How to merge ‘Winter Pool’ sonar data with ‘Summer Pool’ data — C-MAP Genesis Water Level Offsets Primer

This merged map, comprising 45 sonar-log files recorded at various times during a year of wildly varying water levels, is emerging as the most accurate and precise map of Lake St. Croix, because careful attention was paid to offsetting every sonar-log file to a standard water-level benchmark.

One of the best features of C-MAP Genesis is the ability to aggregate sonar data recorded in numerous trips over time in a complete, merged map. However, fluctuating water levels over time can impact the accuracy of aggregated maps if recorded water depths are not offset against a standard benchmark water level.

Naturally flowing rivers, dam-regulated river pools and vast reservoirs can all fluctuate greatly in water level from season to season, due to both natural and man-made forces. Natural forces include snowfall, rainfall and droughts. Man-made forces include flood-prevention measures and power-generation practices. Water levels are measured in feet above sea level.

Throughout a year, water levels on reservoirs – vast, sprawling lakes formed by damming rivers in valleys – fluctuate between “Winter Pool” and “Full Pool” (also called “Summer Pool” in some places). And in the spring, some reservoirs will reach “Flood Pool.” Winter Pool – the water level to which a reservoir is lowered in the late fall through winter – is a reservoir’s lowest water level, outside of drought conditions. Full Pool, as defined by the U.S. Department of the Interior, is the volume of water in a reservoir at “normal water surface,” i.e. the water level “when the reservoir is fully utilized for all project purposes, including flood control.”

Additionally, water levels on a reservoir at Full Pool can fluctuate from week to week, and even day to day, as power-generation and flood-control measures dictate. When water above a dam is released to the pool below it, giant turbines in the dam generate electricity. The process lowers the water level on the pool above the dam and raises it in the pool below.

For anglers who use C-MAP Genesis to create maps made from sonar data recorded during outings throughout an entire year, river-level fluctuations have the potential to make a mess of a master map that merges all their sonar data, because the water depth at any specific contour feature or waypoint could be 10, 20 or 25 feet (for example). So, if you recorded sonar data in the exact same location at three different water levels (resulting in three different depths in that spot), aggregating all your data into a merged map would result in a muddled mess … unless you apply water-level offsets to all your uploaded sonar-data files (AKA “sonar logs” or “trips”). Continue reading to learn how to set water-level offsets to ensure the accuracy of your merged maps.

Using public databases to set water-level benchmarks
The first step to applying water-level offsets to create an accurate merged C-MAP Genesis chart is determining an accurate water-level benchmark against which to compare. In the United States, 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
  • The U.S. Geological Survey via the National Water Information System.
(Click on image to view it full-screen)

The databases above should cover most large U.S. rivers, including the mighty Mississippi and the muddy Missouri, and popular reservoirs. Additionally, privately owned websites such as aggregate and archive water-level data for lakes and reservoirs across the country, including the ever-popular Tennessee River reservoirs of Guntersville, Pickwick, Chickamauga, Wheeler and Kentucky Lake, and the Coosa River reservoirs of Neely Henry, Logan Martin, Lay Lake and Lake Jordan. publishes each reservoir’s daily water level and its Full Pool level, and sometimes also its Winter Pool and Flood Pool levels (see example in the nearby screen shot). In the vast majority of cases, the Full Pool level is the best benchmark.

Once you’ve learned your waterbody’s Full Pool number, you’re ready to apply water-level offsets. Follow these steps:

  1. (Click on image to view it full-screen)
    (Click on image to view it full-screen)

    Upload all the sonar-log files you recorded on your favored waterbody (supported file formats are .slg, .sl2 and .sl3)

    1. Don’t know how to upload sonar-log files? Read this article
  2. After a sonar-log file successfully uploads to the Genesis cloud, open it in your Genesis dashboard – Click the button labeled “View Trip”
  3. Click the button labeled “Data Offset”
  4. Type in an offset number – positive offsets will make each reading deeper; negative offsets will make each reading shallower
    1. If the official water level for the day is 4.5 feet below Full Pool, type in “4.5” … if the level is 4.5 feet above Full Pool, type in “-4.5”
  5. Click the “Apply Offset & Reprocess” button
  6. Repeat the above steps for every sonar-log file recorded on the same date
  7. If water levels were different when sonar-logs were recorded on different dates, repeat the above steps, with the appropriate changes to the offset numbers, to all the files recorded on the different dates
  8. Merge all your sonar files for the same waterbody into a master map
    1. Don’t know how to merge numerous files into one aggregated map? Click HERE to learn how.

After-the-Fact off-sets
Do you have a bunch of uploaded sonar logs from the last couple years to which you did not apply water-level offsets? No worries – offsets for sonar logs recorded on most reservoirs and other regulated river pools can be easily applied after the fact. Simply check the dates on which you recorded (not uploaded) your old sonar logs, then look up the archived water level for that date on the reservoir/river pool. Open each sonar-log file and type in and apply an offset based on the historical info. After applying after-the-fact offsets, your sonar log files will require a short period of time to re-process.

Wild & Scenic Rivers, Natural Lakes
Determining water-level benchmarks for natural lakes and rivers is not as easy as it is for reservoirs. One must locate and analyze as much publicly available water-level data as possible and then make an educated estimate. Google should be able to help you determine if a state or local government body or water authority/district makes such information available online. In Minnesota, for example, the State Department of Natural Resources administers a popular citizen-science lake-level monitoring program.

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. (Click on image to view it full-screen)

Over the course of one year, a C-MAP colleague recorded 45 sonar-data files on Lake St. Croix, the name given a wide spot on the wild and scenic (and not dammed) St. Croix River on the Minnesota-Wisconsin border. In that year, the river level fluctuated a whopping 18 feet (image at right). Without correcting to an established benchmark, sonar data recorded when the river level was very high and very low could not be merged to create an accurate map. To determine a benchmark, we crunched the numbers in Lake St. Croix’s 13-month water-level history (as graphed in image at right) to establish a “spring pool” benchmark of about 676 feet above sea level.

Some natural lakes – especially those in remote areas – are not monitored for water-level fluctuations, and therefore no date exists from which to calculate a benchmark. In such cases, shoreline indicators can be valuable – flooded shoreline brush and vegetation indicate high water; exposed rip-rap and dock posts signify low water. Still, you’ll have to make an educated guess on how many inches high or low the water level is.

Another – and better – method is to establish an arbitrary benchmark depth at a specific location, be it a GPS waypoint near your preferred boat ramp, or even a depth reading at the deep end of the dock at the boat ramp. In this method, you simply take a depth reading at the end of the dock – or an exact GPS waypoint near the dock – before the first trip in which you record sonar data. You’ll need to write it down or type and save a note with your phone. On every ensuing trip, take a depth reading at the exact same spot – if the water is 6 inches higher than your original depth, you’ll subtract a half foot from all sonar logs recorded on that date; if the water is 6 inches lower, you’ll add a half foot to sonar logs recorded on that day.

Tidal Offsets
In tidal area, C-MAP Genesis automatically accounts for water level variations using the nearest buoy data.

4 thoughts on “How to merge ‘Winter Pool’ sonar data with ‘Summer Pool’ data — C-MAP Genesis Water Level Offsets Primer

  1. Greg I have been applying offset to all of my mapping since I commenced in 2014. I get my offset information from the Bureau of Meteorology here in Australia under the heading “River Height Bulletin”. This shows the level below full for all of our dams. It is arranged in state order. Unfortunately I can’t say the same for the social maps as some of it would have variations of 50 ft or more making the job of the mapping guys very difficult I would imagine. My merge maps though would be very accurate

    1. This is a very informative article. I wonder just how many people that contribute to the social maps even bother to make a depth offset. Well I can tell you that answer, none. Nobody makes an offset because there is no way to make one (unless you have a premium membership). This inability to apply a depth offset has been going on for years so I wonder just how accurate any of the social maps are.

      1. If a user recorded sonar logs when the water level was way above or way below the median water level — and did not apply a water-level offset — the log(s) won’t be included in Social Map.

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