around Tangier Sound and the adjacent portion of the Eastern Shore of Maryland and 
Virginia. Using spatial interpolation, chlorophyll a concentrations were estimated 
for all locations in the Tangier Sound area. Based on those estimates, the spatial 
distribution of chlorophyll a is illustrated by shading the area according to the esti¬ 
mated concentration (darker shading represents higher chlorophyll a 
concentrations). The results illustrate the spatial gradients that tend to occur 
throughout an area of this size. Those gradients need to be accounted for in order to 
accurately assess the extent of criteria exceedance. 
The Chesapeake Bay Program spatial-interpolation software (or ‘CBP interpolator’) 
computes water quality concentrations throughout the Chesapeake Bay and its tidal 
tributaries from measurements collected at point locations or along cruise tracks 
(Bahner 2001). It estimates water quality concentrations at all locations in a two- 
dimensional area or in a three-dimensional volume. The CBP interpolator is 
cell-based. Fixed cell locations are computed by interpolating the nearest number ( n ) 
of neighboring water quality measurements, where n is normally 4, but is adjustable. 
Typically an interpolation is performed for the entire Chesapeake Bay for a single 
monitoring event (e.g., a monthly cruise). In this way all monitoring stations are used 
to develop a bay wide picture of the spatial variation of the parameter being consid¬ 
ered. Segment and designated use boundaries can then be superimposed over the 
baywide interpolation to assess the spatial variation of the parameter in any one 
segment’s designated use(s). 
Cell size in the Chesapeake Bay was chosen to be 1 kilometer (east-west) by 1 kilo¬ 
meter (north-south) by 1 vertical meter, with columns of cells extending from the 
surface to the bottom of the water column, thus representing the three-dimensional 
volume as a group of equal-sized cells. The tidal tributaries are represented by 
various cell sizes, depending on the geometry of the tributary, since the narrow 
upstream portions of the tidal rivers require smaller cells to represent the river’s 
dimensions accurately. This configuration results in a total of 51,839 cells for the 
mainstem Chesapeake Bay and a total of 238,669 cells for the Chesapeake Bay and 
its tidal tributaries. 
The CBP interpolator is tailored for use in the Chesapeake Bay in that the code is 
optimized to compute concentration values that closely reflect the physics of strati¬ 
fication. The Chesapeake Bay is very shallow despite its width and length; hence 
water quality varies much more vertically than horizontally. The CBP interpolator 
uses a vertical filter to select the vertical range of data for each calculation. For 
instance, to compute a model cell value at 5-meters deep, monitoring data at 5 meters 
are preferred. If fewer than n (4) monitoring data values are found at the preferred 
depth, the depth window is widened to search up to d (normally ±2m) meters above 
and below the preferred depth, with the window being widened in 0.5-meter incre¬ 
ments until n monitoring values have been found for the computation. The user is 
able to select the smallest n value that is acceptable. If fewer than n values are 
located, a missing value (normally a -9) is calculated for that cell. 
chapter vi 
Recommended Implementation Procedures 
