HYDROGRAPHIC SURVEYING PRACTICES

Written by: Uchenna Omeje - Coordinator, Survey Services

Hydrography is the acquisition, analysis, and management of spatial information concerning all marine/sea/lake/river features, processes and properties. Thus, hydrography literarily covers almost all forms of operations involving measurement and analysis in the aquatic/marine environment.

Some common disciplines in the field of hydrography include; Geodesy, Position fixing, Oceanography, Geophysics, etc.

Data obtained from hydrographic measurements are spatially referenced. This makes all hydrographic measurements to be inclusive of position and depth.

Hydrographic survey has evolved from the most primitive use of lead line, sextant, compass, etc. to the recent use of sophisticated sensors and software for data acquisition, interpretation and presentation. Some current practices in hydrographic survey include the following:

 

Control for Hydrographic Survey

Global Navigation Satellite System (GNSS) has evolved to be the most frequently used reference system for hydrographic surveys. It is a system comprising different satellite positioning systems around the world.

GNSS when used in its Wide Area Augmentation System (WAAS) mode provides accuracy level suitable for all hydrographic survey activities. This is used for real time data acquisition and transmission during surveys. GNSS correction stations are in different parts of the world from where differential corrections are transmitted to users upon subscription. Most hydrographic surveys are controlled using GNSS which includes GPS.

The static and differential modes are used for control extension during hydrographic survey activities.

 

Offshore Positioning

Offshore operators (oil/gas, construction, etc.) require precise positioning of points over water. Since there is no stable platform to operate land-based surveying and positioning system, a combination of navigational and positioning systems is employed over water/offshore.

Basic instruments used for offshore positioning include Global Navigation Satellite System (GNSS), gyrocompass, navigational software and vessel used as platform for the survey.

The GNSS is mounted at a strategic position on top of the vessel so that it can receive satellite signal and corrections, while the compass is positioned in alignment with the vessel and necessary calibrations applied. A common reference point (CRP) is chosen in alignment with the GPS antenna and offset of point marker (e.g. drilling rig) from the CRP is measured and inputted into the navigational software. The navigational software e.g. NaviPac or hypack is used to synchronise the GNSS, gyrocompass and the point marker. VGA cable is used to share the screen of the navigational computer with vessel captain. A model of points or lines of interest are plotted on the navigational software together with a model of the vessel. With this set up, the captain’s task is to navigate the vessel to the points of interest as seen on the computer screen.

Achieved accuracy is a function of the competence of both the surveyor, boat captain, and the kind of equipment used

 

Bathymetric Survey

This is otherwise known as underwater survey. Just like topographic survey represents terrain and features on the land surface, bathymetric survey represents terrain and features beneath the water surface.

In the olden days, it was done using lead line for depth measurement, a combination of compass, sextant, measuring tape; for position fixing and employing the services of divers where water depths are suspicious. The divers go beneath the seabed to check for the presence of debris and features.

In recent times, this is done with sophisticated sensors and software including Global Navigation Satellite System (GNSS), single beam echo sounder, multi-beam echo sounder, side scan sonar, magnetometer, gyrocompass, motion sensor, survey boat, navigational software, laptop computers, velocity probe, etc.  Selection and deployment of these equipment depend on scope of the survey/investigation.

Bathymetric survey is carried out to ascertain how navigable a body of water is, for coastal and offshore construction, for research purposes, dock and harbor engineering, irrigation dam construction, land reclamation, hydroelectricity, flood control, sewage disposal, etc.

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