National Physical Laboratory

TREES

TRaceability in tErrestrial vEgetation Sensors and biophysical products

Data from Earth Observation (EO) satellites are increasingly used to monitor the environment, understand variability and change, inform evaluations of climate model simulations and forecasts, and manage natural resources. Although EO data and products are plentiful, it is still rare for them to have reliable and fully traceable information concerning their quality. Our research aims to provide end-to-end traceability for terrestrial vegetation sensors and satellite-derived products of biophysical Essential Climate Variables (ECVs) through a range of research activities that address:

  • In situ sensor calibration techniques in laboratory and field conditions
  • Field site characterisation
  • Sampling and scaling strategies for satellite product validation
  • 3D radiative transfer modelling and virtual validation site simulations
  • Traceability and uncertainty assessment of product algorithms
  • Developing Quality Assurance frameworks and implementation strategies
  • Assessing user requirements and impacts of improved QA for research and applications

Research Themes

Terrestrial sensor characterisation and development


An important aspect in the drive towards fully traceable satellite products is the provision of fit-for-purpose validation datasets. This means that the sensors contributing to those datasets must be well characterised such that their measured values have an uncertainty traceable to SI (or other community defined standard).

In many cases providing this information is a labour-intensive and expensive task, especially where a large number of sensors are required for field site characterisation. One of our research interests looks to address this through the creation of field calibration standards that can: (a) be used to monitor long term effects without removing sensors from the field; and (b) provide quick, traceable calibration. Similarly, we look to develop sensors capable of addressing deficiencies in current measurement approaches.

Within our group, the primary focus is on the characterisation of sensors used to measure or estimate; photosynthetically active radiation (PAR), leaf area index (LAI), the surface bidirectional reflectance distribution function (BRDF) and shortwave albedo.

 

Spectral response of two PAR sensors

An example of the setup used to characterise the spectral response of two PAR sensors

Field site characterisation


The characterisation of field sites is important for the monitoring of vegetation dynamics. At NPL, we have expertise in traditional field surveys as well as novel 3D LiDAR techniques in both a research and operational context.

3D terrestrial LiDAR animation of the canopy walkway in Wytham Woods. 3D data were collected with a RIEGL-VZ400 terrestrial laser scanner and RGB information was captured with a NIKON D800. The terrestrial LiDAR dataset will be used as a validation dataset for tree and canopy structure.

Summer field campaign at Wytham Woods, Oxford

Six hectares of forest were sampled with a range of optical devices to estimate forest structure, leaf/canopy area and the spectral properties of individual foliage elements, bark and understory. See below for a description of the instruments and measurements taken during the field campaign.

The field team consisted of Kim Calders, Ally Barker, Niall Origo, Joanne Nightingale and Mat Disney (UCL-Geography). The team were supported by Tobias Jackson (Oxford University) and Daniel Fox (NPL summer student).

Wytham field campaign - Terrestrial Laser Scanner (TLS)

 

Terrestrial Laser Scanner (TLS): A ground-based full-waveform LiDAR system which measures the position and reflectance of objects in the scene. This data will be used as input to a reconstruction algorithm which will provide spatially explicit (3D) tree models.

 

LAI-2000 and 2200: These instruments estimate leaf area index from measurements of incoming light made above and below the canopy at 5 angles.

 

TRAC: TRAC (Tracing Radiation and Architecture of Canopies) measures canopy 'gap size' distribution in addition to canopy 'gap fraction'. Gap fraction is the proportion of gaps in the canopy at a given solar zenith angle.

 

Wytham field campaign - DHP Spirit Level  Wytham field campaign - DHP

DHP: Digital Hemispherical Photography, also known as fisheye photography, is a technique to characterize plant canopy geometry using photographs taken below the canopy (looking upward) through an fisheye lens.

 

Wytham field campaign - Spectrometer

 


Spectrometer: Spectral measurements of the leaves, bark and understory were acquired using an ASD field spectrometer. The ASD measures reflectance in the 350 nm – 2500 nm wavelength range. This information will be used in conjunction with the 3D tree models to provide replica scenes for radiative transfer modelling of forests.

 

 

 

Winter field campaign at Wytham Woods, Oxford

From December 2015 – January 2016 we returned to Wytham Woods to re-measure the forest in 'leaf-off' conditions. This allowed us to get better data on the upper canopy branching structure with the TLS, while also helping us to characterise the woody contribution to the DHP and LAI-2x00 measurements.

A comparison between two hemispherical photographs from the same location is shown below.

Winter field campaign at Wytham Woods, Oxford

 

Quality Assurance Frameworks


NPL is the secretariat for the Group on Earth Observations (GEO) Quality Assurance for Earth Observation (QA4EO) framework, making it well positioned to provide the guidance and overarching technical assessment of the quality of any MRV service. Quality Assurance (QA) is defined by ISO (8402: 1994) as:

"The assembly of all planned and systematic actions necessary to provide adequate confidence that a product, process, or service will satisfy given quality requirement."

The provision of reliable quality information on the development of biophysical products enables end users to make informed decisions regarding the fitness-for-purpose of the data for their application. Tools, methods, data and procedures employed in the development of biophysical products are assessed. Provision of such information in a uniform and useful manner can be achieved through the implementation of robust quality assurance (QA) frameworks.

At NPL we are developing QA frameworks based upon good practice for Quality Assurance/Quality Control and Verification outlined in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories and the generic quality assurance principles applied within other industries, and tailoring them to the earth observation community. Projects such as QA4ECV and FOREST have developed QA frameworks which consider Essential Climate Variables (ECVs) product generation and forest products within an MRV system respectively.

The QA framework is organised into six steps which identify how QA should be integrated into MRV services:

Quality Assurance Frameworks - TREES

These QA frameworks may be implemented by:

  1. MRV system developers
  2. customers wanting to assess the quality of the procedures and uncertainty in the product of a service they procured
  3. countries/governments interested in the quality of their forest MRV products. Implementing QA procedures may result in improvements in methodologies, datasets and a reduced error in carbon estimates.

Validation, Sampling, Scaling and 3D RT modelling


Spatial variance is an important factor in upscaling variables inferred from a single sample location to plot level and from plot level to regional level. We report on uncertainties associated with sampling designs and typical field measurements used in the in-situ estimation of biophysical essential climate variables (ECVs) and produce recommendations towards best practice guides.

We use detailed 3D models and radiative transfer modelling for the validation of ECVs. These 3D models can either be existing virtual study sites or they can be derived from detailed in-situ measurements using terrestrial LiDAR, unmanned aerial vehicles (UAVs) and spectral measurements.

TREES validation

Applications

  • Species identification
  • Deforestation
  • Biomass retrieval

Our research also offers opportunities for a more consistent and robust framework to support REDD+ monitoring capacities in developing countries. Our terrestrial LiDAR applications demonstrated the reduction in uncertainties in forest inventories and above-ground biomass assessment.

User requirements - QA for research and applications


To be able to develop suitable research, tools and systems which suit the needs of the community, the requirements of both the generators, and the users, of satellite derived biophysical products need to be understood.

At NPL, we are working to understand the requirements of the users, engaging with them to determine the types of quality assurance information they require and how they would use quality assurance information, among other things. For example, a user survey was conducted under QA4ECV, which suggested that more detailed and easily accessible uncertainty information should be provided to users.

QA4ECV: Developing Metrological Traceability Through ECV Products

Projects

Current Projects

QA4ECV - Quality Assurance for Essential Climate Variables
January 2014 - December 2017

MetEOC-2 - Metrology for Earth Observation and Climate
September 2014 - August 2017

Past Projects

FOREST - Fully Optimised and Reliable EmissionS Tool
January 2014 - December 2015
FOREST project video
Development of FOREST services 

TruDat - Trustable Data for Carbon, Science and Finance
January 2013 - December 2013

Defra - Defra EO site project to establish a test site to help develop methods to use EO measurements to establish a spectral database to identify individual species of tree again in support of the EU Timber Directive

Defra Review of forest carbon measurement techniques to identify ways to help the enforcement of the EU Timber Directive, particularly that relating to the due diligence required within the supply chain to demonstrate that timber products placed on the market within the EU are not the result of illegal logging

Publications

Calders, K., Disney, M., Armston, J., Burt, A., Brede, B., Origo, N., Muir, J. and Nightingale, J., Evaluation of the Range Accuracy and the Radiometric Calibration of Multiple Terrestrial Laser Scanning Instruments for Data Interoperability, IEEE Transactions on Geoscience and Remote Sensing, In Review (2016)

Origo, N., Calders, K., Nightingale, J. and Disney, M., Influence of levelling technique on the retrieval of canopy structural parameters from digital hemispherical photography, Agricultural and Forest Meteorology, In Review ;(2016)

Nightingale, J., Scanlon, T., Origo, N., Calders, K., Fox, N. and Muller, J-P., Development of robust quality assurance procedures for terrestrial essential climate variable data products derived from earth observation satellites., IEEE International Geoscience and Remote Sensing Symposium (IGARSS 2016), 10-15 July 2016, Beijing, China. pp 1920-1923 (2016)

Calders, K., Burt, A., Origo, N., Disney, D., Nightingale, J., Raumonen, P. and Lewis P., Large-area virtual forests from terrestrial laser scanning data, IEEE International Geoscience and Remote Sensing Symposium (IGARSS 2016), 10-15 July 2016, Beijing, China. pp 1765-1767 (2016)

SimpleTree - An Efficient Open Source Tool to Build Tree Models from TLS Clouds
Hackenberg, J., Spiecker, H., Calders, K., Disney, M. and Raumonen, P.
Forests 6, 4245-4294 (2015)

Terrestrial Laser Scanning For Plot Scale Forest Measurement
Newnham, G.J., Armston, J.D., Calders, K., Disney, M.I., Lovell, J.L., Schaaf, C.B., Strahler, A.H. and Danson, F.M.
Current Forestry Reports 1, 239-251 (2015)

Fernandes, R., Plummer, S., Nightingale, J., Baret, F., Camacho, F., Fang, H., Garrigues, S., Gobron, N., Lang, M., Lacaze, R., LeBlanc, S., Meroni, M., Martinez, B., Nilson, T., Pinty, B., Pisek, J., Sonnentag, O., Verger, A., Welles, J., Weiss, M., Widlowski, J-L., Schaepman-Strub, G., Roman, M., Nickeson, J., Global Leaf Area Index Product Validation Good Practices. Committee On Earth Observation Satellites, Working Group on Calibration and Validation, Lan Product Validation sub-group, Edition 2.0 (2014)

Defra: Review of UK applicability of worldwide forest carbon measurement techniques
Barker, A, Beckmann, M, Nightingale, J
NPL Report CCM 1, March 2014
Abstract: This report provides an overview and case studies for the three approaches to forest carbon measurement: in situ, airborne and spaceborne measurement. The advantages and disadvantages of methods currently applied to estimate forest biomass worldwide are discussed, with a combined approach recommended for a UK-based forest assessment. Effective resource management for timber stocks and deforestation in the UK requires accurate forest-non-forest mapping to monitor tree cover before biomass can be calculated.

Quality Assurance Framework for Forest Monitoring Programmes
Barker, A, Origo, N, Nightingale J (2015)

News

  • NPL Field Campaign over a 6 ha plot in Wytham Woods, Oxford commenced 8 June - 4 July 2015. Measurements are being conducted using; an ASD Spectrometer, Tracing Radiation and Architecture of Canopies (TRAC) instrument, Digital Hemispherical Camera (DHC), LiCOR LAI-2000 plant canopy analyzer, LiCOR LAI-2200 plant canopy analyzer, Terrestrial Laser Scanner (TLS) and a leaf gonio (PHYTOS).
  • IGARSS
    Dr Joanne Nightingale attended and presented two papers at the IGARSS conference in Beijing, 10-15 July 2016. Quality Assurance Procedures for Essential Climate Variable Products Derived from EO Satellites invited presentation within the Calibration, Validation and Related Topics in Support of Spaceborne Imaging Spectroscopy Missions session; and Large-Area Virtual Forests from Terrestrial Laser Scanning Data in the Forest Monitoring by Lidar and Multispectral Techniques Session.
  • Supporting C3S QA Development
    The ECO group, in collaboration with the University of Reading and Telespazio-France, have successfully secured funding through ECMWF to support the development of the Copernicus Climate Change Service. Our work will focus on designing and prototyping an evaluation framework that will ensure the data provided through the C3S climate data store is fully traceable, adequately documented and ensures that sufficient attention has been given to the assessment of uncertainties and user guidance.
  • Robert May Prize
    NPL's Dr Kim Calders has won the Robert May Prize for the best paper submitted by an early-career researcher. The prestigious prize, awarded each year by the British Ecological Society, was given to Dr Calders for his paper on non-destructive mass sampling of above-ground biomass of forest ecosystems. The British Ecological Society described the paper as both "a great achievement of international science" and "an important addition to the repertoire of ecological measurement tools." Read abstract here
  • ESA Living Planet Symposium
    The ESA Living Planet Symposium, held in Prague from 9-23 May 2016, was a conference with over 3,000 participants covering all aspects of Earth Observation. NPL was well represented at this conference with oral presentations by Tracy Scanlon on estimating uncertainties through albedo products, Dr Kim Calders on building 3D forest models from terrestrial laser scanning and Niall Origo on validating fAPAR using wireless sensor networks and 3D forest models.

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