C-5 
Eastern Bay, MilesAVye, Little Choptank, Chicamacomico, and Transquaking rivers. 
In Virginia, DataFlow data are available for the Piankatank, York, Pamunkey, and 
Mattaponi rivers. Chapter 7 discusses additional details on plans for monitoring 
shallow-water systems. 
Other alternative monitoring programs have been considered, but not fully imple¬ 
mented for criteria assessment. Beginning in 1990, chlorophyll a concentrations 
have been measured over the mainstem Chesapeake using aircraft remote sensing 
(Harding et al. 1992). Twenty-five to 30 flights per year took place during the most 
productive time periods. In addition, satellite remote sensing data have been consid¬ 
ered for evaluating chlorophyll a concentrations in the Bay (Harding et al. 2004) 
although no detailed evaluation of the feasibility has been completed. Water quality 
sensors and data loggers mounted on buoys have also been evaluated as the best 
means to assess high-frequency dissolved oxygen criteria. This option is expensive, 
however, and only a limited (but growing) number of buoy systems have been 
deployed to date (http://www.cbos.org). 
INTERPOLATION METHODS CURRENTLY 
USED FOR CHESAPEAKE BAY WATER QUALITY 
CRITERIA ASSESSMENT 
The current Chesapeake Bay Interpolator is a grid-based algorithm in which criteria 
measurement data are used to estimate values for all grid cells (see Appendix D for 
a detailed description). Estimates for cell locations are computed by interpolating the 
nearest “n” neighboring water quality measurements for which “n” is normally 4 but 
is adjustable. The interpolation uses an inverse distance weighted (IDW) algorithm 
in which the estimated value of each grid cell is based on the four nearest measure¬ 
ments. Each of the neighboring points is weighted by the inverse of the distance 
squared (i.e., 1 d‘ : ), however, so close neighbors have more influence than those 
farther away. 
The cell size in the Chesapeake Bay interpolation grid is 1 km (east-west) x 1 km 
(north-south) x 1 m (vertical), with columns of cells extending from the water 
surface to the Bay bottom representing the three-dimensional volume as a group of 
equal-sized cells. Each tidal tributary is represented by variously sized cells 
depending on the river’s geometry since the narrow upstream portions require 
smaller cells to model the dimensions accurately. Interpolator grid cells, however, 
remain the same size within individual segments. This designation results in a total 
of 51,839 cells by depth for the mainstem Chesapeake Bay (segments 
CB1TF-CB8PH), and a total of 238,669 cells by depth for all 78 segments making 
up the mainstem Chesapeake Bay and its tidal tributaries and embayments. 
The Chesapeake Bay interpolator is optimized to compute concentration values that 
closely reflect the physics of stratified water bodies such as the Bay. Water quality 
varies much more markedly vertically as opposed to horizontally. To accommodate 
this attribute, each column of data is interpolated vertically to the same depths as the 
centroids of the interpolator cells, (i.e. 0.5, 1.5, 2.5 meters, etc). The interpolator then 
interpolates only in the horizontal dimension. 
appendix c 
Evaluation of Options for Spatial Interpolation 
