Wisconsin Department of Natural Resources

Satellite Derived Water Quality - Metadata

The water clarity datasets represent satellite-derived water clarity measurements for thousands of lakes across Wisconsin. These measurements are derived from remote sensing data from the NASA/USGS Landsat Program with the support of Citizen Lake Monitoring Network (CLMN) volunteers for the collection of on-the-ground data. CLMN volunteers collect Secchi depth data when the satellites are overhead to calibrate a model for the retrieval of water clarity data from satellite images. The calibrated model is used to produce summer water clarity maps and average summer water clarity values for lakes across Wisconsin. The maps provide information to compare the variability in water clarity at the lake and regional level. In addition, the average water clarity values calculated for each lake are used in the Wisconsin Consolidated Assessment and Listing Methodology (WisCALM) assessments to support the reporting requirements for the federal Clean Water Act. The water clarity datasets include annual Satellite Water Clarity layers and the Latest Average Lake Clarity layer.

Satellite Water Clarity

The state of Wisconsin is represented by four different paths of the Landsat reference system and the images for these paths are acquired 2, 7, and 9 days apart. This means the combined summer water clarity maps take four different dates to produce an annual summer water clarity composite for Wisconsin. Therefore, the Landsat Water Clarity layers represent the combined water clarity maps for four different dates mosaicked from Lake Michigan in the eastern part of the state to the Mississippi River in the western part of the state. There are some areas where two paths overlap and a single lake might include data from two different dates. The Landsat Image Date layers represent a list of these dates. The Landsat Water Clarity and Landsat Image Date layers have small gaps in the data where the paths overlap. This is, for example, seen for Lake Winneconne in Winnebago County, where the 2013 data for the eastern part of the lake is from 06/16/2013 and the data for the western part of the lake is from 09/27/2013. The Landsat Water Clarity layers are scaled from < 1 to > 16 ft to emphasize the variability in water clarity.

Latest Average Lake Clarity

The Latest Average Lake Clarity layer represents the average water clarity values calculated for each lake for the latest year with satellite derived water clarity data. These values are potentially different from the values in the annual summer water clarity composite if water clarity data was derived for multiple dates in the same year. The Latest Average Lake Clarity layer is scaled from < 1 to > 32 ft and individual water clarities are accessed through the Get Info tool.

For more information, please visit the remote sensing of water quality topic.