Mapping the Depths: Michigan History Recently Spoke with Bill Virden, an Associate Scientist Who Works for the National Oceanic and Atmospheric Administration (NOAA) through the Cooperative Institute for Research in Environmental Sciences
How long has NOAA been compiling the bathymetric maps of the Great Lakes?
NOAA and the Canadian Hydrographic Service (CHS) have been working collaboratively since the early 1990s to produce the bathymetric maps of the Great Lakes. To date, we have published maps for Lake Michigan (1996), Lake Erie (1998) and Lake Ontario (1999). The map for Lake Huron will be published later this year. At present, we do not have a timetable for when Lake Superior will be complete. NOAA and its predecessor, the Coast and Geodetic Survey, have been collecting Great Lakes bathymetric survey data in support of nautical charting activities since the nineteenth century. We have data that dates back to the mid- to late-nineteenth century. In some cases, this is the only data that is available for certain areas.
Who uses the maps? Why is it important for them to know the depths of the water?
There are users for these maps such as Great Lakes researchers, state and local planners and anglers to name a few. Bathymetry is an essential layer of information for many studies in the lakes. Circulation, fish habitats, geologic history, our understanding of past climate changes are all examples of how this information is used. It should be noted that these maps are NOT to be used for navigational purposes; the National Ocean Service publishes nautical charts for this purpose.
Can you explain the current survey methods? How is the data collected?
Most of the data were collected using sonar technology. Sonar (SOund Navigation And Ranging instruments) transmits a sound wave beneath the ship directed towards the lake floor. The same instrument listens for the return of that sound wave and measures the travel time of that wave from the ship to the lake floor and back. The depth of the water is then calculated using the speed of sound in water. Sonar instruments have evolved from those that send out a single beam or sound wave, to those that send out thousands of beams beneath the ship to map a swath beneath the ship. This newer type of bathymetry is called multibeam bathymetry and it is often collected in a mowing-the-lawn-like pattern that provides a photographic-like representation of the lake floor. Recently light technology has been used aboard aircraft to penetrate the water layer down to the lake floor and reflected back. This is referred to as LIDAR--LIght Detection And Ranging. Since light is used in this method, it is necessary that the water column is clear and not too deep. In a calculation similar to the one used with sonar the depth of the ocean can be determined with resolutions as high as centimeters. …