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CRAMP Rapid Assessment Geographic Coordinates

As part of the ongoing CRAMP quality assurance/quality control effort, an analysis of the accuracy of the Geographic Positioning System (GPS) protocol was undertaken.  Using different approaches, the CRAMP positioning was found to generally fall into the sub-meter accuracy category, with the worst-case scenario being on the order of a few meters.  We have established that GPS co-ordinates provided for the starting point of the Rapid Assessment Technique (RAT) transects have very high positional accuracy. This level of position accuracy is more than sufficient for the intended purposes of location of random points.

Transects within each site are randomly selected by generating 100 random points onto habitat maps using GPS Pathfinder Office 2.8.  To assure adequate coverage of different habitats and full representation of each site, a stratified design is employed.  Points are stratified within depth ranges (<5m, 5 to 10m, and >10m) and habitat types.  Not all habitat types are present at every site.

Habitat map of Waikiki area showing random points generated by GPS Pathfinder in each habitat type.

Habitat map of Waikīkī area showing random points generated by GPS Pathfinder in each habitat type.

An area map with the randomly selected points is generated for field reference.  Coordinates are input into a Trimble GeoExplorer 3.  Navigational GPS is used in the field to determine the position of each point.  Positions are logged and a quick-fix is obtained at the float used to mark the beginning of each transect.  A random numbers table generated in the software program “Excel” is used to determine which point will be surveyed and which direction the transect line will be laid.  Transects follow isobaths to keep the depth consistent within each transect.  Time, diving constraints, oceanic conditions and size of the area define the number of stations surveyed at each site.  In addition, specific stations may be purposely selected due to specific impact, habitats of interest, instrumentation/experiment placement, or prior survey data.

Randomly generated points used in determining locations of transects in the field overlaid on an image of Kaloko/Honokohau, Hawaii

Randomly generated points used in determining locations of transects in the field overlaid on an image of Kaloko/Honokohau, Hawai‘i

Latitude and longitude and UTMs are established at each site using differential GPS.  Data projection is based on UTM, NAD 83 datum.  Field characteristics were input into a pre-composed data dictionary.  GPS data is then processed in Pathfinder Office 2.8 and displayed using ArcView GIS version 3.1 software in the laboratory or laptops in the field.

In order to assure accuracy and precision of GPS coordinates to link with fish and benthic data, several different replicate methods of obtaining positions are used.  This redundancy allows for verification of position averages and quickfixes, with known positions (features and randomly generated points).  Each of the stations at a site have accurately verified random points or documented distances and directions from other random points.  Redundancy is further increased with known coordinates at stations that have established features such as prior survey data or experimentation.  Post processing including deletion of anomalous positions and differential corrections further reduced error.

The vessel or diver navigates to each waypoint using a Trimble GeoExplorer 3.  Each station was marked with a pelican float.  The exact position is therefore known prior to the gathering of 100 averaged positions.  The weight of the float is positioned over the random point that is used to mark the start of each transect.  Boat drift and difficulty in remaining in position is checked against quickfixes and random point coordinates for accuracy.  For accuracy assessment, these are compared to known coordinates from randomly generated points and features with known coordinates.   Quick fixes were also compared to averaged coordinates taken at each station.  Without differential corrections, quick fixes are accurate to within 4 to 5 meters.  These averages are used only as a backup to randomly generated positions and quickfixes.  Where the start of the transect is moved due to depth constraints, a distance and direction to the float is recorded underwater and adjusted for in the post processing of the data. 

The Dept. of Transportation requires all GPS units to be accurate to within 16 meters.  Most of this error is associated with the vertical precision, which is not used in our rapid assessments.  Horizontal precision is accurate to within several meters.

A total of 100 positions are taken at each station.  Pathfinder allows deletion of individual positions.  Positions greater than 3 meters from the first 20 fixes are deleted.  A defined boundary of 3 meters is created and all positions outside this boundary are deleted. Accuracy of the first 20 positions is the greatest as accuracy diminishes after 20 to 30 seconds of data gathering and since the boat maintains better position over the pelican float weight early on.

Differential post-processing corrections are applied to increase accuracy although at a 25m transect scale this is not necessary.  Without selective availability there is presently accuracy within 10m but actual accuracy is usually within 1m (Dr. Everett Wingert, UH Cartographer).

 

Last Update: 04/21/2008

By: Lea Hollingsworth

Hawai‘i  Coral Reef Assessment & Monitoring Program

Hawai‘i  Institute of Marine Biology

P.O. Box 1346

Kāne‘ohe, HI 96744

808-236-7440 phone

808-236-7443 fax

email: jokiel@hawaii.edu