In the 1840s, the Lake Survey (now merged into the National Ocean Service) began collecting data on the bottom topography of the Great Lakes. During the next century and a half, the density of the soundings became increasingly great, revealing with ever-increasing detail the surface of the Earth under the water.
Maps based upon these data were published primarily as navigation aids. They were well suited to helping ship captains avoid hazards, but they did not clearly display the morphology of lake bottoms.
This chart, published in 1981, was an attempt to synthesize available data and present them in an easily perceived format. Depths are shown with both isobaths and bathymetric tints — the deeper the water, the darker the blue. The map also includes other graphs and data of interest to scientists.