36 
neighbors in the spatial unit being considered. Further details regarding the basis 
of spatial interpolation of water quality monitoring data within the Chesapeake 
Bay Program segmentation framework are described in Ambient Water Quality 
Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesa¬ 
peake Bay and Its Tidal Tributaries (Regional Criteria Guidance), pp. 154-157 
(U.S. EPA 2003). 
Recent updates to the interpolator software have been made and the present 
program is a Visual Basic program, version 4.61 August 2006, with customized 
data region and bathymetry files. 
STEP 2.1. Vertical Interpolation 
Specific to Chesapeake Bay, in areas > 12 meters, where deep-water and deep- 
channel designated uses occur, the program uses vertical depth profiles of the 
water temperature and salinity data for each Chesapeake Bay Program water 
quality monitoring station to calculate the upper and lower boundaries of the 
pycnocline. 
The program assigns the data from a particular monitoring cruise number by its 
date and divides the coincidently measured, georeferenced data into separate files 
for dissolved oxygen, salinity, and pycnocline. The result is a set of files for each 
parameter that comprise a set of files for each cruise. 
The Chesapeake Bay Program interpolator’s vertical interpolation function (On 
the Data Import screen), is run in batch mode to vertically interpolate each data 
file. The program is used with default settings beginning with a 0.5 meter START 
DEPTH and applying a 1.0 vertical meter STEP DEPTH. 
STEP 2.2. Horizontal Interpolation 
After vertical interpolation, interpolated data is available at scales below the more 
than 1.0 meter depth-steps from the water quality data collection. To generate a 
horizontal interpolation of the vertically interpolated data set, the program uses 
the Interpolate screen. Data files are again processed in batch mode presently 
using the following settings: 
3D inverse-distance squared model 
Min # Neighbors = 1 
Max # Neighbors = 4 
Horizontal Range (max) = 99000 m (essentially only limited by each 
segment’s data region) 
Vertical Range (min) = 0.1 m 
Vertical Range (max) = 0.1 m 
Vertical step size = 0.1 m 
Missing value = -9 
A file for each water quality parameter-cruise combination (parameters of 
dissolved oxygen, temperature and salinity measured coincidently in space and 
appendix a 
Procedure for Assessing Dissolved Oxygen Criteria Attainment 
