Bathymetric Mapping with Landsat: A Practical Approach

ABSTRACT Landsat's ability to estimate water depths of up to twenty metres promises to greatly reduce both the cost of and time involved in bathymetric surveys. Previous studies have proved the technique in principle but the methods developed fail in normal practice because too much reliance is placed on the homogeneity of the water and atmosphere. In many regions, especially near islands and reefs, a number of disturbing factors make it necessary to look beyond the Landsat data. Work at The Australian National University's Department of Engineering Physics has concentrated on the development of a production oriented approach which utilizes both shipborne and satellite data collection supplemented by meteorological and tidal information. Test surveys off the north-east coast of Australia have verified that this approach, with its integration of Landsat data, interpretation and shipborne survey, is a viable survey method. INTRODUCTION In the face of growing international pressure to complete bathymetric mapping programs, the agencies involved are seeking inexpensive ways of expediting the survey process. Bathymetric surveying is expensive and considerable savings could be realized if Landsat data proved suitable for this application. Early sections of this paper briefly describe the Landsat system and provide a theoretical background to the way in which it may be used for bathymetric mapping like most naturally occurring situations, the ocean is affected by a complex mix of factors and the straightforward approaches usually suggested are unsatisfactory for production use. Studies at The Australian National University have concentrated on developing a realistic approach to production oriented operations. The resulting techniques are based on the notion of segmenting Landsat images into regions for which a consistent set of optical parameters can be found. The approach relies on imagery from multiple passes of the satellite and from multiple bands, along with appropriate survey management and skilled image interpretation. Image interpretation plays a major role but, at the same time, it is imperative that digital rather than photographic imagery be used. Considerable attention has been given to the development of a facile system which permits meaningful man-computer interaction in this application. A full discussion of this project is given in (1). THE LANDSAT PROGRAM Early satellites in the Landsat series, including Landsat-C to be launched early in 1978, have been considered primarily as research and development tools to demonstrate that remote sensing from space is a valuable data source for earth resource management. Each Landsat satellite follows a circular, sun-synchronous, near-polar orbit at an altitude of approximately 950 kilometres. The satellite orbits the earth 14 times per day giving complete global coverage every 18 days. The orbit is trimmed to keep the maximum deviation of the track over the ground to less than 37 kilometres. The payload consists of three systems: the Return Beam Vidicon (RBV) camera; the Multispectral Scanner (MSS); and a Data Collection System (DCS). This discussion concentrates on the MSS data, but the techniques developed here could be applied to any suitable digital multispectral imagery of the earth's surface including that from airborne sensors.