D-16 
the result as the percent change, a .est file can be created that has a percentage 
change value for each cell. This .est file represents a 3D file of “Change Over Time”. 
The plot of this file provides a graphical representation of the change. The categories 
used to display the changes graphically are defined in the pc.mg file. The default 
pc.mg file provides categories of: >+10%, +5 to +10%, 0 to +5%, -5 to 0%,— 10 to 
-5 %, and >-10 % change. These categories should be modified to reflect the needs 
of the analysis. 
Missing values in the analysis can be treated in two ways: 1) included, meaning they 
are propagated through the analysis; or, 2) ignored. The default is to include missing 
values. The result of including missing values is that if one value for a specific cell 
is missing anytime in the times series, then that cell is set to missing. The single 
missing value forces the whole series of values at that cell to be missing and no 
percentage change is calculated. The percentage change value is set to missing (-9 
by default). 
If missing values are set to be ignored, then each missing value in a time series for 
a given cell is ignored and the rest of the time series observations are used to 
compute the percentage change over time. The potential problem with this approach 
is that the trend may be skewed by the lack of having all of the desired data. 
At least two points are required to compute a time series change. If the number of 
observations for any cell is less than 2, the resulting value for the percentage change 
is set to missing. 
MINIMUM-MAXIMUM ANALYSIS 
The “Min-Max” button can be selected to locate the minimum or maximum values 
in a series of interpolated values. For instance, this function could be used to read ten 
interpolated files, and find for Cell 1 the minimum value and write that minimum 
value to the output file Cell 1. This process would be repeated for each cell, so the 
resulting output file would contain the minimum value for each cell in the series. The 
Maximum function could be chosen if desired to find the maximum cell values in a 
series of files. These functions are useful for determining, for example, the lowest 
salinity over a 10-year period, or the highest temperature over a year period, for each 
cell in the interpolated files. (Figures D-18 and D-19). 
appendix d 
User Guide and Documentation for the Chesapeake Bay Interpolator 
