The recent advent of digital technology has had major impacts on acquisition, processing, storage and utilization of hydrographic data These impacts are described and placed in context to show the explosion of the possibilities opened up by this technology An overview of digital hydrography with the links to related techniques and sciences is made in order to demonstrate the future challenges facing us New users and new ways to see and feel the bottom are only examples of what is now a reality with the digital data.

CHARACTERISTICS OF HYDROGRAPHIC DATA

We can describe a typical hydrographic operation as being the flow illustrated in Figure 1. The first step is the data acquisition stage, where the data is collected using a broad variety of sensors The collected data then pass on to the data processing stage where it is cleaned, concatenated, computed, adjusted, etc Once the data have reached a certain quality level, they are kept and managed at the storage stage. Finally comes the application (utilization) stage where the information is built with the data.

The major element of a hydrographic data set is bathymetry Composed of positions associated with related depths, it is characterized as being point data, with no relation to the representation scale in itself. It is the information that requires the major portion of the efforts of a survey. The unknown aspect of the data collection, the physical considerations such as the sensors that have to be used to measure the depths (acoustics), and the practical platforms necessary to pursue surveys render this task very difficult compared to land data gathering.

The shoreline and the line data from the topography are scale related information forming the second significant data group of hydrographic data This information is much more easily gathered using well proven land survey techniques, and its volume in hydrography is relatively minor compared to the previous group.

Other types of data, such as navigation aids, limits, bottom characteristics, form together the rest of the set in a minor proportion. Their collection requires a minimum of efforts compared again to the bathymetry.

THE PAST

What did we do 15 years ago in hydrography? We were very limited in our exploration of the subwater environment We could only guess at what it would look like Our tools based on mechanical, basic electronic or electrical ‘technologies’ could give us some spot information but we had to use our hands and our skills to compute, draw and analyse the results The results seemed impressive at the time.

Data acquisition was, to say the least, tedious. Geodetic computations-for the localization of the ground-based stations-were done by hand, log tables or mechanical calculator. Every single piece of information on the position-even from an electronic positioning system-had to be recorded by hand on a note pad The soundings were recorded on an echogram in analog form by the echosounder, with hand-written annotations We had to record the time from our watches for the tide values, and the patterns that our launches or ships.

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