VegetationIndicesSubmitted by admin on Wed, 2016-12-14 23:09
Calculation of a variety of vegetation indices using Sentinel-2 data
Sep-2017: Note that there are now two separate services: VegetationIndices and VegetationIndicesOLD. The first, VegetationIndices, is a newly revised version designed and tested to work with the current singe-tile Sentinel-2 data format, and supporting the use of the full 10 m resolution. On the other hand, VegetationIndicesOLD can be used with the old format Sentinel-2 data files, and only the use of 20 m resolution is recommended to avoid processing faults. The old format data can be recognized by its size (as well as the date); they are roughly 6-7 GB instead of the new single tile format data files of roughly 1 GB.
Apr-2017: This tutorial has been written for F-TEP Portal version 0.4.3
The measured reflectance in various parts of visible and infrared spectrum (i.e. in various spectral bands of the MSI instrument aboard Sentinel-2 satellites) correlates to various properties of vegetation canopies. When applying arithmetic operations between spectral bands, indices with even better correlation to canopy properties and photosynthetic capacity can be derived. The most commonly-used such index is the Normalized Difference Vegetation Index (NDVI), which correlates very strongly to the proportion covered by leaves when the background is bare soil and when the proportion covered by leaves is not excessively high. In addition to NDVI, several other vegetation indices have been developed. These newer indices make modifications to counter-act effects of soil properties or atmospheric effects. The indices covered in the F-TEP Sentinel-2 Vegetation Index service are:
- NDVI, Normalized Difference Vegetation Index (NIR-Red)/(NIR+Red),
- TNDVI, Transformed Normalized Difference Vegetation Index (sqrt(NDVI + 0.5)),
- RVI, Ratio Vegetation Index (Red/NIR),
- SAVI, Soil Adjusted Vegetation Index (NDVI*(1+S), S = soil adjustment factor),
- TSAVI, Transformed Soil Adjusted Vegetation Index,
- MSAVI, Modified Soil Adjusted Vegetation Index,
- MSAVI2, Modified Soil Adjusted Vegetation Index 2,
- GEMI, Global Environment Monitoring Index, &
- IPVI, Infrared Percentage Vegetation Index.
Sentinel-2 Vegetation Index service data inputs
The Sentinel-2 vegetation Index service within F-TEP requires one or more Sentinel-2 images as its input data. Note that cloud-free images should be selected, as the service does not perform radiometric corrections or cloud masking.
Running Sentinel-2 Vegetation Index service
The Sentinel-2 Vegetation Index service requires defining 5 parameters in the Service dialogue panel shown below:
These parameters should be defined in the following order:
- Select the Input data: The input Sentinel-2 image which has been previously defined. How to search and select satellite images in F-TEP explorer is described here. Drag the input Sentinel-2 image (grabbing from the horizontal lines in front of its name) from the Results tab at the bottom of the screen to the input field.
- Select the Vegetation Index: Using the drop-down list, select a vegetation index. Options include: NDVI, TNDVI, RVI, SAVI, TSAVI, MSAVI, MSAVI2, GEMI, IPVI.
- Select the Target CRS identifier: Write the code for the output Coordinate Reference System (CRS). This must be a string defined beginning with 'EPSG:' -- the following numeric code must be a valid system as defined by the European Petroleum Survey Group. For UTM zones in the 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:32635' (valid in Finland).
- Define the Area of interest: Define 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 in the Well-known text (WKT) format, following the WKT POLYGON specification. 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)).
- Define the Target Image Resolution: Write the requested output image pixel spacing (in metres). For Sentinel-2 optical data, feasible values are between 10 and 100 m. Example: '20'
After all input fields have been filled in, the service is launched by clicking the round play button at the bottom right corner of the input dialogue area. A typical processing time for a single tile product is five minutes. The resulting output can be viewed on top of the map interface, downloaded, or re-used as input in other services, including the GUI applications such as SNAP or QGIS. Tutorials on opening data within the Sentinel Toolbox (SNAP) and QGIS are provided. The image below shows a generated TSAVI index map within SNAP.