CONCLUSION 
If other effects such as skylight reflection, 
atmospheric scatter, and lake bottom effects remain constant, 
then it is indeed possible to ascertain the turbidity of 
water on aerial photos and ERTS images. This can be done 
with a few simultaneously taken water samples and correct 
analysis of the imagery. When the turbidity is caused by 
algal material which is more concentrated in enriched lakes, 
then it is possible to correlate the image brightness to the 
enriched or eutrophic condition of the lake. This technique 
has proven successful on both a series of tannin lakes near 
Ely, Minnesota and a series of non-tannin lakes near Madison, 
Wisconsin. From the Madison area study and studies on Lake 
Superior (Figure 5 and also Figure 2) it is clear that the 
brightness of a lake on one ERTS image cannot be compared to 
the brightness of an ERTS image taken on a different day. 
Various atmospheric conditions cause different correlation 
curves and the position of this correlation curve must be 
established for each image. 
In all of the lakes tested the Secchi disc readings 
were all less than the depth to bottom so the bottom was not 
showing on such images. There are lakes where the bottom will 
show through and effect the brightness of the lake on the 
image. A system is being developed to handle such situations. 
Also lakes where larger weed mats are present must be investi 
gated. 
When a system is finalized which will handle all of 
these types of lakes it appears that aerial photos and ERTS 
imagery will indeed be a useful tool for lake eutrophication 
classification. 
ACKNOWLEDGEMENTS 
The authors gratefully acknowledge the National 
Aeronautics and Space Administration for their support under 
Grant No. NGL 50-002-127; Dr. James L.. Clapp and his staff at 
the University of Wisconsin for their administrative support, 
and the U.S. Forest Service Personnel at Ely, Minnesota, for 
their help in sample gathering and analysis.