Processing GDP Drifter data
Overview
Drifter data from the Global Drifter Program (GDP) is collected from NOAA AOML and inserted into a database at UNC-CH (Xenia schema). Data is served to SECOORA via a WMS feed wrapped by a CGI wrapper for time handling.
Source
Server(s): http://www.aoml.noaa.gov/phod/trinanes/xbt.html
Contacts: Folks at AOML are aware of our collection routines. I have received technical assistance from NOAA contractor Joaquin Trinanes (joaquin.trinanes@noaa.gov) and have also discussed our access with Judy Gray at AOML.
Collection Scripts
This script essentially mimics interaction with the Java Applet GUI to create URL requests for data. Further variables are added to the returned CSV (speed and direction) and a derived text file is posted for HTTP access. A second script collected the AOML test file (HTTP) and calls germane parsing scripts for both AOML GDP drifters and Horizon Marine drifters. The shell script takes output from the parsing scripts to insert into the Xenia database.
Scripts: aoml_drifter_derived.pl running on cromwell.marine.unc.edu: creates an AOML GDP data file
drifter_parse.sh (calls process_file_xenia_aoml.pl) running on cromwell.marine.unc.edu: gets, parses, and inserts the text file into the DB
Timing: Runs hourly
SECOORA Access
Data is served to SECOORA via a WMS feed wrapped by a perl CGI wrapper for time handling.
- http://maury.marine.unc.edu/cgi-bin/drifter_wrapper? accepts TIME, TIME_OFFSET_HOURS and TAIL (# of days to show before latest position) as arguments in addition to normal OGC WMS arguments.
A mapfile on Maury accept the TIME and TAIL arguments from the forwarded WMS request to render particle_tail (position at TIME - TAIL), particle_trajectory (intervening positions between TIME and TAIL), and particle_head (position at TIME).
Possible Improvements
Better pre-filtering of drifter positions. Drifter positions are established via ARGOS (not GPS) so the spatial resolution is very coarse. Limited spatial resolution impacts speed and direction calcs. To date in SECOORA, GDP drifter data is not visualized alongside other surface current layers because of this issue. Methods exist to address this issue, see:
Hansen, D.V. and P.M Poulain, 1996: Quality Control and Interpolation of WOCE/TOGA drifter data. J. Atm. Oc. Tech., 13, 900-909.
