Empirical validation of photon recollision probability in single crowns of tree seedlings

dc.contributorAalto Universityen
dc.contributor.authorHovi, Aarneen_US
dc.contributor.authorForsström, Petrien_US
dc.contributor.authorGhielmetti, Giuliaen_US
dc.contributor.authorSchaepman, Michael E.en_US
dc.contributor.authorRautiainen, Miinaen_US
dc.contributor.departmentDepartment of Built Environmenten
dc.contributor.organizationUniversität Zürichen_US
dc.description| openaire: EC/H2020/771049/EU//FREEDLES
dc.description.abstractPhysically-based methods in remote sensing provide benefits over statistical approaches in monitoring biophysical characteristics of vegetation. However, physically-based models still demand large computational resources and often require rather detailed informative priors on various aspects of vegetation and atmospheric status. Spectral invariants and photon recollision probability theories provide a solid theoretical framework for developing relatively simple models of forest canopy reflectance. Empirical validation of these theories is, however, scarce. Here we present results of a first empirical validation of a model based on photon recollision probability at the level of individual trees. Multiangular spectra of pine, spruce, and oak tree seedlings (height = 0.38–0.7 m) were measured using a goniometer, and tree hemispherical reflectance was derived from those measurements. We evaluated the agreement between modeled and measured tree reflectance. The model predicted the spectral signatures of the tree seedlings in the wavelength range between 400 and 2300 nm well, with wavelength-specific bias between −0.048 and 0.034 in reflectance units. In relative terms, the model errors were the smallest in the near-infrared (relative RMSE up to 4%, 7%, and 4% for pine, spruce, and oak seedlings, respectively) and the largest in the visible wavelength region (relative RMSE up to 34%, 20%, and 60%). The errors in the visible region could be partly attributed to wavelength-dependent directional scattering properties of the leaves. Including woody parts of tree seedlings in the model improved the results by reducing the relative RMSE by up to 10% depending on species and wavelength. Spectrally invariant model parameters, i.e. total and directional escape probabilities, depended on spherically averaged silhouette to total area ratio (STAR) of the tree seedlings. Overall, the modeled and measured tree reflectance mainly agreed within measurement uncertainties, but the results indicate that the assumption of isotropic scattering by the leaves can result in large errors in the visible wavelength region for some tree species. Our results help increasing the confidence when using photon recollision probability and spectral invariants -based models to interpret satellite images, but they also lead to an improved understanding of the assumptions and limitations of these theories.en
dc.description.versionPeer revieweden
dc.identifier.citationHovi, A, Forsström, P, Ghielmetti, G, Schaepman, M E & Rautiainen, M 2020, ' Empirical validation of photon recollision probability in single crowns of tree seedlings ', ISPRS Journal of Photogrammetry and Remote Sensing, vol. 169, pp. 57-72 . https://doi.org/10.1016/j.isprsjprs.2020.08.027en
dc.identifier.otherPURE UUID: dc1d722a-d121-478b-b631-c55bc9570da4en_US
dc.identifier.otherPURE ITEMURL: https://research.aalto.fi/en/publications/dc1d722a-d121-478b-b631-c55bc9570da4en_US
dc.identifier.otherPURE LINK: http://www.scopus.com/inward/record.url?scp=85090558931&partnerID=8YFLogxKen_US
dc.identifier.otherPURE FILEURL: https://research.aalto.fi/files/51571632/1_s2.0_S0924271620302380_main.pdfen_US
dc.publisherElsevier Science B.V.
dc.relation.ispartofseriesISPRS Journal of Photogrammetry and Remote Sensingen
dc.relation.ispartofseriesVolume 169en
dc.subject.keywordEscape probabilityen_US
dc.subject.keywordRadiative transfer modelingen_US
dc.subject.keywordReflectance modelen_US
dc.subject.keywordSpectral invariantsen_US
dc.titleEmpirical validation of photon recollision probability in single crowns of tree seedlingsen
dc.typeA1 Alkuperäisartikkeli tieteellisessä aikakauslehdessäfi