Vegetation fiducial reference test sites for satellite product validation
Satellites are used extensively to monitor properties of the Earth’s vegetation, offering an effective way to obtain frequent, global measurements where ground-based measurements are not practical. Applications of these measurements range from climate, environmental and ecological monitoring, agriculture, forestry and land management.
Ideally, information derived from satellites should be validated against reference measurements (either in situ or airborne) of the same quantity, which are traceable to SI and whose uncertainties are well characterised.
Traceable end-to-end quality assessment of satellite products, that includes uncertainty characterisation of the retrieval algorithm as well as the reference (in situ or airborne) data and methods used to validate the algorithm, is critical for understanding the derived information, as well as the nuances between the wealth of similar satellite derived products that are currently available.
NPL have been working at Wytham Woods site in Oxford, UK to develop and test the methods which should be followed by the international validation community when:
- collecting campaign and permanent fiducial reference measurements, and
- performing validation of satellite derived products, over the vegetated land surface.
This work aims to establish Wytham Woods as one of a network of global CEOS (Committee on Earth Observation Satellites) “supersites” for Earth observation calibration and validation.
Wytham Woods 3D Model
The Wytham Woods 3D model was developed in order to aid the validation of satellite biophysical products by allowing users to test the assumptions associated with their associated processing chains. The model uses terrestrial laser scanning (TLS) data as the basis for the 3D information, and spectral reflectance measurements as the basis for the radiometric information associated with the forest constituents (such as leaves, bark andsoil). The structural components, derived from the TLS data, are built using the quantitative structure modelling (QSM) approach developed by Raumonen et al. (2013). The leaves are added according to the leaf area index (LAI) estimated using the TLS leaf-on data and the algorithm given in Åkerblom et al. (2018).
The model is described in detail in Calders et al. (2018). It is based on data from summer and winter field campaigns conducted in 2015.
Leaf-on >> leaf-addition
Leaf-off >> tree architecture
Wytham Woods test site in 2021 as part of further EMRP grant funding. The 2021 rescan and new model will enable additional exciting research into model validation and change detection at the site.
The Wytham Woods 3D model was developed by NPL and funded through the Metrology for Earth Observation and Climate project (MetEOC-2), grant number ENV55 within the European Metrology Research Programme (EMRP). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.
Please note, our data is free and open access. However, we do kindly ask if you use the data that you reference the source appropriately.
The fraction of absorbed photosynthetically active radiation (fAPAR) is a key parameter in photosynthesis models which inform our understanding of the productivity of vegetation as well as its ability to absorb light and water. Satellites routinely estimate fAPAR from space. Due to the varied nature of the reflectance models that underpin the retrieval of fAPAR from satellites, agreement between different fAPAR products is not common. As such, validation of the output products is required.
At NPL we have created a PAR network which is capable of estimating various types of fAPAR Widlowski, 2010. The network involves placing PAR sensors above and below the canopy to measure the incoming and outgoing PAR at that location. A key component of the data coming out of the PAR network is that it is traceable to the SI and has associated uncertainties. This is done by calibrating and characterising the sensors before they are deployed in the field.
An example of the fAPAR retrieved from the network is shown here:
Wytham Woods 3D model applications
The primary role of the 3D model was to serve as a validation tool. Since satellite retrievals of fAPAR are dependent on a reflectance model, the choice of model and conditions are related to the definition of that specific fAPAR. As such, in order to compare different fAPAR quantities we require a means of normalising these differences. Using the Wytham Woods 3D model and a radiative transfer model (RTM) we are able to allocate a bias associated with each assumption made in the reflectance model. This allows us to validate the fAPAR products fully, as shown graphically below.