. Target CRS identifier: Write the code for the output Coordinate Reference System (CRS) to Input Field 4.  This must be a string beginning with “EPSG:”, and the following numeric code must be a valid system as defined by the European Petroleum Survey Group.  For UTM zones in northern hemisphere and with the WGS84 datum, the numeric code is 32600 + the UTM zone number.  An example of a valid CRS specification is: 'EPSG:32615'  
  5. Area of interest (AOI): Well-known text POLYGON describing the AOI to be processed. This can be copied from the area drawn on the map by pressing the Copy from Map button. Alternatively this can be added as simple text, although it needs to follow the WKT POLYGON format. An example of a valid AOI specification is: POLYGON((-92.906633 16.190411,-92.066559 16.188383,-92.070266 15.376645,-92.907004 15.378567,-92.906633 16.190411)). The AOI should be tight enough so that it only includes pixels in the common overlap area of all input images.  Areas with less radar observations get artificially small stem volumes.
  6. Target image resolution: Write the desired pixel spacing of the biomass map (in metres) to Input Field 6. For Sentinel-1 SAR data feasible values are between 20 and 100m. Example: '20'  

After all input fields have been filled in, the Sentinel-1 biomass service is launched by clicking the round, pink-white play button at the bottom right corner of the input dialogue area.

A typical output biomass map is displayed below using the SNAP applicaiton: