The LPV LAI Intercomparison

At the conclusion of the first organizaitonal meeting in Ispra in 2000, the Land Product Validation participants recommended several activities deserving immediate attention. One prominent need was for an intercomparison of both research and operational LAI products, together with the development and assessment of standards for field measurement of LAI and the scaling of these measurements to remotely sensed imagery.

Intercomparison Sites

Toward this need, investigators with field measurements agreed to share their data sets for the purpose of intercomparison. An October 2000 survey indicated that field data from at least 22 sites were available. Further, remote sensing data from at least 10 different sensors have been collected in association with these field efforts. These data will serve as the basis for the intercomparison activity. To date, most investigator worked exclusively on their own data sets.

Scroll the menu to the left for a list of LAI intercomparison sites and for access to more detailed site information and to available data.

A Coordinated LAI 10 km Intercomparison Activity

Addressing this question requires a methodical, quantitative intercomparison of similar products, such as the pathfinding work by the CEOS Land Product Validation subgroup on the intercomparison of global LAI products, presented at the 2006 Global Vegetation workshop by Sebastien Garrigues. You may download Sebastien's presentation here .

This study investigates the performance of four major global LAI products at 1/11.2° spatial sampling and a monthly time step: ECOCLIMAP climatology, GLOBCARBON (from SPOT/VEGETATION and ATSR/AATSR), CYCLOPES (from SPOT/VEGETATION) and MODIS Collection 4 (main algorithm - from MODIS/TERRA). These products were intercompared during the 2001-2003 period over the BELMANIP network of sites. Their uncertainties were assessed by comparison with 56 LAI reference maps derived from ground measurements. CYCLOPES and MODIS depict realistic spatial variations at continental scale, while ECOCLIMAP poorly captures surface spatial heterogeneity, and GLOBCARBON tends to display erratic variations. ECOCLIMAP and GLOBCARBON show the highest frequency of successful retrievals, while MODIS and CYCLOPES data are frequently missing in winter over mid and Northern latitudes. CYCLOPES and MODIS have consistent temporal profiles over most vegetation types, while ECOCLIMAP does not show any interannual variation, and GLOBCARBON can exhibit temporal instability during the growing season over forests. The CYCLOPES, MODIS, and GLOBCARBON LAI values agree better over croplands and grasslands than over forests, where differences in vegetation structure between algorithms and surface reflectance uncertainties lead to substantial discrepancies between products. CYCLOPES does not reach high enough LAI to properly characterize forests. In contrast, the other products have sufficient dynamic range in LAI to describe the global variability. Overall, the CYCLOPES product is the most similar to the LAI reference maps. However, more accurate ground measurements and better representation of the global and seasonal variability of vegetation are required to refine this result.

These results have been published in JGR (Garrigues, et al., 2007) and can be downloaded here . Links to the intercomparison data sets will be posted here shortly.  This study is currently being extended to a include  a 1 km intercomparison. Look for information and results of this study here in the future. 

A list of intercomparison sites used in the overview paper for the IEEE TGARS Special Issue on Land Product Validation, Validation of global moderate resolution LAI Products: a framework proposed within the CEOS Land Product Validation subgroup is available for view or download . A global map that includes the set of intercomparison sites is also available via the pull-down 'Sites' menu to the left. Select the 'Site'Map' item in the list (first in list) to go to the map page.

A related paper led by Frederic Baret of the INRA/France, is also included in this special issue. In this paper he proposes a network of sites dedicated to the intercomparison of land biophysical products. This network of sites, the CEOS Benchmark Land Multisite Analysis and Intercomparison of Products (BELMANIP), is a globally distributed set of sites built on existing networks such as EOS Core Sites, Bigfoot, and VALERI where direct biophysical measurements were available, FLUXNET and AERONET sites, where some ground data also existed, and finally the set was completed with additional sites selected to improve the representivity of the latitudinal distribution of domiant surface types. See the manuscript for details of the selection process. A list of the BELMANIP sites can be viewed here.

A flow diagram of the LAI Intercomparison activity is also available.

Sites for which there are LAI maps are currently available:

Background

The basis for this intercomparison activity was formed at an initial meeting hosted by ESA, in Frascati, Italy, in June of 2001.  This meeting provided an opportunity to share insights and to evaluate progress on LAI product validation and to place this work in the context of the Global Observation of Forest Cover (GOFC) program.  Based on the discussion and presentations at the meeting, the LPV subgroup would like to prepare a manuscript of best-practice strategies for validating satellite-derived LAI products.

This ESA kick-off meeting was documented in The Earth Observer article "Summary of the International Workshop on LAI Product Validation."

A follow-up LAI Intercomparison meeting was held in the summer of 2004 at the University of Montana prior the the MODIS Vegetation Workshop II (also found on the Biophysical Meetings page via the 'Biophysical' pull-down menu).  See the Earth Observer summary article of the meeting in the Sept/Oct 2004 issue.

At the 2006 Global Vegetation Workshop in Missoula, Sebastien Garrigues presented his initial results of the 10 km intercomparison effort.   A link to that presentation is provided in the associated paragarph above, where one can also find the link to the 2007 paper published in JGR that Sebastien led.

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Curator: Jaime Nickeson
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Last Updated: December 3, 2008