Standards for Hydrographic Surveys

3 Vertical Datum

3.1 Sounding Datum

All depths must be reduced to a low water datum, which can be defined as “a level where the water level will but seldom fall below it during the navigation season”.

In tidal waters, soundings are reduced to Lowest Low Water Large Tide (LLWLT) whereas in non-tidal waters the soundings are reduced to a low water datum determined from long term (period) water level records.

Sounding datum must be referred to a minimum of 3 vertical benchmarks whose elevations must be determined to the accuracy stated in the Canadian Tidal Manual.

3.2 Datum for elevations

All elevations and clearances must be referenced to a specific datum. In tidal waters, elevations and clearances are referenced to Higher High Water Large Tide (HHWLT). In non-tidal waters, elevations are referenced to sounding datum (ex. IGLD ’85).

3.3 Water Level Observations

Water level observations should be made throughout the course of a survey for the purpose of:

  1. Providing water level reductions for soundings.
  2. Providing data for tidal analysis and subsequent tidal constituent determination and prediction. For new sites or sites with poor historical records, observations should extend over the longest possible period and preferably not less than 29 days.
  3. To establish the vertical datum, (both Low Water and High Water), of an area. For this purpose, observations should extend over the longest possible period.

Water level heights should be observed so that the total measurement error at the gauging station, including timing and filtering errors, does not exceed +/- 5cm at 95% confidence level for Exclusive Order and Special Order surveys, for tidal analysis and for the determination of vertical datum. For other orders +/- 10cm at 95% confidence level should not be exceeded.

In order for the bathymetric data to be fully exploited in the future using advanced satellite observation techniques, water level observations and consequently sounding datum should be related both to a low water datum (usually Chart Datum) and also to a geocentric reference system, preferably the World Geodetic System 84 (WGS 84) ellipsoid or the North American Datum 1983 (NAD83) ellipsoid.

Independent water level measurement techniques should be used to verify the calibration and operation of the water level gauge. As a minimum, these are to be made at the beginning and the end of the tide gauge deployment and if possible at both high and low stage and more frequently during sounding operations.

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