Band registration of tuneable frame format hyperspectral UAV imagers in complex scenes

Eija Honkavaara,Tomi Rosnell,Raquel Oliveira,Antonio Tommaselli

doi:10.1016/j.isprsjprs.2017.10.014

Abstract

A recent revolution in miniaturised sensor technology has provided markets with novel hyperspectral imagers operating in the frame format principle. In the case of unmanned aerial vehicle (UAV) based remote sensing, the frame format technology is highly attractive in comparison to the commonly utilised pushbroom scanning technology, because it offers better stability and the possibility to capture stereoscopic data sets, bringing an opportunity for 3D hyperspectral object reconstruction. Tuneable filters are one of the approaches for capturing multi- or hyperspectral frame images. The individual bands are not aligned when operating a sensor based on tuneable filters from a mobile platform, such as UAV, because the full spectrum recording is carried out in the time-sequential principle. The objective of this investigation was to study the aspects of band registration of an imager based on tuneable filters and to develop a rigorous and efficient approach for band registration in complex 3D scenes, such as forests. The method first determines the orientations of selected reference bands and reconstructs the 3D scene using structure-from-motion and dense image matching technologies. The bands, without orientation, are then matched to the oriented bands accounting the 3D scene to provide exterior orientations, and afterwards, hyperspectral orthomosaics, or hyperspectral point clouds, are calculated. The uncertainty aspects of the novel approach were studied. An empirical assessment was carried out in a forested environment using hyperspectral images captured with a hyperspectral 2D frame format camera, based on a tuneable Fabry-Pérot interferometer (FPI) on board a multicopter and supported by a high spatial resolution consumer colour camera. A theoretical assessment showed that the method was capable of providing band registration accuracy better than 0.5-pixel size. The empirical assessment proved the performance and showed that, with the novel method, most parts of the band misalignments were less than the pixel size. Furthermore, it was shown that the performance of the band alignment was dependent on the spatial distance from the reference band.

Highlights

Hyperspectral imaging employs tens to hundreds of contiguous bands in order to accurately reconstruct the spectral signature of the target of interest (Goetz, 2009)
In order to obtain the estimated uncertainty of the band alignment, we introduced the interior orientation parameters (IOPs) and exterior orientation parameters (EOPs) of the reference bands and the control points (CPs) as errorless parameters and calculated the full variance-covariance matrix for the EOPs calculated by the space resection
The present study developed a solution for one of the greatest bottlenecks in hyperspectral frame format imaging based on tuneable filters, namely a rigorous and efficient approach for band registration in complex 3D scenes in the case of unmanned aerial vehicle (UAV) imaging

Summary

Introduction

Hyperspectral imaging employs tens to hundreds of contiguous bands in order to accurately reconstruct the spectral signature of the target of interest (Goetz, 2009). ⇑ Corresponding author at: National Land Survey of Finland, Finnish Geospatial. Research Institute FGI, Department of Remote Sensing and Photogrammetry, P.O. Box 84, FI-00521 Helsinki, Finland. Of these sensors are the AVIRIS (Vane et al, 1993) and HyMap (Cocks et al, 1998). Pushbroom scanners, such as CASI (Babey and Anger, 1989) and the AISA series (Specim, 2017), capture spectra in lines. The use of sensors operating in the frame format principle (for example, those based on filter wheels or tuneable filters) have been rare due to the associated processing challenges (Schaepman, 2009)

Methods

Results

Discussion

Conclusion