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Again we see the same general shaped curves as in 
Figure 2 with the ERTS image curve being about 5% higher than 
the laboratory curve. These shifts can be attributed to 
atmospheric effects. For the lakes sampled it is apparent 
that an excellent correlation exists between reflectance and 
turbidity, solids, and the lake's classification. 
Madison Area Lakes : During August of 1973, these 
techniques were applied to a series of lakes in the Madison, 
Wisconsin area. Water sampling was done almost simultaneously 
with the 5 August 1973 ERTS overpass. The lakes near Madison 
are generally warm water lakes, and in several lakes accelerated 
eutrophication is occuring due to large nutrient influxes 
from agricultural and urban storm runoff. 
Water samples were collected and Secchi disc readings 
taken to assure that the bottom reflection was not affecting 
the images. The water samples were analyzed for turbidity, 
solids and reflectance. ERTS image brightness values were 
determined for all of the lakes in question. 
This image analysis was accomplished for the overpass 
of 5 August 1973 and for an overpass a year earlier in 1972. 
Figure 4 represents two ERTS images of these lakes. Figure 5 
shows the resulting correlation curves. The fact that the 
curve for 5 August 1973 is higher than the curve for 28 August 
1972 merely indicates that there were more atmospheric effects 
on 5 August 1973. The greater scatter for the 1972 image data 
is because water sampling was not simultaneous with the 1972 ERTS 
overpass. 
Both the tannin lakes near Ely, Minnesota; and the 
non-tannin lakes near Madison, Wisconsin, appear to have good 
correlation between reflectance and algal-caused turbidity. 
It appears that it is not possible to classify a tannin lake 
with a curve for non-tannin lakes (See Figures 3 and 5). Like 
wise it is not possible to classify a non-tannin lake by use 
of a curve for tannin lakes. Further research is needed to 
identify tannin and non-tannin lakes on an ERTS image before 
they are classified as to eutrophic level. A method must also 
be devised to eliminate the effects of bottom reflection in 
shallow lakes and to identify those lakes where these effects 
are significant. Weed mats also cause image interpretation 
errors and some method must be developed for identifying or 
handling such situations. It is hoped that by computer wave 
length ratioing techniques all these types of lakes can be 
identified, and once the identification is made they can be 
appropriately classified as to eutrophic level.