Water depth determination from satellite data
Dr. Mohd Ibrahim, Seeni Mohd
Centre for Remote Sensing Faculty of Surveying
University of Technology Malaysia, 80990 Johor Bahru .
Abstract
The coastal waters of importance in navigation. However, inspection of hydrographic charts of these waters reveal a large number of doubtful soundings. The task of updating hydrographic charts using conventional techniques alone is costly and time-consuming. Remote sensing from satellites appears to hold promise for obtaining depth information in shallow coastal waters.
In this paper the physical principles, an optical model and algorithm for water depth determination from remote sensing techniques are given. The results from a study carried out in the coastal waters of Penning island in Malaysia using the Landsat -3 Multispectral Scanner data are also presented.
Introduction
According to the International hydrographic Bureau's estimates, sufficiently adequate sounding to determine sea floor topography only exist for about 16% of the area covered by the world's oceans. Another 22% of the area has data sufficient only for the determination of major sea floor features; while for the remaining 62% there is not enough data for deterinining sea floor topography (Kapoor 1976). Watson (19860 notes that the situation is still very much the same since bathymetric surveying by conventional shlpborne sounding techniques is slow. Hazardous and expensive. Interest has been generated in the application of remote sensing techniques at least in the critical shallow areas, which are frequently used, by ships approaching or leaving ports or harbours.
The possibility of using remote sensing technology was addressed as early as the late 1960s (Brown et at, 1971). These studies led to the NASA /Cousteau Ocean Bathymetry Experiment in 1975 which demonstrated the feasibility of using landsat high-gain multispectral scanner (MSS) data in bathymetry (Polcyn 1976). Since then, a number of studies have been carted out
Physical principles, optical model and algorithm for depth determination from remote sensing.
- Physical principles
The radiation reaching a satellite or aircraft sensor is a function of some radiative transfer processes in the ocean-atmosphere system. These processes are described by Sorensen (1980) as follows (see Figure 1):
- single or multiple scattering of solar photons by the atmosphere into the field of view of the sensor.
- Reflection of unscattered or scatiered photons from the surface and subsequent scattering into the field of view of the sensor
- Reflection of unscattered photons by the instantaneous field of view (IFOV) under examination and subsequent propagation (with losses) to the sensor
- Reflection of single-and multiple-scattered solar photons ()with losses to the sensor
- Scattering of single-and multiple-scattered or unscattered solar photons (which have penetrated the sea surface) by water and suspended particulate and subsequent propagation (with losses ) to the sensor.
The total radiance seen by the remote sensor is the sum of the radiance's due to processes 1 and 2 (the path radiance. Rp) plus the sun of the radlances due to processes 3,4 and 5 (the surface radiance, Rs) after accounting for the transmission losses, l.e.
Figure. 1: Processes contributiong to radiance at the sensor(after Sorensen 1980)
R = Rs.ta+Rp -----------------------(1)
Where ta is the appropriate transmittance through the atmosphere. The su surface radiance Rs is the sum of the reflected radiance Rr and the ocean-leaving radiance Ro, i.e.
Rs=Rr + Ro -------------------------(2)
In applications over land areas, Ro is absent and Rr is the quantity which is desired Conversely. In most applications over water. Information concerning sub-surface conditions is sought . in requited. Thus. For land viewing it is necessary to remove Rp and multiply the remaining signal by ta -1. in the ocean viewing it is necessary to remove Rp+ta. Rr and multiply the remaining signal by ta-1. In the ocean case the remove cont :.ibution from process 3 is usually called sun glitter (or glint). Also, the radiance Ro can be derived from the upwelling radiance lust beneath the sea surface. Rw. Through
-----------------(3)
In which p(v) is the reflection from beneath the rough ocean surface (in the case of a smooth surface, p(v) is the Fresnel reflectivity of the interface) and n is the refractive index of water. Only 5-20% of the signal received over water by a sensor on board a satellite or a high-flying aircraft stems from the atmosphere and the surface, all of which must be quantified and subtracted over eater by the total signal (Sorensen 1980). The interesting part of the radiance value measured over water by the remote sensor is therefore the total signal minus that contains the oceanographic information. Will at most comprise values from three sources the bottom suspended and dissolved, matters in the water and the water itself. For bathemetry. It is the signal from the bottom that is important.
