Passive Multispectral Bathymetry Mapping of Negril Shores, Jamaica

A new method was tested using SPOT images for mapping of variations of the reflectances and depth of sea-bottom French Polynesia, where the standard error on computed depth was less than 0.2 m for average depths of 2 m. The method uses "soils line" for zero-depth calibration and "brightest pixels line" for diffuse attenuation coefficients and actual value of K as linear functions of band ratios, thus providing bottom depth and reflectances without field data. This method has been blind-tested using Landsat TM data. TM should improve the possibilities of mapping deeper waters thanks to blue wavelength band. Preliminary results show very promising correlations with the echo-sounded sections along the Negril coast of Jamaica, where the bottom is a mixture of sand, corals and sea grass and 2K values of at 0.179, 0.251, 0.899 m-1 are estimated from the image at 486, 570 and 660 nm respectively. The outlines and performances of the PMBM process are presented, and the results of its application at Negril are presented and discussed. 1.0 INTRODUCTION, LOCATION AND PREREQUISITES Jamaica, located in central Caribbean with a size of about 11000 km2 and a population of 2.5 mill. inhabitants, has an economy that is strongly dependent on operations in the coastal area. The foremost national income emanates from beach based tourism. One of the latest, highly tourist developed areas is Negril (Fig. 1), with mile long beaches and nice coral reefs outside. This area is dominated by a flat lowland coastal zone with a marsh back land. The beach and the near shore bottom consist of white sand. The bottom elsewhere is of very complex nature, with a mixture of sand, sea grass, scattered coral heads and reefs (Fig. 1). Most of the coastal waters around Jamaica are clear, nutrient and suspended poor. The Negril area has been heavily expanding, causing hygienic water problems, but the exchange of water in this region is large, making the water still quite clear. During recent years the EU has contributed to restoration measures in Negril. Sea-charts of the area are very rudimentary and need updating. During a coastal zone planning project (Norrman et. al.,1997), one of the tasks was to map the bottom characteristics around Jamaica. For this purpose, a number of Landsat TM scenes were used in combination with aerial photos and extensive field data sampling. Many different approaches from the literature were used to try to efficiently make depth charts of the coastal zone, but with limited success. For more than two decades, many efforts have been applied to operate various types of models to map bottom depths, starting with the early research at ERIM (Polcyn et al.,1970; Lyzenga, 1978). Much of this research was based on the assumption that the diffusion and reflectance in the water and the atmosphere followed models, neglecting all types of disturbances like suspended matter etc. and therefore using a simple "darkest pixel" correction. Mapping of water depths in areas with complex and varying bottom conditions has been performed by Luczkovich et al.(1992) using separate training areas for different types of bottom cover. However, few constructive solutions have been presented where models for reflectance conditions for different surfaces have been used. One of us, a marine geologist (Morel, 1996), spent the last four years reviewing recently published contributions and testing new concepts on a large data bank of SPOT images of Pacific islands: the purpose of this paper is to provide a brief outline a new method, called PMBM, to conduct a blind test using a Landsat TM image, and then evaluate it with field data. 2.0 THE PASSIVE MULTISPECTRAL BATHYMETRY MODELER (PMBM) This expression refers to the modeling of the variations of the bottom depth and reflectance in clear and shallow waters using bottom-reflected sun light captured in two or more wavebands by an airborne or space borne imager. In contrast with all other methods in use to-date, the PMBM process does not rely, for the calibration of the model parameters, on the acquisition, reduction, classification and statistical analysis of a preliminary set of field data which are costly to obtain and difficult to process. The basic equation used is directly derived from the simplified radiative transfer equation for optically shallow water. The theory behind this equation has been discussed in detail by Philpot (1989), and Maritorena et al.(1994) found it to be consistent with detailed measurements of diffuse irradiance in the underwater light field in the clear waters of French Polynesia, and also with a Monte Carlo simulation of bottomreflected photon paths. Following is its transcription for nadir radiances at the water-air interface (BOA): L = Lw [1 exp(-KZ)] + LB exp(-KZ)(1)