Bathymetry of Lake Michigan

Bathymetry of Lake Michigan has been compiled as a component of a NOAA project to rescue Great Lakes lake floor geological and geophysical data and make it more accessible to the public. The project is a cooperative effort between investigators at the NOAA National Geophysical Data Center's Marine Geology & Geophysics Division (NGDC/MGG) and the NOAA Great Lakes Environmental Research Laboratory (GLERL). was compiled utilizing the entire historic sounding data base. This bathymetry resolves physiography of the lake floor to an extent that known features are revealed more accurately and features never before seen are revealed for the first time. Bathymetry has been compiled using the entire array of good-quality historical hydrographic soundings collected in support of nautical charting over a 120-year period by the NOAA National Ocean Service and its predecessor agency for Great Lakes surveying, the Army Corps of Engineers. More than 600,000 bathymetric soundings were employed, of which approximately 60 per cent were already in digital form, 25 per cent were digitized in conjunction with this effort, and the remaining 15 per cent were available only on paper survey sheets. Bathymetry was compiled at a scale of 1:250,000, with a contour interval of 5 meters. Density of tracklines is generally about 2000m for the open lake and ranges from 200m to 600m for nearshore areas. Soundings collected since 1903 were already reduced to the Lake Michigan mean low water datum; these were used for bathymetric contouring without further calibration or adjustments. Soundings collected prior to 1903 were reduced to the mean low water datum. In preparation for bathymetric contouring, digital soundings were converted to metric units and plotted in color; separate colors were assigned to the various depth ranges. From the paper sheets, contours in metric units were generated directly on overlays; these contours were then reduced to the compilation scale of 1:250,000 and incorporated into the map. Compilation sheets were then scanned and vectorized; and the resulting digital vector bathymetric contour data were used to generate the imagery shown on the large color plate. Images were constructed using the publicly-available software Generic Mapping Tools (GMT).