Horizontal Position Fixing

process of determining the horizontal position of a survey vessel or sounding point while depth measurements are being taken.

ensures that every sounding is associated with its correct horizontal position.

uses a hand-held optical square and a float line to establish survey lines perpendicular to a straight beach baseline.

typically at least 100 m long and marked with ranging rods at both ends.

driven into the ground along the baseline

• ___ — irregular terrain or rocky coastline.
• ___ — flat, long, straight beaches.

positioned offshore at right angles to each peg using an optical square or theodolite.

anchored between each shore steel peg and its corresponding offshore buoy.

should ideally extend about 100 m on either side of the proposed shelter or landing area.

involves lowering a calibrated sounding chain at 5 m or 10 m intervals to create a grid of spot levels.

automatically records soundings on a thermal paper roll, producing a continuous seabed profile.

Echosounder runs — ___ are preferred.

surveys should be conducted in

Run starting point — begins approximately ___ to improve boat alignment.

Parallel Line Method Specifications
• Maximum survey distance offshore: approximately ___.
• Maximum recommended float-line length: ___.
• Float-line rope diameter: ___.
• Coaxial colored float spacing: ___.
• Large red spherical floats: placed every ___ to make the line easier to identify and read.
• Preferred rope material: ___.
• Rope property: ___ is preferred because excessive stretching can affect survey-line accuracy.

uses shore-based theodolites to determine the horizontal position of a survey vessel offshore.

determines the vessel's position by observing its location along a known direction or ray from the shore.

Maximum offshore distance: approximately 300 m.

uses two shore-based transit/theodolite observers.

uses microwave-ranging Electronic Positioning Systems (EPS).

the system determines position using measured distances from reference points.

developed as an alternative to the triangulation method.

has since been superseded by GPS-based positioning systems.

primary modern reference method for engineering and construction hydrographic surveys.

stationary receiver located on a known benchmark.

receiver installed on the survey vessel.