Hydrographic also known as bathymetric survey is the science of measurement and description of the seabed features.

Hydrographic surveys use sonar to collect highly accurate water depth and seabed contour information. These echo sounding systems work on the fact that when sound is sent into water, it will be reflected back after striking an object. An instrument called a transducer sends a sound signal into the water, and then it will be reflected back after colliding with the sea floor. The transducer also picks up the reflected sound. The sonar sensor precisely measures the total time taken by the sound signal to travel to and fro the seabed. The water depth and consequently the seafloor topography is determined by knowing the speed of sound in water which is approximately 1500ms^{-1 (Syqwest, 2021)} This method of seafloor mapping is known as echo sounding.

There are majorly two types of echo sounders normally used today namely;

• Single beam echo sounders
• Multiple beam echo sounders

 Hydrographic surveying is very essential in the following fields;

• maritime navigation
• marine construction
• dredging or reclamation
• offshore oil exploration/offshore oil drilling
• Cable planning
• Earth quake and fault studies, etc.

The purpose and accuracy of the project required highly influences the methodology to be used and therefore the equipment to be used.

In ascending order of accuracy, below are the classes of hydrographic survey.

• Class 3 or reconnaissance
• Class 2 or Project condition or design
• Class 1 or Contract payment

Maximum allowable errors in hydrographic surveys

Type of survey	Class 3	Class 2	Class 1
Vertical Accuracy	500mm	300mm	150mm
Horizontal Accuracy	100m	12m	6m

NB. The same equipment may be used to achieve the different levels of accuracy depending on the methodology or field procedure used. i.e. The accuracy is primarily a function of the field procedure used.

There are various methodologies in use today to carryout hydrographic or bathymetric survey. As earlier on discussed, these depend entirely on the level of accuracy required, but also on the size of the project area.

For vertical measurements, the following methods may be employed;

• Hand held calibrated lead sounding line
• Simple engineering echo sounder recording on paper
• Advanced engineering echo sounder recording on a data logger and linked to a position fixer via integrated software (mostly used for modern Class 1 Hydrographic surveys)

 Meanwhile, for Horizontal positioning, the following methods may be employed;

• Hand-held optical square in conjunction with a floatline
• Single theodolite in conjunction with a float line or twin theodolites
• Constant range tracking electronic Positioning system (EPS)
• Differential GPS
• Realtime Kinematic GPS(RTK)