Hydrography, traditionally focused on supporting navigation, has undergone a significant evolution, transforming into a comprehensive scientific discipline that extends its applications to crucial areas such as coastal zone management and environmental monitoring. This expansion is a direct consequence of rapid technological advancements and the escalating needs of a growing human population.

The Evolving Definition and Scope of Hydrography

Historically, hydrography was primarily focused on maritime navigation. However, over the past two decades, its scope and applications have shifted fundamentally. The term "hydrography" itself originates from the Middle French "hydrographique," referring to the nature and measurement of water bodies, including depth and currents. Lexical definitions further elaborate its meaning:

• The art and science of compiling and producing charts or maps of water-covered areas of the Earth's surface.
• The science of measurement, description, and mapping of the Earth's surface with special reference to navigation.
• The art of measuring and describing the sea, lakes, rivers, and other waters, along with their phenomena.
• The branch of surveying that encompasses the determination of the contour of the bottom of a harbor or other water body, the depth of soundings, the position of channels and shoals, and the construction of charts.

The International Hydrographic Organization (IHO), an international body responsible for managing and publishing navigation-related materials, first defined hydrography academically in its Special Publication Number 32 (SP-32) in 1970. Later, the Group of Experts on Hydrographic Surveying and Nautical Charting, in 1979, further defined it as "the science of measuring, describing, and depicting nature and configuration of the seabed, geographical relationship to landmass, and characteristics and dynamics of the sea".

This definition was further elaborated by the Expertise Group on Hydrography (KK Hidrografi) at Bandung Institute of Technology (ITB) in 2004, stating that hydrography is "the branch of science concerned with the measurement and description of the characteristics and dynamics of water bodies". This definition explicitly includes the study of seabed features, such as bathymetry (or "topography" of the seabed), seabed material types, and seabed morphology. It also encompasses the dynamics of water bodies, which involve tides and currents.

The primary purposes of hydrographic information are extensive and include:

• Navigation and maritime safety.
• Delimitation of maritime boundaries or areas at sea.
• Study of coastal dynamics and marine resource management.

Beyond these, hydrography supports various other marine activities and purposes, such as offshore industries, research, environmental protection, and prediction services. Indonesia, as a large maritime nation, has actively participated in hydrographic development, particularly in education, with ITB's Geodesy Engineering Department pioneering the development of hydrography as a scientific discipline since 1982.

Core Activities in Hydrographic Surveying

Surveying is the most critical activity in generating hydrographic information. The fundamental activities of hydrographic surveys typically include:

• Positioning at sea  (1) and the use of reference systems (7).
• Depth measurement (sounding) (2).
• Current measurement (3).
• Sediment measurement (sampling and analysis) (4).
• Tide observation (5).
• Measurement of detailed situations and coastlines (6).

The data obtained from these activities are presented as maps or non-map information and compiled into marine spatial databases.

Key Techniques and Technologies

1. Positioning The location of an object at sea is a primary activity in hydrographic surveys. A position is determined by combining two or more "Lines of Position" (LOPs). An LOP represents the location or presence of points from an observer with a fixed observation quantity (distance, direction, angle, or distance difference from a reference point to the determined point). Optical (visible light) and electronic (radio waves) instruments are used to obtain these fixed observation quantities. LOPs can take various geometric forms, including straight lines, concentric circles, eccentric circles, and hyperbolas.

Global Positioning System (GPS) has revolutionized positioning at sea.

• Characteristics and Principles: GPS is a satellite-based radio navigation and positioning system operated by the United States. It is designed to provide precise 3D position, velocity, and time information continuously, worldwide, regardless of weather conditions, for multiple users simultaneously. Its core principle is measuring distances to several satellites with known coordinates. GPS signals consist of three components: range information (C/A-code and P-code), satellite position information (navigation message), and carrier waves (L1 and L2).
• Receiver Types: GPS receivers for positioning can be categorized into navigation type (handheld), mapping type, and geodetic type. Navigation type receivers provide absolute positioning instantly and are typically used where high precision isn't required (5-10 meters for civilian, 3-5 meters for military).
• Positioning Methods: GPS offers several positioning methods:
  • Absolute Positioning: A single GPS receiver determines its position relative to the Earth's center. Real-time absolute positioning accuracy improved significantly from 50-100 meters to 5-10 meters after Selective Availability (SA) policy was abolished on May 1, 2000.
  • Differential Positioning (DGPS): This method uses at least two receivers (one fixed reference station, one moving) to determine positions relative to other known points. It can achieve higher precision by using pseudo range data (for medium accuracy, up to a meter level) or phase data (for high accuracy, centimeter level). DGPS is commonly used for real-time applications, while Real-Time Kinematic (RTK) systems (using phase data) offer even higher accuracy (1-5 cm) and are suitable for both static and kinematic positioning.
• Advantages: GPS provides a wide spectrum of accuracy, from millimeters to tens of meters, allowing users to select the optimal precision for their needs, making it efficient and cost-effective for various applications. It operates globally, is not affected by topography or distance from the coast, and positions are referenced to a global datum (WGS-84), which is highly beneficial for a vast archipelago like Indonesia. Additionally, GPS equipment and software have become more affordable and user-friendly, and the technology itself is generally free to use.
• Limitations: GPS relies on antenna visibility to satellites, requiring careful antenna placement on survey vessels to minimize signal obstruction. Positions are determined relative to the WGS-84 ellipsoid, meaning heights obtained are ellipsoidal, not orthometric (relative to the geoid or Mean Sea Level), which is commonly used for practical purposes and terrestrial measurements. Furthermore, a lack of human resources skilled in GPS technology in Indonesia poses a challenge, emphasizing the need for skilled personnel alongside hardware and software acquisition.

2. Depth Measurement (Sounding) Sounding, is the process of mapping the shape (topography) of the seabed surface.

• Methods:
  • Mechanical Sounding: This was the earliest method, involving manual depth measurements.
  • Optical Method (Laser Airborne Bathymetry - LAB/LIDAR/HALS/LADS): A newer method using laser beams from aircraft to measure water depth, suitable for clear, shallow waters (up to 50 meters deep). It relies on the transmission and reflection of light through water, accounting for refraction due to density changes. Systems like LADS from the Royal Australian Navy have been tested in Indonesia.
  • Acoustic Method: The most popular technique in modern hydrography for measuring underwater features, including depth, currents, and sediments. It uses echosounders that emit sound waves from a transducer; the depth is calculated from the time it takes for the sound to travel to the seabed and return, multiplied by the sound velocity in water. Acoustic waves with frequencies of 5 kHz to 100 kHz can maintain intensity up to 10 km depth, while 500 kHz waves lose intensity at depths greater than 100m.
• Survey Design and Data Quality: Sounding lines (lajur perum) can be straight, concentric circles, or other patterns, designed to detect extreme depth changes by running perpendicular to the general direction of coastlines. Data quality is affected by pulse length and beam width. Calibration using a "bar check" is essential to correct for instrument errors. Sweeping techniques, such as mechanical sweeping with steel bars or acoustic sweeping with side scan sonar, are employed to detect depth anomalies, like coral reefs or piles, that might be missed by regular sounding lines.

3. Current and Sediment Measurement

• Currents (Arus): Currents are the movement of water bodies, generated by tides, waves, and winds. Understanding current dynamics is crucial for environmental and regional engineering studies.
  • Measurement Approaches:
    • Lagrangian Approach: Involves tracking floating buoys to observe surface water mass movement over time.
    • Eulerian Approach: Involves measuring currents at a fixed position within the water column as a function of time.
  • Technology: While early methods used mechanical current meters (e.g., USGS Price Type AA, Neyrpic Dumas), modern techniques utilize Acoustic Doppler Current Profilers (ADCPs). ADCPs emit acoustic waves and measure the frequency shift of the returning echo (Doppler effect) caused by moving particles in the water, providing depth-resolved current profiles. ADCPs are non-intrusive and offer better spatial and temporal resolution than traditional current meters.
• Sediments: Sediments are fundamental components of seabed morphology.
  • Characteristics: Sediments are characterized by their grain size, density, fall velocity, composition, porosity, and shape. Grain size, often represented by diameter (d), is particularly important as it indicates the sediment's resistance to transport.
  • Sampling and Analysis: Sediment samples are collected using methods like grab samplers. Suspended sediment concentration can also be measured indirectly using optical (measuring turbidity) or acoustic (measuring backscatter intensity) techniques, which require calibration with direct samples. Sediment transport (q) is calculated from sediment concentration (c) and current velocity (u).
  • Importance: Sediment transport studies are vital for understanding seabed elevation changes (erosion/deposition) and managing coastal stability, especially where rivers introduce significant sediment loads.

4. Tide and Vertical Datum

• Tides: Ocean tides are the periodic rise and fall of sea level caused by the gravitational forces of celestial bodies, primarily the Moon and the Sun. While the Moon's mass is smaller than the Sun's, its closer proximity to Earth makes its gravitational influence on tides significantly greater.
  • Types: Tides are categorized as diurnal (single high and low water per day), semi-diurnal (two high and low waters per day), or mixed.
  • Phenomena: Spring tides occur during new and full moons when the Sun, Moon, and Earth are aligned, resulting in the maximum tidal range. Neap tides occur during quarter moons when the Sun and Moon are at right angles to each other relative to Earth, resulting in the minimum tidal range.
  • Tidal Currents: These are the horizontal water movements associated with the rising and falling of the tide.
• Vertical Datum: The sea surface serves as the vertical reference (datum) for depth measurements. Since the sea level continuously changes, a specific tidal datum must be chosen to define the "zero" depth.
  • Mean Sea Level (MSL) is a commonly used vertical datum, obtained by averaging hourly water levels over a 19-year period.
  • Lowest Astronomical Tide (LAT), the lowest predicted water level, is recommended as a reference for navigation charts.
• Observation and Prediction: Tide observations are conducted manually using tide staffs (palem) or automatically using tide gauges. Tide prediction relies on harmonic analysis to determine tidal components (amplitude and phase). Non-harmonic factors, such as meteorological (rainfall, wind) and hydrological conditions (river discharge), also influence real-time sea level and must be considered for accurate predictions. Tide predictions are critical for navigation and coastal infrastructure development.

5. Coastal Mapping and Detail Surveys

• Coastal Details: These surveys collect information on natural and man-made features along the coastline, such as coconut trees, mangroves, rocks, cliffs, harbors, lighthouses, and buildings. This information is crucial for visual navigation and determining a vessel's position.
• Coastline Definition: The coastline is the boundary between land and sea. Although it continuously changes with tides, a fixed water level, typically the high water line, is chosen as the coastline for marine charts. For depth references, the low water line is used.
• Survey Challenges and Methods: Determining the coastline on the ground faces challenges due to various coastal characteristics (e.g., mud, sand, rocky, coral). GPS technology can be effectively applied for measuring coastal details and coastlines due to its efficiency and the open view of the sky in marine areas.

Hydrography in Coastal Zone Management and Environmental Monitoring

Hydrography plays a pivotal role in managing extensive aquatic territories. Its evolution is driven by the increasing demand for data to support various marine and coastal applications, including:

• Maritime Boundary Demarcation: Hydrography is fundamental for defining and implementing national and regional maritime boundaries based on international conventions like the United Nations Convention on the Law of the Sea (UNCLOS) 1982. This involves surveying land and sea, observing tides, and conducting bathymetric surveys to define baselines (normal and straight baselines), territorial seas (up to 12 nautical miles), contiguous zones, Exclusive Economic Zones (EEZ) (up to 200 nautical miles), and continental shelves. Indonesia, as a signatory to UNCLOS, is obligated to consistently implement these provisions.
• Coastal Zone Management: Hydrography provides essential data for integrated coastal zone management. This includes understanding coastal morphology, sediment transport, and the interaction of water bodies with the seabed, which is critical for managing coastal erosion and deposition.
• Environmental Monitoring: Hydrographic surveys support environmental protection by providing data on water body dynamics, sediment characteristics, and potential pollution. This data can be used for modeling oil spills and assessing environmental impacts.
• Infrastructure Development: Hydrographic data is vital for planning and executing marine infrastructure projects, such as port dredging operations.
• Spatial Data Infrastructure (GIS): Hydrographic information, including survey data, is integrated into Geographical Information Systems (GIS) for various purposes, including mapping, precise positioning, and navigation systems.

Standards and Publications in Hydrography

To ensure consistency and quality, the International Hydrographic Organization (IHO) publishes standards and guidelines for hydrographic surveys. The IHO Standards for Hydrographic Surveys, Special Publication Number 44 (SP-44 IHO), particularly the 4th edition published in April 1998, serves as a comprehensive guide for IHO member states, including Indonesia.

The SP-44 standard has evolved to accommodate a broader range of data collection needs beyond traditional navigation charts. The 4th edition emphasizes gathering data to meet diverse user requirements, including:

• Hydrographic data for coastal zone management.
• Environmental monitoring.
• Implementation of national maritime boundaries.
• Offshore exploration and exploitation.

Accuracy standards for hydrographic surveys are defined within SP-44, varying by "survey order" (Spesial, 1, 2, 3), which are dependent on the area's characteristics and depth. These standards specify horizontal and vertical (depth) positioning accuracy, as well as data density requirements for features. For instance, total vertical error for depth measurement (total propagation of fixed and depth-dependent errors) is set with a 95% confidence level, with different thresholds for various survey orders.

Conclusion

Hydrography has truly transformed from a specialized field focused on navigational support into a dynamic and interdisciplinary science. Driven by technological innovation and the escalating demands for marine and coastal information, it now underpins critical activities in coastal zone management, environmental monitoring, resource exploitation, and maritime boundary delimitation. The ongoing evolution of hydrographic techniques, particularly with advancements in GPS, acoustic sounding, and current profiling, ensures its indispensable role in ensuring maritime safety, facilitating sustainable development, and promoting the responsible stewardship of our vast oceans and coastlines. As the world increasingly looks to its oceans for resources and management, hydrography will continue to adapt and expand its vital contributions.

Refference

Poerbandono dan Djunarsjah, E. (2005). Bandung: Refika Aditama. ISBN-10: 979-3304243