GPS Methodology
Global Positioning System (GPS) Tracking


A GPS track is conducted as part of a site study

  1. to gain an overview of the reef system as a whole by collecting observations over a larger area than the Vitareef and transect sites encompass; and
  2. to generate a set of georeferenced observations for potential future use in conjunction with satellite imagery.


The following equipment is used to conduct a GPS track: a Zodiac small boat with outboard engine; a Garmin GPSmap 76 handheld unit, configured to record track points every 10 seconds; a HawkEye Digital Sonar handheld depth sounder; a Sony CyberShot DSC-P8 camera inside a Sony Marine Pack 40m housing; and waterproof slates and pencil.

A team of at least three individuals is required to conduct the GPS track: a driver, observer, and recorder/GPS operator. At the beginning of the track, the GPS track log is cleared to commence recording points from that location onward. The driver maneuvers the boat to follow the reef contour as precisely as possible at a constant depth of three to five meters. The observer is towed in the water on the reefward side of the small boat making observations on the underwater area below and in the direct vicinity of the boat. The recorder takes GPS waypoints and depth soundings in conjunction with observations as they are reported. At the end of each segment of reef being tracked, the GPS track log is saved. The process is repeated, tracking segments of reef, until the desired area is covered for the study.

Underwater observations may include a percentage breakdown estimate of the bottom composition (e.g. live/dead hard coral and genus where possible, soft coral, sand, rock, rubble, filamentous/macroalgae, etc.), comments on coral health, reef slope, presence and abundance of fish or other marine animals, and other information as appropriate.

The bottom composition is reported as follows: While being towed, the observer formulates an estimation of bottom composition over a distance while the composition remains relatively constant, however long or short this may be. When the observer judges the bottom composition to change noticeably, the observer reports this to the recorder. The recorder notes the estimated bottom composition for the previous distance up to that point. At this moment the recorder also takes a GPS waypoint and depth sounding and notes the two in association with the bottom composition observation. Therefore, an individual bottom composition observation refers to the distance covered by the track previous to the waypoint with which it is associated (and following the last waypoint with an associated bottom composition observation).

The observer reports discrete observations (e.g. on presence of notable species or anomalies such as evidence of bombing) along the track as they occur, at which points the recorder takes a waypoint and depth sounding to record in association with the observation.

Photographs may be taken along the track in association with a waypoint and depth sounding.

When long stretches pass without reported observations, the recorder may take depth soundings and waypoints to maintain a roughly constant record of the depths along the track.

For reference, waypoints may be taken at permanent or semi-permanent landmarks, such as buoys and markers, and tracks may be taken along beaches or other distinct landmarks in the vicinity of the reef under study.