Lake mapping in Minnesota
What is lake mapping?
Lake mapping is a way to learn what a lake is like under the water's surface. Conducted by the DNR's Division of Ecological Resources Lake Mapping Unit, lake mapping provides a visual representation of the shape and depth of Minnesota lakes. It also provides important information about the lake ecosystem.
Fisheries and wildlife managers require information provided by lake maps to effectively manage the state's natural resources. Anglers, hunters, and other outdoor enthusiasts rely on lake maps to fully enjoy and pursue some of their favorite outdoor activities.
Fisheries managers use lake maps to determine survey locations.
Who uses lake maps?
- Fisheries managers to help determine fish stocking quotas, provide an estimate of lake volume for chemical rehabilitation projects and vegetation control, and to calculate potential yield of fish lakes;
- Wildlife managers to determine the feasibility of lake outlet structures for managing water levels;
- Anglers to find sunken points, drop-offs, mud flats, and other structures within the lake;
- Hunters to find points and passes for waterfowl hunting;
- the U.S. Geological Survey to incorporate lake contours into its quadrangle maps;
- Publishers of outdoor newspapers and atlases to inform their readers about hunting and fishing opportunities; and
- DNR lake ecologists to document lake depths and contours to determine the rate of sedimentation in the basin.
Lake mapping method prior to 1950.
When did lake mapping begin?
Minnesota's lake mapping effort began in the 1930s. At that time, most mapping was done by dropping down a weighted line to determine the lake depth. In the early 1950s, echo sounders were first used for mapping. Echo sounders emit a signal and then time how long it takes for the signal to reach the bottom and return to the boat, resulting in a depth reading.
How are lakes mapped today?
Since the 1950s, lake mapping has evolved to include more precise echo sounders, on board computers, and GPS systems. The two methods that are currently used for lake sounding are line of sight and GPS. The method used on each lake is determined by the lake's size. For smaller lakes, the line of sight method is used. For this method, the boat is driven along a path called a transect between visually selected landmarks on shore. Transect location and direction is recorded on the hardshell, which is a drawing of the lake outline and surrounding features. Hardshells are drawn from aerial photographs or United States Geological Survey (USGS) quadrangle maps.
Boat currently used for mapping large bodies of water.
For lakes over 2,000 acres, the line of sight technique may be inaccurate due to the inability to see the shoreline across large bodies of water, and dangerous depending on weather conditions. Therefore, a larger boat and GPS equipment are used to map large lakes. For this method, transects are run based on latitude and longitude readings. Depth and position data are recorded by on-board computers and an echo sounder, and are referenced to a base station that is located on shore.
For both methods, location of transects depends on the scale of the map and the lake basin's complexity. For instance, a lake known to have steep drop-offs, submerged reefs, islands, and points requires more (and more closely spaced) transects than a lake with a bowl shaped basin. The number of transects and the spacing between them is determined during the sounding process.
Hazards within the lake and aquatic vegetation are recorded on the hardshell. Water surface elevation is measured, and various other biological information is collected.
Contouring method used for lakes mapped with line-of-sight method.
How is mapping information converted into a lake map?
For lakes sounded using the line of sight method, depth information from the graphs produced by the echo sounder are plotted along transects drawn on the hardshell (see figure at right). Contour lines, which connect points of similar depth, are drawn at 5-foot intervals. Once the contours are drawn, the map is captured and stored electronically, or digitized.
For lakes sounded using the GPS method, the data collected by the boat during sounding is combined with the data collected from the base station, resulting in data that provides accurate locations. After checking the data for quality, it is loaded into a computer program, and the contour lines are automatically drawn (see figure at right). Rivers, roads, landmarks, and other geographic features are added to the map, and the map is edited to ensure accuracy. The lake's surface area, littoral area (area 15 feet or less in depth), volume, mean depth, and shoreline length are calculated.
Computer screen display of contouring method used for lakes mapped with GPS method.
Copies of the completed map are distributed to Fisheries and Wildlife personnel. Maps are scanned and placed on the DNR LakeFinder, and are released to the Minnesota Bookstore where they can be purchased by the public.
Purchase printed lake maps at Minnesota's Bookstore.
Commercial use of lake maps and data
The DNR retains the copyright on these lake maps. To re-produce, re-publish, or utilize any lake information commercially, you must file a license agreement form with the Lake Mapping Program. Please download, fill out and send in the Lake Map License Agreement form
Send this form to:
DNR - Ecological Resources
500 Lafayette Road, Box 25
St Paul, Minnesota 55155