Bottom Characterization from Hyperspectral Image Data

Abstract

In optically shallow waters, i.e., when the bottom is visible through the water, a tantalizing variety and level of detail about bottom characteristics are apparent in aerial imagery (Figure 1a). Some information is relatively easy to extract from true color, 3-band imagery (e.g., the presence and extent of submerged vegetation), but if more precise information is desired (e.g. the species of vegetation), spatial and spectral detail become crucial. That such information is present in hyperspectral1 imagery is clear from Figure 1b, which illustrates the Remote Sensing Refl ectance spectra for several selected points in the image. Spectral discrimination among bottom types will be greatest in shallow, clear water and will decrease as the depth increases and as the optical water quality degrades. Discrimination can also be complicated by the presence of vertical structure in the optical properties of the water, or even if there is a layer of suspended material near the bottom (see Box on opposite page). Despite these diffi culties, bottom characterization over the range of depths accessible to remote sensing is important since it corresponds to a signifi cant portion of the photic zone in coastal waters. Mapping bottom types at these depths is useful for applications related to habitat, shipping and recreation. The purpose of this paper is to present the issues affecting bottom characterization and to describe various methods now in use. Given space limitations, we refer the reader to the references for results and examples of bottom type maps