Hydrographic surveys are essential for collecting detailed information on water depths in various aquatic environments such as ports, anchorages, rivers, inland waterways, dams, and reservoirs. These surveys support a wide range of applications including:

• Navigation

• Project and engineering planning

• Irrigation and tidal power

• Tourism and inland waterway development

• Pre-, progressive, and post-dredging assessments

• Monitoring siltation, erosion, and pollution

• Flood control initiatives

The data obtained from hydrographic surveys informs critical decision-making in areas such as:

• Vessel navigation

• Resource management and development

• Marine infrastructure and construction

• Coastal development

• Environmental monitoring

• Tactical and strategic military operations

Survey Techniques: Single Beam vs. Multi-Beam

While single beam echo sounder (SBES) surveys have long been the industry standard due to their simplicity and lower cost, multi-beam echo sounder (MBES) surveys are increasingly being adopted. Advances in hardware, software, and computing power have made MBES systems more accessible and cost-effective. Compared to single beam systems, MBES offers:

• Faster seabed mapping

• Higher precision

• Greater spatial resolution

IHO S-44 Standards: Survey Orders

The International Hydrographic Organization (IHO), through its S-44 4th Edition standard, classifies hydrographic surveys into four types based on their intended use:

1. Special Order

   • Reserved for areas where vessels operate with minimal under keel clearance.

   • Focuses on locations with potentially hazardous seabed features like boulders or rock outcrops.

   • Requires the highest level of positional and depth accuracy.

   • Typically designated by national hydrographic authorities.

   • Mandates "100% bottom search" using techniques such as:

     • Closely spaced survey lines

     • Side-scan sonar

     • Multi-transducer or multi-beam echo sounder arrays

1. Order 1a

   • Applicable to areas where under keel clearance is critical but not as stringent as for Special Order areas.

   • Typically used in ports, harbors, coastal routes, and busy shipping lanes where vessels operate with moderate under keel clearance.

   • Requires high survey accuracy, though not to the same extent as Special Order.

   • The seafloor must be examined for significant features that could pose hazards to navigation.

   • May involve the use of:

     • Multi-beam or single beam echo sounders

     • Side-scan sonar

     • Densely spaced survey lines (though less dense than for Special Order)

2. Order 1b

   • Designed for areas where vessels do not routinely operate, or where there is significant under keel clearance.

   • May be used for coastal development, fisheries, anchorage planning, or general maritime resource management.

   • Focuses on detecting larger obstructions that could affect deep-draft vessels.

   • Accuracy and detection requirements are lower than Order 1a, reflecting the reduced risk.

   • Allows for wider line spacing and may not require full bottom coverage.

3. Order 2

   • Suitable for general maritime or engineering purposes in deep or low-risk areas.

   • Commonly used for inland waters, recreational navigation, or initial planning stages of marine infrastructure projects.

   • Accuracy and object detection requirements are the lowest among the four orders.

   • Often conducted with:

     • Single beam echo sounders

     • Wider line spacing

     • Minimal or no side-scan sonar use

   • Typically used in areas where:

     • Navigation is infrequent

     • Safety of navigation is not a critical issue

Summary of IHO S-44 Survey Orders

Note: The term “100% bottom search” replaced the earlier, ambiguous term “100% personification” after extensive discussion among professionals.