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Twenty southeastern Wisconsin lakes were identified on individual frames 
for the following dates: 9 August 1972, 11 June 1973, 17 July 1973, and 
22 August 1972. All of these lakes had known fertility problems, and as 
would be expected the exposure in band 5 increased for almost all the lakes 
as the algal turbidity levels increased as the summer progressed (see 
Figure 9). Exposures were consistently significantly higher in August 1972 
than in August 1973. This increase in exposure could be attributed to a 
light atmospheric haze covering the frame. 
Densitometry of Wisconsin Lakes Greater Than 100 Acres 
Using ERTS Band 5 70mm Imagery 
Based on the preceeding studies, it was decided to densitize ERTS 
band 5 70mm lake imagery to develop a trophic status ranking of all lakes 
greater than 100 acres in the State of Wisconsin. This classification is 
based on the relation between band 5 exposure and turbidity caused mainly 
by phytoplanktonic algae. Energy detected in band 5 may come from as much 
as 5 feet below the lake surface, and submerged rooted aquatic macrophytes 
are probably registered by the satellite's sensors. This study, however, 
has not directly addressed itself to the relation between lake exposure 
and the extent of these macrophyte growths. 
One hundred acres was selected as the minimum lake size to be densi- 
tized based on the need for the microdensitometer measurement spot to be 
wholly within the lake. The 50 micron measurement spot used covers an 
area approximately 550 feet across on a 70mm image. A round 100 acre 
lake is 2300 feet across, which was felt to be the minimum lake area that 
could be found and measured with a reasonable degree of accuracy. 
Theoretically 17 ERTS images from one 6-day overpass period would 
provide complete coverage of the State of Wisconsin. However, because 
of cloud cover and missing imagery, this project used 26 images from 
four different 6-day overpass periods. The 5-day period from 3 through 
7 August 1973 provided the majority of the imagery used. 
Densitometer readings for each lake were punched on IBM cards for 
computer calculation and manipulation. In addition to densitometer 
readings, IBM cards were also punched with each lake's name and an 
arbitrary identification number, the lake's latitude and longotide, 
county location, secchi disc depths when available, maximum water depth, 
an arbitrary 0,1,2 ranking for atmospheric haze, and an arbitrary 0,1,2 
ranking for evaluating cases where the lake shape was such that difficulty 
was encountered insuring that the measurement spot of the densitometer was 
wholly within the lake. 
Computer programs developed for this project were used to calculate 
lake exposure, and to rank the 1000 lakes by exposure by county, DNR 
district, or the state as a whole. In addition, sorting routines will 
sort the lakes by depth, haze, or size for analysis purposes. The computer 
printouts presented with this report include: 1) a sort of all lakes by 
district with the lakes ranked in descending order of exposure; 2) a sort 
FIGURE 9 — BAND 5 EXPOSURE TIME SERIES F0R 4 SELECTED LAKES 
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