The Georges Bank Broad-Scale Survey Synthesis
Grid Group Report


The Grid Group met on Friday, December 13, 2002, in Room 204 Redfield at WHOI.  Attending were Jeff Runge, Dennis McGillicuddy, Peter Wiebe, Dezhang Chu, and David Mountain.  The primary charge to the group is to recommend a consistent procedure for gridding the various data sets in the Broad Scale component of the program.  This requires deciding upon 1) the area (or box) to be gridded, 2) the distribution of the grid points within the box, 3) the interpolation scheme to estimate values at the grid points and 4) the data sets to be used in the gridding exercise.

The Box

The strawman box suggested at the October workshop was felt, at that time, to extend too far beyond the actual Broad Scale station locations.  David Mountain was charged with recommending a smaller box and did so for the Grid Group meeting.  The discussion reflected two desires.  The first to keep the box limits near the data points so that any plots or calculations would represent areas of meaningful data.  The second was to include within the box areas that might be of interest to the modelers, such as likely nearby source or fate regions for variables measured on the Bank  (since the data on the Bank provide information concerning of the conditions in those upstream or downstream areas at a different time).  In the second case areas where little or no information actually exists could be eliminated (or not displayed) on the basis of the error estimate from the interpolation scheme.  After much discussion, the first approach won consensus.  The smaller box submitted by David Mountain was felt to be acceptable for the Broad Scale needs.  While some modest changes were considered, in the end, the feeling was to use the box as submitted.   In addition, it was decided that a wider area grid that included all of Georges Bank and the Gulf of Maine, the MARMAP2 grid, would be appropriate for the modeling interests and that copies of all of the data sets use to make gridded products should be retained for later use in any modeling investigations.  The gridding of selected data sets to the MARMAP2 area would be a simple task, that could be done when needed.

The Grid Points

At the October workshop the value of using the finite element grid points or a regularly space grid pattern was discussed.  That discussion continued in the Grid Group meeting.  A consensus was reached that the regular spaced grid was preferable.  The finite element grid spreads points further apart where the bottom is flat, on the basis of the physics.  However using a regularly spaced grid was felt to offer a more consistent representation of the biological fields, particularly when the gridding/interpolation is done on log transformed data.  The grid spacing for the initial testing by the Group will be 0.05 degrees in latitude and longitude. The format for the grid will be: grid point #, latitude, longitude, area represented by point.  This grid will be referred to as the Georges Bank Broad-Scale Standard Grid.

The Interpolation Method

Two primary methods of interpolation are to be tested B krigging and objective analysis.  The krigging will be done using easykrig developed by D. Chu.  The objective analysis will be done using OAX developed by C. Hannah.  Some estimate of the spatial statistics for the variable being interpolated is needed B in the form of a variogram for easykrig and a correlation length scale ellipse for OAX.  The standard OAX package includes the correlation values appropriate for the physics within the grid node file.  To develop the needed variogram(s) for each parameter, the associated investigator will meet with D. Chu to use easykrig for determining the appropriate variogram.



The list of variables to use in the grid testing includes

A physical variable - surface salinity (D. Mountain)

A phytoplankton variable - chlorophyll (D. Townsend)

A plankton variable- biovolume (P. Wiebe through J. Green)

A zooplankton variable - Calanus egg production rate (J. Runge)

A fish variable - larval cod (D. Mountain)

A nutrient variable - to be determined (from D. Townsend)

An along track variable - to be determined (P. Wiebe)


A number of test sets (i.e., cruises) for each of these variables will be created in a format:  latitude, longitude, value.  For each data set and for each variable, the output of the interpolation will be a file containing:  gird point #, latitude, longitude, value, value*area for the grid point, error.


Once the test data sets are created and the appropriate variograms are developed, the Grid Group will meet again to decide how (who, when, Y) the gridding will be done and how the results will be reviewed and evaluated.  The data sets both original and mapped will be stored on the GLOBEC database.

The Grid data

The Grid picture