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Figure D-1 5. Math screen with files chosen to "Recode Parameter 
Value" of Dissolved Oxygen by recoding values from 0-3 to the new 
value of "1", recoding values of above 3 to the new value of "0" and 
retaining missing values as -9, to create the new output file. Choosing 
a Range File is provided to load an existing set of ranges, which can 
then be modified on this screen for specialized analyses. 
volume of these categories can also be computed to show critical ranges for habitat 
analysis. 
Missing values are handled in a special way. Since missing values have no real value, 
they are not used in math operations. If a cell in either “Input File 1’’ or “Input File 
2” are flagged as missing (normally -9), then no math is done, and the “Output File” 
value for that cell is set to missing (The “Missing Value” is set on the Interpolate 
screen). 
TRENDS ANALYSIS 
Interpolated values can be analyzed for trends. The “Change Over Time” button 
allows the user to create a 3-dimensional (.est) file with linear percentage changes 
over time for each cell in the bathymetry (Figures D-16 and D-17). 
As a simple example, a station may be sampled several times over a period of time. 
The measured values can be plotted with time on the x-axis and value on the y-axis. 
The resulting linear regression line can be plotted through these points and the slope 
and intercept can be used to compute the percentage increase or decrease between 
the beginning and end ot the time series. 
This same technique can be used with the interpolator. Each cell value from a series 
of .est files can be used to compute a linear regression, so that each cell has its own 
regression and resulting percentage change, either up or down, over time. By coding 
appendix d 
User Guido and Documentation for the Chesapeake Bay Interpolator 
