270
of all lakes by district, with the lakes ranked in order of descending
exposure by county within each district; and 3) a sort and ranking by
DNR district of all lakes greater than 20 feet in depth, with no haze
or clouds obscuring the imagery, and whose size and shape is such that
the microdensitometer measurement spot is wholly within the lake. i'i- 11
nive
A rough cost estimate of labor and computing costs for future densi- 1'in
tometry of lakes greater than 100 acres in band 5 comes to about $3.00 acre
per lake analyzed. Based on the cost of analyzing the 13 lakes from the 1111 *
ERTS digital tapes, it is hoped that any lake greater than 25 acres can i- nc ^
be analyzed in all four ERTS bands for a comparable cost per lake.
CONCLUSIONS
There seems to be a relationship between the exposure value for a
lake as measured by ERTS band 5 and secchi disc depth. The exact rela
tionship between exposure and secchi depth is still to some extent in
doubt. A better relationship should result from an analysis of the data
from last summer.
If a photographic product is to be analyzed, the 70mm positive
transparencies are the best approximation to the scene brightness as
measured by ERTS. The most reliable data from ERTS can be derived from
the ERTS digital tapes. For lakes less than 100 acres, assessment of
lake conditions should be made from the digital tapes.
The costs of analysis for lakes in Wisconsin from ERTS are less than
$3.00 per lake. Costs for analysis from the digital tapes should be
comparable or less than the analysis of the photographic product.
ACKNOWLEDGEMENTS
This work has been supported by the Wisconsin Department of Natural
Resources and NASA Grant NGL #50-002-127, Office of University Affairs.
ZUSAMMENFASSUNG
Es wurde eine Durchführbarkeitsuntersuchung des Einsatzes photographischer
Wiedergaben der ERTS-Aufnahmen bei der Klassifizierung von Seen im ameri
kanischen Bundesstaat Wisconsin nach ihrer trophischen Einstufung durchge
führt. Im Band 5 der 70-mm-ERTS-Aufnahmen wurden densitometrische Messungen
für alle Seen in Wisconsin von mehr als 100 acres (40 ha) ausgeführt, also
für insgesamt etwa 1000 Seen. Nach Feldvergleich wurde ein Algorithmus für
die Vorhersage des trophischen Zustandes aus Satellitenaufnahmen ausgearbei
tet.
