IHO REQUIREMENTS

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IHO REQUIREMENTS

• Steve Shipman
• Professional Assistant (Hydrography)
• International Hydrographic Organisation
• Monaco

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Safety of Navigation

• Conduct surveys and publish charts
• Publish tide-tables
• Issue Maritime Safety Information

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The Vertical Dimension 1

• A datum level is required
• Mean Sea Level?
• Hydrographers driven by safety of navigation
• Low water datum (Chart Datum)

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The Vertical Dimension 2

• The IHO has resolved (TR A2.5) that
• Lowest Astronomical Tide (LAT) should normally be
  used for Chart Datum.

LAT is defined as the lowest tide level which can
be predicted to occur under average
meteorological conditions and under any
combination of astronomical conditions. It is
recommended that LAT be calculated either over a
minimum period of 19 years using harmonic
constants derived from a minimum of one years
observations or by other proven methods known to
give reliable results. Tide levels should, if
feasible, reflect the estimated error values
obtained during the determination of these levels
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Chart Datum / LAT
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The Vertical Dimension 3

• Safety of Navigation also requires that there is
  a High Water Datum
• Clearance heights of bridges and power cables are
  referenced to this High Water Datum
• The IHO has resolved that Highest Astronomical
  Tide (HAT) should be used
• HAT has a similar definition to LAT
• Relevant for flood evaluation

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High Water Datum
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Variation in Datum 1

• MSL
• a relatively small / slow variation both
  spatially and temporally
• LAT and HAT
• Significant spatial variation
• Also temporal variation in time of high / low
  water across medium to small scale charts

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Variation in Datum 2

• Reasons for the variation in the range of tide
  include
• Seabed topography
• Water body resonance
• Coriolis

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Spring Tidal Ranges
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Variation in Datum 3

• Datum will vary across all but large scale
  charts
• Need to model this variation in datum
• The co-tidal chart or model

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Variation in Datum 4

• The Co-tidal model usually describes
• Variation in range of tide
• Variation in time of tide
• The model may need to allow for
• Variation between High and Low water
• Variation with the Spring / Neap cycle

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Requirements

• Data to populate the co-tidal model
• Density relative to variation
• A lack of offshore data

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Tidal Data

• Publication S-44IHO Standards for Hydrographic
  Surveys
• Tidal heights should be observed so that the
  total measurement error at the tide gauge,
  including timing error, does not exceed /- 5 cm
  at 95 for Special Order surveys. For other
  surveys /- 10 cm should not be exceeded.
• Special Order surveys - Harbours, berthing
  areas, and associated critical channels with
  minimum under keel clearances

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Charts

• Paper
• Raster Navigation Charts
• Electronic Navigation Charts (ENCs)

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Tides

• Predicted
• Real

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Printed Tide-tables
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UKHO EasyTide Example
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Predicted Tides

• Quality depends on data available for analysis
• Publication S-44IHO Standards for Hydrographic
  Surveys
• providing data for tidal analysis and subsequent
  prediction, for which purposes the observations
  should extend over the longest possible period
  and not less than 29 days

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Predicted v Real

• Predicted tides do not account for Meteorological
  Factors.
• In critical situations real time tidal
  information is required or at least information
  on variation from predicted.

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Tidal Warnings

• Issued as part of Maritime Safety Information
  (MSI)
• Local and Regional Warnings
• Vessel Traffic Services
• Possible inclusion in AIS

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Seamless Vertical Reference 1

• RTK GPS now allows the conduct of hydrographic
  surveys without any requirement to measure tides!

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Seamless Vertical Reference 2

• We have removed one problem and introduced a
  new one!
• No need to measure tides -therefore no need for
  a co-tidal chart
• But we now needa spheroid chart datum model

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Seamless Vertical Reference 3

• There is a lot of work going on in the geodetic
  community to determine a seamless vertical
  reference
• A MSL / Geoidal surface would appear to be
  preferred
• The maritime community might prefer an
  ellipsoidal reference?

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Seamless Vertical Reference 4

• The geodetic community want a model at the
  sub-centimetre level
• The maritime community, in general, is interested
  at the decimetre level although there are always
  some special cases with higher requirements

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Conclusions 1

• More data collection for longer periods in order
  to have better predictions
• Greater provision of real time tides / variation
  from predicted particularly in restricted areas
• Greater variety of delivery mechanisms with
  integrity checking

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Conclusions 2

• Increasingly integrated in VTS
• All referenced to a global vertical reference
• IALA are also developing a Risk Management
  Model for assessing amongst other things the
  likelihood of groundings, this requires a tidal
  statistical input

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Conclusions 3

• MUST NOT FORGET
• The horizontal movement of the water mass is also
  of great importance to Safety of Navigation