Gordon W. Frazer

A comparison of digital and film fisheye photography for analysis of forest canopy structure and gap light transmission

Due to the scarcity and high cost of conventional film-based hemispherical photographic systems, some forest scientists are now using multi-purpose, consumer-grade digital cameras for the analysis of forest canopy structure and gap light transmission. Although the low cost of digital cameras and direct capture of digital images appear to offer significant advantages over film camera systems, relatively little is known about their technical differences from an applications perspective. In this study, we compared the performance of a popular digital camera (Nikon Coolpix 950 with FC-E8 fisheye) with a conventional film camera (Nikon F with Nikkor 8 mm fisheye) under different stand structures and sky conditions. Our findings show that the Nikon Coolpix 950 digital camera produced hemispherical canopy photos with substantial color blurring towards the periphery of the exposure. We believe that chromatic aberration associated with the camera’s lens optics may be the source of this phenomenon; however, other factors may have also contributed to the diminished image sharpness. Color blur influenced (i) the size, shape, and distribution of canopy gaps; (ii) the accuracy of edge detection and the binary division of pixels into sky and canopy elements, and (iii) the magnitude, range, and replication of canopy openness, leaf area, and transmitted global radiation results. The Nikon Coolpix 950 produced canopy openness measures that were 1.4 times greater than film estimates in 22 of the 36 photo pairs. Cloud cover and sky brightness also influenced the spectral characteristics of the lateral chromatic aberration (halos), and thus had an added and unpredictable effect on canopy openness. Setting the Nikon Coolpix 950 to record in black and white, and shooting only under uniformly overcast skies will help to minimize the unpredictable effects of chromatic aberration. Nevertheless, we recommend a cautious approach when undertaking canopy measurements with the Nikon Coolpix 950, particularly when stands are dense and canopy openness falls below 10%. High-quality (1:4) JPEG compression had no significant influence on mean canopy openness; however, lower XGA and VGA image resolutions

Exploring Small Footprint Lidar Intensity Data in a Forested Environment

Lidar intensity, a quantity analogous to backscatter, has yet to be fully exploited as an information source in the characterization of forests. Intensity images appear noisy due to complex laser interactions in a forested environment; however, we expect that these data are inherently spatially structured. We found that (1) the scale and variability of global estimates of spatial autocorrelation derived from raw intensity data were markedly different between stands of different age, and (2) the magnitude of local estimates of spatial autocorrelation varied with canopy height. We believe that these height and age-related differences in intensity are due to both structural and compositional features unique to each stand.

LiDAR-guided analysis of airborne hyperspectral data

This paper describes a new framework to the collection and fusion of multisensor airborne LiDAR and hyperspectral data. We describe a data fusion philosophy that provides a spatially precise positioning of hyperspectral data based on discrete first and last return LiDAR data. Three dimensional objects defined by the LiDAR data are then used to sample optimal spectra for subsequent analysis. The sampled spectra retain their positioning metadata and so can be mapped back into geographic space for further analysis. While the paper presents this philosophy within the context of a species classification, other analytical analysis can be performed.

Integration of first and last return LiDAR with hyperspectral data to characterize forested environments.

The fusion of active LiDAR and passive optical hyperspectral data allows us to characterize the forest environments in ways that have not been possible previously with only one data source. This paper describes an airborne platform configured to collect data from multiple sensors simultaneously. Data from the platform have been applied to describe forest environments both in terms to species and structure. Integration of the data yields information and characterization of forest environments than has been possible in the past.

View Angle-Dependent Clumping Indices for Indirect LAI Estimation

The leaf area index (LAI) of forest canopies can be rapidly estimated by the canopy gap fraction measured using in situ optical instruments. Gap fraction analysis, however, will routinely underestimate LAI when forest canopies exhibit a strong pattern of aggregation at one or more spatial scales. Recent advances in optical-based measurement techniques and gap fraction analyses show that inclusion of a view angle-dependent clumping index Ω(θ) may substantially improve optical-based estimates of LAI. Here we introduce four different estimates of Ω(θ) and demonstrate how these indices can be extracted directly from hemispherical canopy photographs. Our findings suggest that estimates of Ω(θ) can improve LAI estimation in the presence of spatial non-randomness, although their overall effectiveness was strongly dependent on the clumping index chosen, quantity of leaf area, severity and complexity of foliage clumping, and the view angles used for LAI integration.