The small boat weaved in and out of coves, along the shoreline, and to the deepest water. It was July 7, 2015, when MassWildlife was spotted on Manchaug Pond. The mission? Bathymetry: the measurement of the depths, the bottom topography, of the pond for the purpose of updating the state’s map of Manchaug Pond.
Anglers and boaters have accessed these maps through the Division of Fisheries and Wildlife’s Department of Fish and Game Office of Boating Access which highlights the lakes characteristics and fisheries listing the public access ramps of the great ponds throughout the Commonwealth. Developed in the 1940s and 1950s, the maps were in need of updating as modern technology in the surveying could more accurately portray Manchaug Pond’s higher waterlevel and topography and reflect the much higher level the reservoir has been managed in recent decades.
The new pond map for Manchaug utilized 22,000 sonar soundings to show the contours and depth of the water from shore to 37 feet at the time of the summer survey. In color, with details as to islands, submerged rock walls, feeding streams, topography of surrounding hillsides as well as the fisheries information makes the update a great resource for all who enjoy Manchaug Pond.
Mass Wildlife explains* that “to create the new pond maps, biologists use GPS sounders to record depth measurements and GPS coordinates at a user-defined interval; in our case every second. Depth readings are stored in the depth sounder as a boat is driven in a grid pattern on the lake or pond. Depth information is then imported into a geographic information system and plotted; any gaps are assigned a depth using a statistical model. While the original pond maps were created using anywhere between 50 and 150 data points, new maps often incorporate 8,000 to 12,000 points. Modern statistical and data collection techniques using GPS, combined with a roughly 100-fold increase in the amount of data, result in pond maps that are far better representations of actual bathymetry. Additionally, the use of modern equipment such as GPS measures depths and locations with pinpoint accuracy unlike past methodologies which were largely dependent upon a biologist’s approximation of location on the pond when the sounding was taken.”
60 years ago, biologists created the original maps “by motoring (or in some cases rowing) a boat in a straight line across a lake at a constant, known speed. Echo soundings were taken at various points along this line and a timer was used to measure the time it took to travel the transect and the amount of time between each sounding. Depths were assigned to locations on a map by first hand drawing each transect then using average speed and time signatures from each sounding to estimate its location along a specific transect. Depth contours were then estimated and drawn often simply following the shape of the shoreline.”
Visit our ManchaugPond.org website’s “About the Lake” page to view the Mass Wildlife map and fact sheet PDF as well as other maps of the entire watershed and the Blackstone River System. Check out the Pinterest map board of new and old maps of the watershed towns of Douglas, Oxford and Sutton which include Manchaug Pond.
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