One of the best features of BioBase EcoSound and its sister technology for anglers, C-MAP Genesis, is the ability to aggregate partial maps created over time into a complete map later. The recent blog post on Ten Mile Lake in Minnesota, USA, details a notable example of the power of aggregation. However, changing water levels over the course of time can impact the accuracy of aggregated maps if recorded water depths are not offset against a standard benchmark water elevation.
45 uploaded sonar logs; 18 feet of water level variation
Forty-five sonar logs recorded on Lake St. Croix (a wide spot on the St. Croix River in Minnesota, near Stillwater) were uploaded over the course of a year. Over that time, river levels ranged 18 feet (Figure 1). So a sonar log recorded in trip from June 2014 combined with a log from a trip in April 2015 will not produce a good map without a water-level correction. The more difficult decision facing water-resource professionals and anglers is what benchmark to use for their aggregated map.
 |
| Figure 1.Water levels from the Army Corps of Engineers Water Control District in St. Paul MN. The USACE is one of several free online sources of real-time water-level data for dammed rivers and reservoirs. |
Using public databases to retrieve water level information
In the U.S., three Federal Government Agencies collect, maintain and distribute real-time water-level data for thousands of lakes, rivers and reservoirs: The U.S. Army Corps of Engineers (USACE) through its RiverGages website, the National Oceanic and Atmospheric Agency (NOAA) via the National Weather Service’s National Advanced Hydrologic Prediction Service, and the U.S. Geological Survey via the National Water Information System. These databases should cover most large U.S. rivers and reservoirs.
For inland lakes, you might need to hunt a bit more for information on water levels, but Google is an amazing product and will help you determine if a state, local unit of government or water authority/district houses this information for your lake of interest. If they do, they likely publish the data on its website. In Minnesota, for example, the State Department of Natural Resources administers a popular citizen science lake-level monitoring program and I found a 10-year hydrograph for a lake I’ve mapped in the past (Figure 2). Note that despite some anomalies, water levels rarely vary more than 1 foot in this isolated headwater lake, which has a water-control structure at the outlet. So in this case, offsetting trips to lake elevation isn’t hugely important.
 |
| Figure 2. Water levels from Square Lake, Washington Co. Minnesota USA from citizen collected water level gauges. |
But let us return to Lake St. Croix, where offsets are important. Now we have to make a decision about what our benchmark water elevation should be against which all other trips should be adjusted. In the case with the St. Croix River, we needed a map that would be accurate at “normal” pool elevations during fishing outings in spring 2015. As such, my partner with whom I am cooperatively mapping the river, looked at the Army Corps hydrograph and we chose a benchmark of 676 feet above sea level as our mapping benchmark. My partner offset his May 2014 trips by -9 feet during the high-water periods on the St. Croix and documented the date, water-level during the trip, benchmark elevation, and the source of the data in the trip offset tab in EcoSound (also found in C-MAP Genesis; Figure 3).
 |
| Figure 3. Example offset and documentation for a high-water period on the St. Croix River near Stillwater, MN. Offsets to benchmark elevations and proper documentation are critical for accurate merges when trips are combined over periods of time when water elevations are changing. Transducer offsets can also be added if users desire to bring the water level up to the surface of the water |
 |
| Figure 4, This merge consisting of 45 sonar logs (and growing) spread over a period of a year of wildly varying water levels is emerging as the most accurate and precise map of the St. Croix on the planet because careful attention was paid to offsetting every trip to a standard water-level benchmark. |
Using observation and relative benchmarks for offsets
Often in rural areas, lake levels are not formally monitored and anglers and aquatic managers need different tactics to properly offset EcoSound or Insight Genesis trips. First, users could use a physical, temporary mark (crayon?) on the dock at the launch for instance to indicate the water level during each trip and offset each trip the distance between the current trip and the chosen benchmark trip. Alternatively, but perhaps less precisely, users could use shoreline indicators to understand the location of the lake’s high water level and where the current elevation is relative to that high water mark.
How Offsets Affect Vegetation Maps in BioBase
EcoSound renders aquatic plant height as percent of the water column that is filled with vegetation (% biovolume). Plants that grow very close to the face of the transducer or to the surface of the water will show as red and register a 100% biovolume value (Figure 5a and b).
 |
| Figure 5a. Percent of water depth fill with vegetation in Gibbs Lake, Rock Co., WI, USA with no depth offset. Don’t use an offset to create vegetation maps in scenarios when vegetation is actually growing to the surface. |
 |
| Figure 5b. Summary statistics from automated vegetation report |
If you add a positive offset, plant height will not change, but the proportion of water volume fill of vegetation will decrease and the map will get “cooler” (Figure 6a and b). If you add a negative offset, as is typical with automated tidal offsets to Mean Lower Low Water (see this blog), plant height will decrease (biovolume increase) if the offset water depth is less than the plant height. Lake Managers conducting vegetation surveys may use the offset tool strategically if plants were growing just to the transducer face but not above it. Adding the transducer-depth offset will change 100% values to more accurate sub-surface values.
 |
| Figure 6a. Same map as Figure 5a but with a 1-ft depth of transducer offset. Use the transducer offset to characterize sub-surface vegetation growth just below the transducer |
 |
| Figure 6b. Vegetation summary statistics in Figure 6a adjusted with the offset. Notice the maxium biovolume values increase with depth but always remain lower than 100% (surface of the water). |
The concept and practice of offsetting your maps with EcoSound and C-MAP Genesis is simple, but will make a world of difference in the quality of your maps for fishing or conservation. We hope you’ve found these tips useful.
One thought on “Offset Tool and Public Water Databases to Create Accurate Depth Maps”