Comparison of vertical and inclined camera observations on the validation results of remote sensing phenological parameters
In the fields of agricultural production management and climate change research,monitoring large-scale plant phenology with satellite-based remote sensing is crucial to reveal the interaction of biology and nature environment.During validation on remotely sensed phenology information,near-surface digital cameras are often employed as main data sources.However,more efforts were focused on the scale difference between ground and remotely sensed data and rarely on the difference of sensors viewing zenith,i.e.,the satellites mainly adopted the near-nadir observation while cameras were mostly inclined in arrangement.Vertical(PhotoNet)and inclined(PhenoCam)camera observations were acquired at the similar latitude for the same vegetation type,and then these observations were compared with phenological parameters extracted from Sentinel-2 data to assess systematically the effects of camera observation angles on the results of satellite phenological verification.For 16 locations,we compared a Greenness Chromatic Coordinate(GCC)series derived from digital cameras and Sentinel-2.A double hyperbolic tangent model was fitted for each series.The threshold method was applied to the annual complete modeled data,and the curvature extremum method was used for incomplete data to estimate the onset of greenup,the maturity of the green canopy,the peak of season,the end of greenness,and the dormancy of the green vegetation(OG/MG/PS90/EG/DG).Results showed that the viewing zenith of cameras is one of the uncertain sources to validate phenology information from satellite imagery.In most cases,the vertically observed camera showed improved agreement with the phenological parameters extracted by the satellite-based method,with an average bias of 9 days,while a larger bias of 19 days was observed for inclined camera observation.Therefore,the two camera observation methods result in the verification deviations of up to 10 days on average.However,the deviations might be vegetation type and growth stage dependently.The bias of vertical observation was remarkably higher than that of inclined observation during the end to dormant period for maize.The different results of the vertical and inclined cameras on the similar vegetation can be partly explained from the directional reflection characteristics of vegetation canopy and the difference of target components(e.g.,different fractions of soil and vegetation,photosynthetic and nonphotosynthetic components of vegetation)within the camera field of view.Results demonstrate that the viewing zenith angle of the near-surface cameras is an important factor in the validation of satellite phenological parameters.In addition,the uncertainty of verification results caused by the phenological period extraction method,illumination,and satellite observation geometry is limited,which is not the main factor affecting the verification of satellite phenology parameters.As a result,the verification error introduced by the angle effect should be fully considered while near surface cameras are deployed in the field to provide more reliable verification data for satellite remote sensing monitoring.
万方数据
维普
