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Upper Midwest Environmental Sciences Center


Frequently Asked Questions

What does bathymetry mean?
Bathymetry is the measurement of water depth at various places in a body of water. In addition, the term also refers to the information derived from such measurements.
Why is bathymetric research important for the Upper Mississippi River System?
Bathymetric data is an essential tool scientists use to monitor the ecology of the river, and to assess the possible impact of hydrodynamic changes upon the riverine ecology. Bathymetry was identified as an important data need during the development of the Long Term Resource Management program and continues to be ranked high in data needs assessments among river managers.
How are bathymetric maps generated?
Details of the methodology are included in this web site.
the bathymetry boat prepares to launch At left is the boat currently used to survey the river and capture the bathymetric data. The inset outlined in yellow is the Global Positioning System receiver. Its locoation on the boat can be seen on top of the craft's cabin.
In general, water depth measurements are collected using an echo sounder. At the same time, geographical positions are collected with a Global Positioning System. These data are collected as computer files onboard the survey vessel and brought back to the office. "Maps" are generated through interpolation at geographical locations between the locations of measured depths.
echo sounder equipment
The echo sounder is on the right side of the above photo; the transducer is located underneath the hull of the craft. The data generated by the device is digitized and captured by a laptop computer visible to the left of the sounder. This equipment is housed inside the cabin of the boat.

How was bathymetric data generated prior to these current technologies?
Although echo sounders for rapid collection of depths have been around for a long time, the integration with automated positioning systems in a computer hardware/software setup has only recently become commonly used. Prior to echo sounders, water depths were measured and recorded using a "sounding pole". Prior to Global Positioning Systems, multiple range distance measuring devices or traditional survey traverses were used to determine geographical location. The generation of maps through computer interpolation between data points has nearly replaced "human interpolation" accomplished through plotting point data and drawing contours.
What different types of maps can be generated?
In the past, the most common type of map was a contour map, with the contour lines representing selected water depths located across the map. As computer use of these data evolved, the "line" data become less important. "Areas" of selected depth ranges were needed to assess the structure of a body of water, particularly when using a GIS. These types of maps are often displayed as color-coded maps. A very detailed map could be created with small depth range intervals. However, the 2-dimensional maps using colors to display depth are not a natural-looking representation of bathymetry. To get a more eye-pleasing map, 3-dimensional maps can be created.
contour map 2-D color-coded map 3-D color-coded map
This contour map exhibits a portion of Mississippi River Pool 13. The contours on the bottom half of the map reflect the prominent wingdam features of this portion of the pool. This 2-D color-coded map shows the same features within Pool 13 as the previous map. The bright spots on the bottom are areas scoured by the currents generated by wingdams. This 3-D color-coded map again reflects the same features of Pool 13 as the previous maps - this perspective provides a view looking down the channel of the river.
These same features of Pool 13 are transformed from initial survey data into a bathymetric map in the Methods - Interpolation section of this web site.
How accurate are the maps?
The water depth collected in the field has an accuracy of about 0.15 meters. However, because the maps are generated by interpolation between data points, the estimated water depths in most locations of the map are much less accurate. In some cases, the water depths between transects used for data collection may be much different than along the transects. For this reason, and for the reasons discussed below, these maps should not be used for all navigational purposes.
Is bathymetric research important for commercial and recreational navigation?
The information displayed by the LTRMP bathymetric maps is a general representation of the bathymetry at one point in time. More accurate maps are required to determine if the main channel is suitable for barge passage, and these specific data are collected regularly by the U.S. Army Corps of Engineers at selected navigation channel locations. The LTRMP maps are useful to recreational users for general navigation, but due to frequent changes in the river bottom, water levels, and location of submerged snags, the maps should not solely be relied on for safe navigation.

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