Mean Lower Low Water (MLLW): Water level data derived from averaging the lower of two low tides per day over a long period of time or epoch. The purpose of this document is to detail the technical specifications and requirements for the conduct of hydrographic surveys by the Hydrographic Branch of the National Resource Mapping and Information Authority (NAMRIA).
List of Major Changes from March 2019
The specifications described in this document are largely based on the International Hydrographic Organization's (IHO) Standards for Hydrographic Surveys – Special Publication No. The references in this document also include the hydrographic survey standards of various international hydrographic organizations.
It should also be noted that the provisions of this document do not discredit or invalidate previous hydrographic surveys or nautical charts, but rather seek to improve the conduct of future hydrographic surveys. This paper also attempts to respond to the advancement and advancement of technology and principles associated with hydrographic surveying.
Tides and Water Levels Requirements
Hourly heights and high and low forecasts in the tide and current tables will be referenced to MLLW. A series of staff observations will be carried out initially after installation, during deployment and before removal from the tide station. The tide gauge to tide gauge differences must be averaged and applied to the height of the Primary gauge for calculating the tide datum.
When restoring the tide reference points of previously occupied slave tide gauge stations, the PBM height above the tide reference point shall be used. Data editing and gap filling is done and documented according to established POD guidelines. The patch test results, along with procedures, will be discussed in a narrative report.
Cross control lines will be obtained and processed in addition to the regular or development lines. Raw digital files of bathymetric data will be submitted in the original format from the acquisition software. Manual and/or automatic filtering for noise reduction and smoothing of the projected/predicted surface should be done.
When portable tide gauges are used for the duration of the survey, the downloaded data must be submitted.
The NAMRIA Hydrographic Survey Standards will be reviewed annually by the Technical Working Group and will be revised as necessary. For suggestions, comments or questions related to the NSHS, please contact the Hydrographic and Geomagnetic Data Management Division, Survey Support Division by or email firstname.lastname@example.org.
Coordinated Universal Time (UTC) is used as the reference time for all data collection, including water level data. All horizontal positions refer to the World Geodetic System 1984 (WGS84), realization G1762 or later.
- Position Accuracy
The general misconception is that the accuracy of the positioning equipment alone corresponds to the positional accuracy. Primary coastal control points established by ground survey method with a relative accuracy of 1 part in 100,000 shall be used.
- Survey Order Pre-Qualification
Evidence of quality monitoring of recording operations, such as DOP and RMS values, should also be documented and discussed in the Descriptive Report.
The uncertainty of the geodetic control used in the positioning of soundings and features during hydrographic survey contributes to the total horizontal uncertainty which must not exceed the values in Table 1. NAMRIA-established geodetic controls—Zero Order, First Order, Second Order, Third Order, and Fourth Order—can be used in hydrographic surveys, provided the positions of these controls are referenced to WGS84(G1762) or later.
General Data and Reference Datum Requirements
Error Budget Considerations
When the Universal Transverse Mercator (UTM) projection is used, the data coverage must be defined under the appropriate zone definition (Zone 50 to 52) in the Northern Hemisphere. The positional accuracy refers to the closeness of the measured feature or sound to its exact position within the reference frame.
Tidal Data collection
- Tide Station Requirements
- Tide Gauge
- Station Installation
- Tide Staff
Components of the tide station include the attached tide gauge, a tide staff and at least five (5) benchmarks. Personnel observations after installation and before removal shall be conducted concurrently with the specified tide gauge sampling interval for at least three (3) hours. The tide bar observations must be made to calculate the difference between the tide staff and the tide gauge.
At least two (2) previously established reference points must be recovered and verified to be undisturbed and still suitable for tidal reference recovery. A tide gauge station can then be installed in the area and leveled to determine the difference in elevation between the reference points, the water level sensor and the tide gauge staff. Rappers are described according to the type of sign used, inscription, access and position in relation to at least two more visible signs nearby.
Tidal Data Processing
- Quality Control
- Data Processing
The PPE is usually the most stable mark in the vicinity of the water level station. If the PBM is found to be disturbed or unstable, another benchmark from the five (5) previously established BMs should be used. The newly designated PBM must then be associated with the tide datum by determining its previous elevation compared to the original PBM.
Any deviations by the field party from these requirements will be subject to approval by the CSSD. Movement of tide gauge or tide rod relative to PPE compared to previous leveling of more than 1 cm will be verified by rerunning leveling between tide staff or tide gauge to PPE. Tidal data that has undergone quality control will be used to calculate for different tidal dates and generate hourly, maximum and minimum water height to MLLW.
Computation of Tidal Datums
- Datum Computation
- Tidal Datum Recovery
- Quality Control
General Standards for Depth
The investigating party must make preliminary calculations of the total propagated uncertainty (TPU) to ensure that the planned investigation meets the standards of horizontal control. The field survey party must ensure that all components of the tide station are safe and secure throughout the observation length. The tide rod should be mounted on a separate pole independent of the water level sensor to maintain the independent stability of the rod and sensor.
The field survey party will consult with the POD regarding the existence of previously observed subordinate tide stations in the vicinity of the survey project. If a subordinate tide station has previously been installed in the area, the party will retrieve previously used gauges and use the elevation data from said gauges for retrieving the tide datum. If possible, the GNSS position of the mark (in degrees, minutes and seconds) should also be determined for easier recovery in the future.
Leveling of the tidal station, tidal staff and the five (5) benchmarks shall be carried out after installation and before dismantling the tidal station, after modification of the station and at least every six (6) months during operation. Areas shallower than 100 m where the general view of the seabed is sufficient and the clearance under the keel is not a problem at all.
Multibeam and Single-Beam Echosounders
- Line Spacing
Corrections to Soundings and Uncertainty Analysis
- Corrections for Instrument Error
- Vessel Static Offsets
- Corrections for Draft
- Corrections for Sound Velocity
- Corrections for Vessel Motion
- Uncertainty Analysis for Depths
In general, draft is the vertical distance between the water surface and the transducer reference point. Dynamic draft refers to the vertical displacement of the transducer when the vessel is underway. Any deviation from the standard must be reviewed and investigated and, if necessary, additional cross-check lines must be procured to help resolve the discrepancies.
Lower sample frequency must be carefully determined to obtain adequate characterization of the survey area. Progress reports will be submitted weekly or daily, if reasonably practicable, to HGDMS and to the command directly supervising the field work for the duration of the operation. Navigation surfaces in the form of Bathymetric Attributed Grid (BAG) must be submitted as one of the formats of the final depth information of the survey.
Cross-line comparison – results of the cross-check analysis, including the graphs and the order in which the survey falls. Results of the survey – chart comparison, seabed samples, aids and hazards to navigation and coastal demarcation.
- Multibeam Sonar Calibration
- Cross Check Analysis
General Standards for Features
- Aids to Navigation (ATONs)
- Dangers to Navigation (DTONs)
- Submerged Installations
- Seabed Characteristics
- Shoreline Construction
- Current Observation
- Elevations and Clearances
Positions of navigational aids shall be obtained using survey-grade GNSS equipment and shall meet the accuracy requirements in Table 1. Anomalous features previously reported in the survey area and those detected during the survey shall be thoroughly examined to provided accurate and detailed description of the contours. Seabed characteristics will be obtained in ports, anchorage areas and potential anchorage areas, as well as other critical areas.
The high and low water lines defining the intertidal zone shall be determined to the accuracy given in Table 2. The low water line shall be derived by conducting hydrographic surveys with a beam during high water. Manual depth measurements will be recorded using Manual Depth Measurement Data Sheet NAMRIA-HNC-Form51 Rev00.
Data Processing and Analysis
- Progress Reports
- Navigational Warnings/Notice to Mariners
- Bathymetric Data
- Tidal Data
- Descriptive Report
- Smooth Sheets
- Calibration Forms
Horizontal controls – description of established and/or restored geodetic control points and base station configuration parameters. Comments and Recommendations – This includes notable circumstances and field experiences that may have influenced the results of the study, and recommendations that may assist future projects that may find themselves in a similar situation. Latitude and longitude should only be used specifically to position the details when recorded by astronomical observations or GPS and a full description of the method, equipment and date used should be given.
If it is preferred to show the changes on a tracing of the largest scheme (rather than the map itself), these should be in red as above, but sufficient detail from the scheme should be traced in black ink to enable the changes to be fitted correctly. Electronic Navigational Charts (ENCs): A screenshot of the largest usage band ENC with changes and additions shown on it in red. The time, date and time zone should be given to allow corrections for tidal height where necessary, or a statement of what corrections for tides have already been made.
Older electro-mechanical echo sounders often record signals from echoes received back after one or more rotations of the stylus have been completed. However, soundings received after one or more rotations of the stylus may still be useful and should be submitted if they show significant differences from the mapped depths.