Figure 5 shows a plot of band 5 exposure versus secchi depth for tree 
tannin colored lakes. While exposure values for tannin lakes don’t 
correlate with secchi depths, at a given secchi depth they consistently have 
lower exposure values than non-colored lakes. One possible explanation for 
this phenomenon is that turbidity caused by the brown colored tannin dye 
dissolved in the water absorbs light while the particulate turbidity caused 
by phytoplankton increases lake reflectivity. 
Analysis of 11 Lakes Using Digital Gray Level Data 
From ERTS Computer Tapes 
Using computer programs developed for this study, digital brightness 
values for 13 lakes in south central Wisconsin were extracted from ERTS 
computer tapes. The technique involves the use of a Princeton Electronic 
Products (PEP) interactive graphics terminal to display a representation 
of the area of interest in band 7. The high contrast between the lake 
image and the land surrounding it in this band allows for easy and positive 
identification of the lake to be analyzed. Using an electronic "joy stick," 
a cursor is positioned on the lake surface and the scene brightness value 
from 0 to 63 in all four bands at that location is accessed and stored on 
a high speed disk for further manipulation and analysis. This procedure 
eliminates measurement errors due to densitometer spot size and positioning, 
and data degradation due to photographic processing. In addition, any lake 
larger than several pixels (200 feet across each) can be analyzed with a 
high degree of accuracy. Graphs of scene brightness versus secchi depths 
for bands 4 and 5 are shown in Figures 7 and 8 respectively. 
The relationships between scene brightness and secchi depth as found 
by this computer analysis are to be compared with Figures 2 and 3. The 
relationship found for band 5 seems to be comparable. The standard deviation 
is less, but significant scatter occurs. The scatter in the regression 
suggests that there are either measurement errors or that the conditions 
in the lakes changed between the time of sampling and ERTS passover. 
Measurement errors are almost non-existent when using the PEP terminal. 
This would suggest that the scatter is due to changing lake conditions. 
This past summer secchi depth and other ground truth measures were 
taken in a number of Wisconsin lakes on the same day as the ERTS overpass. 
A more reliable relationship between scene brightness and secchi depth 
should be available after data from this work has been analyzed. The 
similarity between densitometric-derived exposures and computer-derived 
exposures lends confidence to the densitometric measurement of all lakes 
greater than 100 acres in Wisconsin. When the new exposure versus secchi 
relationship is derived, secchi depth will be predicted for all lakes 
greater than 100 acres from the exposure values derived from the measure 
ments on the 70mm imagery. 
Time Series Analysis 
A time series analysis was performed to evaluate the variability of 
lake exposure as the algae growing season progresses through the summer.