Thoughts on the Limitations of S-44
Bundesamt für Seeschifffahrt und Hydrographie (BSH)
1 Chart Focused
S-44 only focused on nautical chart, with depth classification
The general agreement at the BSH is that hydrographic surveying not only provides data for the production of navigation charts for the purpose of safety of navigation, but more generaly provides multi-purpose base-geodata. A widening of the scope of S-44 is therefore a welcome adaptation of the standard.
In the comments received from the HSPT survey, someone commented that S-44 is a standard for bathymetry, not a hydrographic standard. According to the IHO defintion, hydrography is indeed not limited to bathymetry (see IHO definition of Hydrography below), whereas S-44 focuses for the most part on position and depth measurements. Chapter 4 - Other Measurements of the standard deals of course with other aspects than bathymetry. However to be truly balanced, perhaps it would be worthwhile to segment S-44 into specific sections, each dedicated to a type of measurement.
Hydrography is the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary purpose of safety of navigation and in support of all other marine activities, including economic development, security and defence, scientific research, and environmental protection.
Below is an example of a possible structure for the 6th edition of S-44:
IHO Standard for Hydrographic Surveys 6th Edition - Section 1: Classification of Surveys - Section 2: Sub-standard on Bathymetry - Section 3: Sub-standard on the Nature of the Seabed - Section 4: Sub-standard on Chart and Land Survey Vertical Datums Connection - Section 5: Sub-standard on Tidal Predictions - Section 6: Sub-standard on Tidal Stream and Current Observations
2 See All Requirements
Difficulty to have an overview of all requirements
No comment.
3 Limited Definitions
Limited number of definitions
A short review of the following documents was done in order to verify if the terms and definitions in S-44 Ed 5 are in accordance with the publications of the Bureau International des Poids et Mesures (BIPM):
- International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (known as the VIM)
- Guide to the Expression of Uncertainty in Measurement (known as the GUM)
Table 1 presents equivalences between terms used in S-44 edition 5 which may need to be modified to fully comply with VIM and GUM. The following subsections explain the rationale behind the choice of rewording.
| S-44 Ed. 5 | S-44 Ed. 6 |
|---|---|
| Uncertainty / Confidence Interval | Standard Uncertainty |
| Total Propagated Uncertainty | Combined Standard Uncertainty |
| Total Vertical Uncertainty | TBD (not defined in GUM + VIM) |
| Total Horizontal Uncertainty | TBD (not defined in GUM + VIM) |
| Confidence Level | Level of Confidence |
| True value (of a measurand) | Value (of a measurand) |
3.1 Uncertainty / Confidence Interval
In S-44, Uncertainty is defined as being the interval around the measurement that contains the true value. In this context, Uncertainty and Confidence Interval are considered as being equivalent. This is not the case with the GUM. According to the GUM, the Uncertainty may be expressed either as a standard deviation (see definition of Standard Uncertainty below) or it may be expressed as an Expanded Uncertainty at a specified Level of Confidence. The Expanded Uncertainty "…defines an interval about the measurement result that encompasses a large fraction p of the probability distribution characterized by that result and its combined standard uncertainty, and p is the coverage probability or level of confidence of the interval." (GUM, p. 23).
In order to be in agreement with the GUM, S-44 should follow the definitions of the GUM (See definitions below) and it should state that Uncertainty may be expressed either as:
- a Standard Uncertainty
- an Expanded Uncertainty at a specified Level of Confidence
(Measurement) Uncertainty
non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used.
Standard (Measurement) Uncertainty
measurement uncertainty expressed as a standard deviation
Coverage Probability (or Level of Confidence)
probability that the set of true quantity values of a measurand is contained within a specified coverage interval
3.2 Total Propagated Uncertainty
The VIM and GUM do no use the term Total Propagated Uncertainty but rather Combined Measurement Uncertainty (see definition below). The Law of Propagation is nevertheless used to comupute Combined Uncertainties.
Combined Standard (Measurement) Uncertainty
standard measurement uncertainty that is obtained using the individual standard measurement uncertainties associated with the input quantities in a measurement model
3.3 Confidence Level
The difference between Confidence Level and Level of Confidence is not clear from reading the GUM.
3.4 True Value (of a measurand)
There sometimes seems to be a contradiction with the use of the term true value in S-44. While it is stated that: "…The true value can never be known…" (p.17, definition of Error), the term true value is nevertheless used to define fundamental terms such as Uncertainty and Confidence Level. Below is the clarification in GUM:
D.3.5 The term “true value of a measurand” or of a quantity (often truncated to “true value”) is avoided in this Guide because the word “true” is viewed as redundant. “Measurand” (see B.2.9) means “particular quantity subject to measurement”, hence “value of a measurand” means “value of a particular quantity subject to measurement”. Since “particular quantity” is generally understood to mean a definite or specified quantity (see B.2.1, Note 1), the adjective “true” in “true value of a measurand” (or in “true value of a quantity”) is unnecessary — the “true” value of the measurand (or quantity) is simply the value of the measurand (or quantity). In addition, as indicated in the discussion above, a unique “true” value is only an idealized concept.
4 Misalignment CATZOC
Misalignment between S-44 and CATZOC
No comment.
5 Grid Resolution Surfaces
Grid resolution and bathy surfaces not addressed
Scale-independent bathymetric models (i.e. bathymetric surfaces) are now a standard survey product and should therefore be fully addressed in S-44. Bathymetric surfaces should be directly addressed in Chapter 3 - Depths and not only in Chapter 5. The modified Chapter 3 should state how a bathymetric surface can comply with S-44 based on an assessment of the total vertical uncertainty of its grid nodes. This will require the clear distinction between Soundings and Grid Nodes. The IHO S-32 definition of a Sounding (not defined in S-44) is presented below.
Sounding
Measured or charted depth of water, or the measurement of such a depth.
One important question to answer will be whether or not to accept that a Grid Node, by definition, does not have a horizontal uncertainty. From a first conceptual point of view, this makes sense since a grid is essentially defined and overlaid on a chosen datum.
The choice of grid resolution should also be recommended in the standard as a compromise between a minimum grid node density (e.g. 5 soundings per grid node) and the median footprint size of the echosounder at the mean survey depth.
6 TPU
Confusion between a-priori TPU and a-posteriori Qualification
GUM helps clarify the confusion between a-priori TPU (which should actually be refered to as Combined Standard Uncertainty. See above.) and a-posteriori TPU (which should actually be refered to as Standard Uncertainty):
3.4.3 In order to decide if a measurement system is functioning properly, the experimentally observed variability of its output values, as measured by their observed standard deviation, is often compared with the predicted standard deviation obtained by combining the various uncertainty components that characterize the measurement. In such cases, only those components [..] that could contribute to the experimentally observed variability of these output values should be considered.
There is a problem with S-44 in this regards. Edition 5 states that the "Vertical uncertainty is to be understood as the uncertainty of the reduced depths". This makes perfect sense. However S-44 does not explain how Check lines of overlapping swaths can be used to build an observed frequency distribution of reduced depths. S-44 Edition 6 should therefore make clear that for the pupose of comparing observed and predicted standard deviations, it's important to compare only the same contributing sources. As an example, the uncertainty of a geoid model used for sounding reduction will not be part of the combined standard uncertainty calculation used for comparison with the observed standard deviation of check lines. The geoid uncertainty will be added (in a root-sum-square sense) to the "final" combined uncertainty only afterwards.
The above paragraph was related to the assessment of soundings. The same concern also applies to the assessment of grids. That is to say that the Standard uncertainty (i.e. Standard deviation) of a grid node should be compared with the Combined Uncertainty (i.e. TPU) of the grid node with only the same contributing sources. Moreover, there is concern as to how a sounding's horizontal uncertainty contributes to the vertical uncertainty of the surrounding grid nodes (such as in the CUBE algorithm). The method used should perhaps be explicitely stated in the metadata of the grid.
7 Tech Agnostic
S-44 should stay away from specific technologies
New technologies should be addressed to the extent that they may have an impact on a specific part of the standard. For example, use of Water Column (WC) data should be addressed in the section of S-44 that deals with depth measurements to the extent that WC data may be used to derive new measurements that are not obtained directly by other means.
8 Metadata
Confusion of attribut in metadata
9 Oudated
Outdated chapters
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