It’s a little depressing to admit it, but all our measurements are approximate and therefore wrong. We are doing our best, following the technology that puts increasingly accurate or, to put it another way, less and less imperfect instruments in our hands.
Accepting that our work is necessarily imperfect is unsatisfactory to the intellect and to our clients. Fortunately, hydrographers know both how to improve the accuracy of their measurements, and their inherent limitations, because we have the means to qualify the data and to estimate the uncertainty1.
And so there is a wide variety of options, both technical and methodological, for as much quality of surveys as there is, if by “quality” we mean of course “accuracy”. For example, bathymetry with complete bottom insonification using multibeam echosounder or resulting from single-beam interpolation are two different ways of restoring the same situation in a completely correct manner. There is no one method more valid than the other. There are different approaches.
For the same area, a design office that asks us to collect data in order to model the flow of a river will not have the same needs as a dredging company that has to determine cubature. And if these measurements in the river are intended for cartographic use for navigation, then our hydrographers must comply with the highest IHO standards2.
For each approach, there are means and time, and therefore a specific cost. And the discriminating criterion is as much precision as price. We understand that the best survey is the one that is best suited to the project.
But how do you place the cursor in the quality precision/price ratio?
Some clients have clear requirements that are generally expressed rigorously in the technical specifications of their consultation. Others are sometimes uncertain about a lot of things, and may wonder about the result, about good practices or feasibility, and more often still about what it will cost.
This is why it is essential for our clients in the conduct of their project to :
1. Define the technical specifications well
2. Before defining the technical specifications, solicit the specialists.
We cannot require our clients to master the norms and standards, or to be informed of the latest state of the art. On the contrary, we owe them the advice that is an essential dimension of our business, so that we can finally deliver the best service: the one they need.
“Good job done”
Raphaël Pacot
1. By combining all sources of error contributing to the measurement uncertainty and using a statistical method, its uncertainty can be estimated from the error budgets or Total Propagated Uncertainty (TPU).
2. IHO Standards for hydrographic surveys S-44
https://iho.int/uploads/user/pubs/standards/s-44/S-44_5E.pdf