A second search radius filter is used to limit the horizontal distance of monitoring data 
from the cell being computed. Data points outside the radius selected by the user 
(normally 25,000 meters) are excluded from calculation. This filter is included so that 
only data near a specific location are used for interpolation. In the current version of 
the CBP interpolator, segment and region filters have been added (Bahner 2001). 
The Chesapeake Bay Program segments are geographic limits for interpolation. For 
instance, the mainstem Chesapeake Bay is composed of eight segments (see Figure 
VI-1 and Table VI-4). The tidal tributaries are composed of 70 additional segments, 
using the Chesapeake Bay Program 1998 segmentation scheme (CBP 1999). Each 
segment represents a geographic area that has somewhat homogeneous environ¬ 
mental conditions. Segmentation enables users to report findings on a 
segment-by-segment basis, which can reveal localized changes compared to the 
entire Chesapeake Bay ecosystem. 
As stated above, the CBP interpolator uses monitoring data to fill in the three-dimen¬ 
sional space of the Chesapeake Bay. The CBP interpolator assumes a linear 
distribution of the data between points. Given the dynamic nature of estuaries, this 
is obviously a conservative assumption. However, the spatial limitations of the data 
make the simplest approach the most prudent. The strength of the CBP interpolator’s 
output is directly related to the quality and spatial resolution of the input data. As 
sample size increases, interpolation error decreases. For more detailed documenta¬ 
tion on the Chesapeake Bay Program interpolator and access to a downloadable 
version, refer to the Chesapeake Bay Program web site at http://www. 
chesapeakebay.net/tools.htm. 
STEP 2. COMPARISON OF INTERPOLATED WATER QUALITY 
MONITORING DATA TO THE APPROPRIATE CRITERION VALUE 
To quantify the spatial extent of criteria exceedance, the interpolated water quality 
monitoring data must be compared to the appropriate criteria value. In all cases, the 
water quality criteria are defined within specific spatial limits and with varying spatial 
values. In order to define the spatial extent of criteria exceedance, the appropriate 
criteria values must be aligned with the water quality measures throughout the spatial 
assessment unit. Accordingly, the spatial definition of each criterion is superimposed 
on the interpolator grid structure to assign a criteria value to each cell. Criteria 
assessments can then be made on a cell-by-cell basis using the water quality estimate 
from the interpolator and the criteria value defined for each cell. Figure VI-3 illus¬ 
trates a schematic of the process for spatially defined criteria assessment. Chlorophyll 
a estimates generated from the interpolator (such as that for Tangier Sound, Figure 
VI-2) are combined with the grid-based definition of criteria values. The integration 
of those two layers allows the comparison of ‘measured’ chlorophyll a to the appli¬ 
cable criteria value in each cell to determine if that cell exceeds the criterion for the 
time period for which data were collected (Figure VI-3). 
chapter vi • Recommended Implementation Procedures 
