259 
THE USE OF ERTS IMAGERY FOR LAKE CLASSIFICATION 
F. L. Scarpace, R. E. Wade and L. T. Fisher 
University of Wisconsin 
Madison, Wisconsin U.S.A 
Data 
4. 
ABSTRACT 
c 
ugust. 
The feasibility of using photographic representations of the ERTS 
imagery to classify lakes in the State of Wisconsin as to their trophic 
level was studied. Densitometric readings in band 5 of ERTS 70mm imagery 
were taken for all the lakes in Wisconsin greater than 100 acres (approx 
imately 1000 lakes). An algorithm has been developed from ground truth 
measurements to predict from satellite imagery an indicator of trophic 
light 
status. 
Quality 
. ERTS 
INTRODUCTION 
The Wisconsin Department of Natural Resources (DNR) is required to 
classify the lakes in the state as to their trophic level in response to 
the federal legislation "Federal Water Pollution Control Act Amendments 
Dm 
' Pro- 
; Sensing 
Ontario, 
of 1972," section 314. This project represents an attempt to evaluate 
the feasibility of using photographic imagery from the ERTS (Earth 
Resources Technology Satellite) to accomplish this classification. The 
ERTS satellite passes over the same location on the ground every 18 days. 
Each ERTS image covers a rectangle on the ground 115 miles by 115 miles. 
The satellite's sensor systems (multispectral scanner) gather data in 
four different wavelength bands simultaneously: Band 4 (.5-.6y); Band 5 
(,6-.7y); Band 6 (.7-.8y); and Band 7 (.8-l.ly). 
Die Tool 
*gs. 
World, 
Densitometric readings in band 5 of ERTS 70mm imagery were taken for 
all lakes in Wisconsin greater than 100 acres (approximately 1000 lakes). 
For 37 of these lakes, DNR water quality ground truth data was correlated 
with density readings in all four ERTS bands. The lakes in the remainder 
of the state were classified as to the level of eutrophication by an 
algorithm developed by a statistical analysis of this correlation. 
METHODOLOGY 
This project involved four separate experiments: 1) Densitometric 
analysis of 37 lakes in each of the 4 ERTS bands using 70mm positive 
transparencies; this data was then correlated with secchi depth readings 
taken by the Wisconsin Department of Natural Resources; 2) Using specially 
developed computer programs and an interactive CRT terminal, ERTS digital 
tapes were accessed and the actual 64 scene brightness values sent back 
by the satellite were obtained for 14 of the above lakes; 3) A time series 
densitometric analysis of 20 lakes in southeastern Wisconsin on four 
different ERTS overflight dates; and 4) Densitometric analysis of approx 
imately 1000 lakes in Wisconsin greater than 100 acres on band 5 of ERTS 
70mm imagery.