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An evaluation of the Earth Resources Technology Satellite (ERTS-1) multispectral scanner as a tool for the determination of lacustrine trophic state Public Deposited

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  • This study evaluates the Earth Resources Technology Satellite- One (ERTS-1) multispectral scanner (MSS) as a means of predicting lacustrine trophic state and the magnitude of selected trophic state indicators. Numerical classificatory methods are employed to ascertain the trophic character of 100 lakes in Minnesota, Wisconsin, Michigan, and New York using the trophic indicators: chlorophyll a, conductivity, inverse of Secchi disc transparency, total phosphorus, total organic nitrogen, and an algal assay yield. A complete linkage clustering algorithm is first used to examine the lakes for natural clusters. The hyper-dimensional cloud of data points is then reduced in dimensionality through the ordination technique of principal components analysis. The first three principal components explain, respectively, 68 percent, 14 percent, and 8 percent of the variation in the data; the three-dimensional ordination is expressed in the form of a "ball and wire" model. The two complementary classificatory techniques show the existence of poorly defined clusters. A multivariate trophic state index (PC1) is derived from the principal component analysis. A binary masking technique is used to extract lake-related MSS data from computer-compatible digital magnetic tapes. Data products are in the form of descriptive statistics and photographic concatenations of lake images. Linear digital contrast stretching is demonstrated. MSS color ratio regression models are developed for the prediction of Secchi disc transparency and chlorophyll a in selected Minnesota and Wisconsin lakes. The models give good estimates of Secchi disk transparency and fair estimates of chlorophyll a levels. Lake area estimates derived from MSS pixel counts are generally within ten percent of area values obtained from topographic maps. The synoptic view of the sensor is conducive to lake enumeration work. The trophic state of lakes, as defined by lake position on the first principal component axis (PC1 value), is predicted using MSS color ratio regression models; each date of ER TS-1 coverage has its own model. There is a general tendency for the MSS ratios (GRNRED, GRNIR1, GRNIRZ, REDIR1, and REDIRZ) to decrease as the manifestations of eutrophication become more evident. A less rigorous approach to the study of MSS data-lacustrine trophic state relationships is undertaken using three-dimensional color ratio models. The mean IR1 intensity levels of several "hypereutrophic" lakes exceed their mean RED levels and effectively isolate them from other lakes in the models. An automatic image processing technique is employed to classify a group of Wisconsin lakes using MSS colors (GRN, RED, and IR1) in conjunction with the lakes' trophic state index values. The results are depicted in the form of both gray-scale and colorenhanced photographic concatenations. The ERTS-1 MSS has utility in the assessment of the lacustrine resource. Its usefulness is most apparent when the seasonal contrasts between lakes at different points on the trophic scale are at a maximum. Excessive cloud cover, faulty or missing MSS data, and the need for some ground truth impair, but do not preclude, its use in lake monitoring and classification. The use of computer-compatible tapes in conjunction with digital image processing techniques is essential if the maximum benefits are to be derived from the ER TS-1 MSS in lake-oriented studies.
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