D-2 
INTERPOLATOR DESCRIPTION 
The Chesapeake Bay Interpolator is a cell-based interpolator. Fixed cell locations are 
computed by interpolating the nearest n neighboring water quality measurements, 
where n is normally 4, but this number is adjustable. Cell size in Chesapeake Bay 
was chosen to be 1km (east-west) x 1km (north-south) x lm (vertical), with columns 
of cells extending from surface to the bottom of the water column, thus representing 
the 3-dimensional volume as a group of equal sized cells extending throughout the 
volume. The tributaries are represented by various sized cells depending on the 
geometry of the tributary, since the narrow upstream portions of the rivers require 
smaller cells to accurately model the river’s dimensions. This configuration results 
in a total of 51,839 cells by depth for the Main Bay (Segments CB1TF-CB8PH), and 
a total of 238,669 cells by depth for all 77 segments which comprise the Main Bay 
and tributaries. Computation time on a Pentium 2 ghz PC running Windows XP is 
approximately 15 seconds for the Bay and tributary interpolator model. 
The Chesapeake Bay Interpolator is unique in the way it computes values in 3- 
dimensions. The interpolator code is optimized to compute concentration values that 
closely reflect the physics of stratified water bodies, such as Chesapeake Bay. The 
Bay is very shallow compared to its width or length, hence water quality varies much 
more vertically than horizontally. The Chesapeake Bay Interpolator uses a vertical 
filter to select the vertical range of data that are used in each calculation. For 
instance, to compute a model cell value at 5m deep, monitoring data at 5m deep 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 cl (normally +/-2m) meters above and 
below the preferred depth, with the window being widened in 0.5m increments until 
n monitoring values have been found for the computation. The smallest acceptable n 
value is selectable by the user. If fewer than n values are located, a missing value 
(normally a -9) is calculated for that cell. 
A second search radius filter is implemented to limit the horizontal distance of moni¬ 
toring data from the cell being computed. Data points outside the radius selected by 
the user (normally 25,000m) are excluded from calculation. This filter is included so 
that only data that are near the location being interpolated are used. 
In this version of the Interpolator, Segment and Region filters have been added. 
Segments are geographic limits for the interpolator model. For instance, the Main 
Bay is composed of 8 segments (CB1TF, CB20H, ...,CB8PH). The tributaries are 
composed of 69 additional segments, using the CBP 1998 segmentation scheme 
(Figure D-l). These segments divide the Bay into geographic areas that have some¬ 
what homogeneous environmental conditions. This segmentation also provides a 
means for reporting results on a segment basis that can show more localized changes 
compared to the whole Bay ecosystem. To replicate the segmentation scheme, the 
segment boundaries were used to cookie-cutter out the Interpolator cells that fall 
within each segment. Each set of these cells are then identified inside the corre¬ 
sponding *.bth file that contains the bathymetry definitions. To compute the 
interpolated values for the Main Bay, the corresponding bathymetry file is named 
“cbay8.bth”. This file contains the cell locations for the cells in the Main Bay Inter¬ 
polator. A similar file, ‘ > bay_trib.bth” contains the cell definitions for the Main Bay 
appendix d 
User Guide and Documentation for the Chesapeake Bay Interpolator 
