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.

FLW Bass Fishing mag: C-MAP Genesis gives anglers ‘serious advantages’

C-MAP Genesis is making headlines again. FLW Bass Fishing magazine, in its Nov-Dec 2017 issue, says Genesis Edge, our premium custom-mapping service,  offers “serious advantages for tournament anglers who need to decipher patterns quickly and line up on prime spots more efficiently.”

We couldn’t agree more.

Genesis Edge is a premium upgrade to our free marine-mapping service, C-MAP Genesis (formerly known as Insight Genesis). While the free version includes unlimited downloads of community-sourced Social Map charts and custom-color depth shading, Genesis Edge offers even more. Its key features include sonar replay, an option to keep personally mapped areas private and creation of bottom-hardness and submerged-vegetation map layers.

Although several companies “have flirted with the concept” of custom mapping, FLW said, “perhaps none has figured it out as well as C-MAP Genesis.” Continue reading

How to subscribe to Genesis Edge, C-MAP’s premium mapping service

While you can’t make your own luck, you can chart your own success. For those with drive, there’s better maps — Genesis maps. To catch more and bigger fish tomorrow, subscribe to Genesis Edge today.

Genesis Edge is a premium upgrade to the free marine-mapping service C-MAP Genesis (formerly known as Insight Genesis). While the free version includes downloads of community-sourced digital Social Map charts, the ability to add personally mapped areas to Social Map, and custom-color depth shading, Genesis Edge offers even more. Its key features include sonar reply, an option to keep personally mapped areas private and creation of bottom-hardness and submerged-vegetation map layers. For recreational and tournament anglers alike, that’s a game-changer.

This article includes step-by-step instructions for subscribing to Genesis Edge, as well as more information about how you can use Genesis Edge features to catch more and bigger fish. Continue reading

Genesis Edge: How to opt out of subscription auto-renewal

In order to prevent interruption of service, annual and monthly Genesis Edge subscriptions default to auto-renew. This means that on the first day after your initial subscription period ends, a new subscription period and billing cycle of the same duration as the original will begin. You may, however, opt out of auto-renew by following these steps: Continue reading

C-MAP and Fishbrain partner to integrate maps and app

An innovative integration of map and app enables anglers to use historical catch locations to catch more and bigger fish in the future.

C-MAP, the leading supplier of digital marine cartography and cloud-based mapping solutions, and Fishbrain, the world’s largest angler-focused app and social network, launched a partnership that will show Genesis Edge subscribers tens of thousands of public Fishbrain catch locations on C-MAP Genesis charts online and let Fishbrain Premium subscribers see waypoints and their GPS position overlaid on C-MAP Genesis maps on their phone.

Genesis Edge is a premium upgrade to the free marine-mapping service C-MAP Genesis (formerly known as Insight Genesis). Fishbrain premium, an upgrade to the free app, provides anglers additional tools and insights designed to help them catch more and bigger fish. Continue reading

How to create and use a C-MAP Genesis bottom-hardness map layer to catch more and bigger fish

Many fisherman “beat the bank” because they have more confidence targeting visible cover. But the fish in those areas get pounded, making them harder to catch. With an C-MAP Genesis bottom-hardness map, however, you can fish the bottom as confidently as you do the bank, targeting offshore structure like hard-to-soft-bottom transitions and the fish-holding sweet spots on humps, reefs, and shoals, etc. This article will teach you how.

Although it’s a great fish-finding tool, StructureScan alone can’t really show you the size and shape of hard-bottom areas like gravel patches, clay slicks and shellbeds. But the C-MAP Genesis (formerly Insight Genesis) bottom-hardness layer can, making such areas stick out like “Fish Here!” signs on your map. It overlays onto your contour lines a color-coded representation* of the size, shape and location of areas of different bottom hardness: dark orange areas show the hardest bottom; a lighter orange indicates the next-hardest bottom; tan is the next-hardest bottom; and pale yellow shows the softest-bottom areas.
Continue reading