Hydrographic Survey Management Guidelines

5 Mobilization

Before any hydrographic data can be collected, whenever possible, it will be necessary to recover or re-establish horizontal and vertical control, install (if necessary) and calibrate the positioning (reference) system and install the necessary instrumentation required to record the water level fluctuations.

5.1 Horizontal Control

Horizontal control points may be required in order to reference the position of bathymetric soundings, fixed and floating aids, any conspicuous objects useful for navigation and for surveying shoreline.

To accomplish the tasks mentioned above it may be necessary to install and establish new control points in the survey area. New control points are to be established to the accuracy stated in the CHS Standards for Hydrographic Surveys.

If GPS is to be used to obtain positions, the procedures and specifications as stated by the official regional or national Geodetic Agencies must be followed.

For more information on GPS positioning, refer to the appropriate national and/or provincial geodetic agencies.

If conventional ground survey methods are to be used (angles and distances) to obtain positions, methods and procedures from national and/or provincial geodetic agencies should be followed in order to ensure that the accuracies stated in Table 1 and Table 2 of the CHS Standards for Hydrographic Surveys are obtained.

5.1.1 Permanently marked horizontal control points

Unless otherwise stated, all new primary shore control points must be permanently marked using a rock or soil post and a tablet stamped and identified using the numbering convention used by the National Geodetic Data Base.

Secondary stations can be marked by a drill hole, iron rod, pipe etc. as they are considered to be of a semi-permanent nature. They should nevertheless be described the same way as permanent control point. (see § 5.1.2 Station description).

All Horizontal permanent control markers are to be identified by SERVICE HYDROGRAPHIQUE - CANADA - HYDROGRAPHIC SERVICE inscribed on the face of the marker tablets. The words should be spaced around and near the perimeter of the tablet face.

Each tablet should be identified using the National Geodetic Data Base (NGDB) numbering system which consists of 8 characters MYRA9XYZ where:

• “M” is a fixed letter which denotes that the station was established from the year 2000
• “YR” is the last two digits of the year the station was established (e.g. 87, 99, 01)
• "A" is the single digit number that identifies the province or territory (see Table below) where the control point is established
• “9” is the single digit number that identifies the organisation that established the station (CHS=9)
• “XYZ” is a three digit number (001 to 999) that identifies the specific marker identification (given by regional HDC offices)

For example, the station number 8739412 signifies: 87 = year of establishment; 3 = Ontario, 9 = CHS, and 412 = individual marker identification digits. Note that the first character “M” is left blank because the station was established prior to year 2000.

The horizontal control point number is to be stamped on the tablet with dies, across the central open area of the marker tablet face centred above "-CANADA"-" and read from the south facing north, the same as "-CANADA-".

ALL station markers are to be placed with “CANADA” to the south so that the markers are read from the south and the “twelve o’clock” position is oriented north.

Listing of Provinces and Territories and their identifiers for Horizontal Control:

In cases where reference markers are placed to aid future location of the horizontal control point, they should be referenced and stamped Ref. l, Ref. 2, Ref. 3, etc. They must be described in relation to the actual station.

Each Region is assigned, on an annual basis, a block of numbers 001-999 to use for the XYZ digits.

Each Region will have the responsibility of assigning XYZ numbers for the following areas of work:

• Maritimes Regional Office should assign numbers for Newfoundland, Nova Scotia, Prince Edward Island and New Brunswick.
• Quebec Regional Office for the province of Quebec.
• Central and Arctic Regional Office for Ontario, Manitoba, Saskatchewan, Alberta, Yukon coast, NWT and Nunavut.
• Pacific Regional Office for British Columbia and Yukon.

The numbers or blocks of numbers may be assigned from one region to another when it is necessary for a region to conduct surveys in areas under the responsibility of another region.

5.1.2 Station description

In order to help in the recovery of permanently marked stations, a clear description of each must be written. All control points must be described as specified in QMS documentation. The general area sketch should show the general location of the marker in relation to adjacent coastline, directions to other markers, etc. The detailed sketch should give distances from the survey marker to conspicuous features and objects so as to easily recover the control point. Photographs should also be taken (see § 6.13 Use of photographs) and included with the written description.

5.2 Vertical Datum

It will be necessary to recover or establish sounding datum in order to properly reduce the soundings to a low water datum (which can be defined as a level which the water level will seldom fall below during the navigation season).

It will also be necessary to recover or establish the datum of elevations in order to reference the elevations of objects (fixed aids and conspicuous objects) and clearances under aerial obstacles (bridges, power lines, etc.).

In tidal water:

• Soundings are reduced to Lower Low Water Large Tide (LLWLT)
• Elevations and clearances are reduced to Higher High Water Large Tide (HHWLT).

In non-tidal waters,

• Soundings, elevations and clearances are reduced to sounding datum.

In some cases vertical datum will have been previously established and it will only be necessary to verify the existing gauge or install a temporary gauge and set it to sounding datum by levelling it from existing bench marks.

In other cases it will be necessary to establish sounding datum in the survey area by water level transfer which involves obtaining simultaneous water level readings from a temporary gauge location and readings from an existing permanent gauge at a distant location.

Specific instructions should have been given by a Tides and Water level representative and included in the Survey Project Instructions.

5.2.1 Water level gauges and bench marks

The permanent water level gauges in the vicinity of the area to be surveyed must be verified to ensure that the water levels recorded are accurate. The existing bench marks and the water level staff in the vicinity must be leveled to ensure that they are stable and have not shifted in elevation. Any significant discrepancies should be reported to a CHS Tidal Officer before sounding operations start.

Temporary gauges, including water level staffs, should be installed and referenced to at least three permanently established bench marks. These bench marks should be referenced to sounding datum, CGVD28 (MSL), WGS84 or GRS80 ellipsoids.

For digital water level gauges, the methods and procedures set out in the owner’s manuals should be followed.

All gauges, temporary and permanent, must be checked on a regular basis (daily if possible) to ensure that they are recording the correct water level readings.

All work is to be carried out using the methods, procedures and accuracies set out in the latest edition of the Canadian Tidal Manual so as to satisfy the standards established in the CHS Standards for Hydrographic Surveys.

5.2.2 Permanently marked vertical control points (e.g. bench marks)

Unless otherwise stated, all new bench marks are to be permanently established using a metal tablet stamped and identified using the numbering convention used by the National Geodetic Data Base.

All vertical bench mark tablets are to be identified by SERVICE HYDROGRAPHIQUE - CANADA - HYDROGRAPHIC SERVICE inscribed on the face of the marker tablets. The words should be spaced around and near the perimeter of the tablet face.

Each tablet should be identified using the National Geodetic Data Base (NGDB) numbering system which consists of 8 characters MYRx9XYZ where:

• “M” is a fixed letter which denotes that the station was established after the year 2000
• “YR” is the last two digits of the year the station was established (e.g. 87, 99, 01)
• "x" is the letter that identifies the province or territory (see Table below) where the control point is established
• “9” is the single digit number that identifies the organisation that established the station (CHS=9)
• “XYZ” is a three digit number (001 to 999) that identifies the specific marker identification (given by regional HDC offices)

For example, the station number 87L9412 signifies: 87 = year of establishment; L = Quebec, 9 = CHS, and 412 = individual marker identification digits. Note that the first character “M” is left blank because the station was established prior to year 2000.

The bench mark number is to be stamped on the tablet with dies, just below and parallel to the groove inscribed in the tablet face. The letters BM should be stamped and centred above the groove.

Listing of Provinces and Territories and their identifiers for Vertical Control (VC) - e.g. BM – Markers:

Each Region is assigned, on an annual basis, a block of numbers 001-999 to use for the XYZ digits.

Each Region will have the responsibility of assigning XYZ numbers for the following areas of work:

• Maritimes Regional Office should assign numbers for Newfoundland, Nova Scotia, Prince Edward Island and New Brunswick.
• Québec Regional Office for the province of Quebec.
• Central and Arctic Regional Office for Ontario and Manitoba, Saskatchewan, Alberta Northwest Territories, Nunavut and Yukon coast.
• Pacific Regional Office for British Columbia and Yukon.

The numbers or blocks of numbers may be assigned from one region to another when it is necessary for a region to conduct surveys in areas under the responsibility of another region.

5.2.3 Bench mark descriptions

In order to help in the recovery of bench marks, a clear description must be written and a horizontal position must be taken. A sketch should indicate the direction and distance between the BM and other prominent structures or features (see Canadian Tidal Manual, § 6.2.2 Benchmarks – descriptions). Photographs should also be taken (see § 6.13 Use of photographs) and included with the written description.

5.3 Positioning system

5.3.1 Real-time GPS method

Real-time differential GPS corrections can be provided by the CCG DGPS, WAAS or commercial networks. But in certain areas where the signal cannot be received or when higher accuracy is required, it may be necessary to install a temporary DGPS or RTK reference station. This temporary station must be installed on a permanently marked control point of sufficient accuracy required by the survey order.

5.3.2 Post-processing GPS method

Post-processed solutions can be obtained by utilizing techniques such as: Post-processed Kinematic (PPK), Precise Point Positioning (PPP), Inertially Aided PPK, etc.

5.3.3 Positioning Quality Assurance

Before collecting data, it may be necessary to verify the parameters and the accuracy obtained by the positioning system. This could be achieved by doing different tests (reference bottom surface, comparison with independent systems, control points, etc.).

The positioning system must be monitored to ensure that the quality of horizontal positioning falls within specifications. GPS software provides monitoring capabilities which can be done on the vessel in real time. Most systems show and log residuals, number of satellites, Dilution Of Precision (DOP), age of DGPS corrections, etc. As long as the Hydrographer monitors this information and acts when accuracy conditions are not met, this should be sufficient. The position of the reference station should be initially verified against a known control point, and a check of the vessel’s system, relative to a point surveyed on a wharf or accessible feature, should be done.

5.4 Final check

A final check should be performed to ensure that all equipment is working properly and that the logging systems are correctly logging positions, depths and in some cases water level values and any other data. The positioning system should be checked on a regular basis.