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Focus Area on Vegetation Index Product Validation


Tomoaki Miura, University of Hawai'i, USA
Else Swinnen, VITO, Belgium

 

Vegetation Index Definition

Satellite-based vegetation index (VI) products products are used to monitor temporal and spatial variations in vegetation density and health. VIs are optical measures of vegetation canopy ‘greenness’, a direct measure of photosynthetic potential resulting from the composite property of total leaf chlorophyll, leaf area, canopy cover, and structure. They are also widely used as proxies in estimating canopy state variables (leaf area index, fraction of absorbed photosynthetically-active radiation, chlorophyll content, vegetation fraction) and canopy biophysical processes (photosynthesis, transpiration, net primary production).

In general, a vegetation index combines surface reflectance in two or more spectral bands, often red and near-infrared. They vary from simple ratioing to linear combination of multiple bands. The aim of combining bands is to minimize the effect of external factors, such as the atmosphere, or internal, such as sensor noise, from the satellite signal. More commonly used satellite vegetation indices are the normalized difference vegetation index (NDVI) and the enhanced vegetation index (EVI) while there are many others.

Units: unitless [ratio]

 

Highest Validation Stage Currently Reached for Satellite-Derived Vegetation Indices

Validation stage 3 (LPV validation stage hierarchy) - Satellite VI products have been validated into various validation stages at their respective institutions. For example, MODIS VI products have reached validated stage 3, whereas LTDR AVHRR NDVI product is considered at validated stage 1 or 2. The CEOS LPV Vegetation Index focus area is currently developing a validation good practice protocol with which every VI product’s validation status can be measured and established in a universal fashion.


Validation Good Practices

The CEOS LPV Vegetation Index focus area is currently developing a good practice protocol for the validation of satellite-derived VI products. The three components listed below are considered needed to characterize VI uncertainties and to satisfy the user needs:

  • uncertainty of VIs in their units
  • characterization of VI value changes with respect to changes in actual vegetation conditions (biophysical and/or physiological)
  • long-term stability of VI time series datasets

Approaches and methodologies used in previous studies are being evaluated and considered for incorporation into the protocol. These include:

  • Inter-comparison or cross-comparison of VI products from similar satellites (e.g., MODIS, VIIRS, SeaWiFS, and SPOT VGT) or from satellites with differing spatial resolutions (e.g., MODIS vs. Landsat)
  • Using AERONET-corrected surface reflectance from the same sensor data as a reference
  • In situ reflectance data (e.g., ground-, tower-, and UAV-measured reflectance)
  • In situ time series measurements of vegetation productivity (e.g., biomass, GPP, and crop phenology) or environmental condition (e.g., precipitation)
  • Error analysis/error budget analysis

Vegetation Index Validation Reference Data Sets

  • NEON Airborne Hyperspectral Data
  • AERONET-based Surface Reflectance Dataset
  • Ground and Drone Observational Reflectance Data
  • FLUXNET

LPV Focus Areas

 

Meetings

EGU 2017, 23-28 April 2017, Vienna, Austria. Abstract submission due 11 Jan 2017, details here.

ESA Annouces first gathering of the Sentinel-2 Validation Team (S2VT), 8-29 November 2016, at ESRIN, Frascati, Italy.

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