Ray Valley(a) and Eli Kersh(b)
(a) BioBase Product Expert and Aquatic Biologist
(b) BioBase Ambassador, Certified Lake Manager, and President of LakeTech Inc.
The term “Fit for Purpose” has recently become a popular term in our circles in the context of mapping and surveying water bodies, and it refers to utilizing the appropriate equipment, methods, and budget for the task at hand.
This principle acknowledges that one type of equipment and method may not be suitable for every scenario. For instance, whether you’re assessing the storage volume of municipal stormwater ponds or maintaining a river channel for commercial navigation, the required level of accuracy will and should be different. While high accuracy and precision may be crucial for certain projects, it might not be necessary for others.
The cartoon by the webcomic Randall Munroe is particularly relevant (and funny).
When we are mapping waterbodies, are we interested in finding Waldo or mapping the grains of sand? Evaluating the economic Return On Investment (ROI) and considering the costs associated with different levels of accuracy is essential. In some cases, the assurance provided by expensive survey-grade equipment may be warranted, but in many others, it may not be necessary.
Even with investments in survey-grade equipment with published assurances of high levels of accuracy, the Army Corps of Engineers in their 700 page 2013 epic “Hydrographic Surveying” acknowledges actually achieving the published levels of both vertical and horizontal accuracy are difficult due to a myriad of environmental variables that cannot be controlled (Appendix 1).
Further, recent advancements in consumer and computing technology (including crowd-sourcing and other novel data collection and analysis approaches) have narrowed the gap in data quality between consumer and commercial survey devices. Thus quality data can be acquired more frequently by more people at a low cost (Figure 2).
“Map Off” on California Park Lake
In May 2023, investigators from UC-Davis and LakeTech with funding from Kasco Marine were interested in doing detailed modeling of diffused aeration and the near and far field flow imposed by a dual-loop diffuser in a small strongly stratified lake (California Park Lake in Butte Co. CA). Part of this modeling required a bathymetric study to get estimates of the total volume of the different layers of the water column in the lake. Also, a bathymetric study is critical to understand where specifically to place aeration systems and the physical size of the system needed to do an adequate job of aerating the water column. So bathymetric analysis is really a “first principles” type of exercise for any management that calls for aeration.
Through some fortuitous circumstances, the opportunity arose to conduct multiple bathymetric surveys with different types of survey equipment and processing software and compare the outcomes.
Single Beam, Side-scan, and Multibeam
Traditional sonar systems like those processed by BioBase, emit a single beam of sound that bounce off objects in the water and return to the sensor, providing information about the depth and location of those objects.
In contrast, side-scan (2 beams) and multibeam (many beams) sonar systems emit wide swaths of sound waves covering a wide area beneath and to the side of the survey vessel. By covering wide swaths of the bottom, these sonar systems and analysis software can create detailed, high-resolution maps of the seafloor or other underwater structures much faster and more efficiently than single-beam sonar systems. These sonars typically thrive in open ocean environments or for inspection of man-made submerged structures in highly engineered waterways. However, for a basic bathymetric survey on a lake as small as California Park Lake, one could attain a similar coverage with single beam compared with multibeam in almost the same amount of time.
Method 1: Consumer Sonar and BioBase’s EcoSound
On May 31st 2023, we used a Lowrance HDS-9 Live with an Active Imaging 3 in 1 single beam transducer along with BioBase’s EcoSound cloud processing to create a bathymetric map of the southern basin of California Park Lake (Figure 3).
Because of the small size of California Park Lake and the ease at which Lowrance is deployed from the boat, it took longer to commute to the lake than it did to survey it with the Lowrance. We were able to comprehensively cover the lake while monitoring features and depth changes on the Lowrance screen. Where complexities in the bottom revealed themselves, we adjusted our data collection intensity to ensure detailed coverage of small features. When processed with BioBase’s EcoSound, point densities are typically separated by 1 meter along tracks. We used an interpolation setting in BioBase of 1 m to produce a continuous grid of 1 m grid cells across the lake. Water depths were adjusted to an elevation benchmark (Full Pool) in BioBase. Data collection using this method took only one hour and twenty minutes and approximately 30 minutes to process in the cloud with BioBase and produce the required data sets. Provided the Lowrance and transducer is installed correctly, anyone who knows how to drive a boat and press record on the sonar, could produce the same quality output. The low cost of the equipment and software might be the most attractive quality of this method. An all-in-one Lowrance kit for instance is $2000 (a lower cost option was used in this study) and a single waterbody BioBase EcoSound Subscription is only $499.
Method 2: Side-scan Sonar System (Boat Based)
In order to focus on the differences in detail and cost and to avoid critiquing any one brand, we keep the non-BioBase systems anonymous. Partners at UC Davis used a high-end dual-beam, interferometric side-scan sonar device deployed from a jonboat.
Time on the water with the UC Davis system was a bit quicker than the BioBase survey, but not by much (1 h vs 1 h 20 min with BioBase). Processing these high volume datasets required advanced desktop software and a high level of expertise to apply the various calibration corrections prior to creating the bathymetric map and summary statistics. Side-scan systems like the one used by UC-Davis do indeed create beautiful maps of incredible detail (Figure 4).
Method 3: Survey-grade Multibeam System 2 (USV)
Finally, Unmanned Surface Vehicles (USV’s) are exploding in popularity and becoming easier to equip and deploy for a variety of water assessment needs. A manufacturer of USV’s equipped with a multibeam system and high resolution, Real-Time-Kinetic (RTK) differentially corrected GNSS also participated in the study. Compared with the UC-Davis system, costs were modestly lower with the USV system. Base station corrections of GNSS position was required to achieve the cm-level position accuracy. The map detail using the USV system was very similar to the UC-Davis System.
Bathymetric Analyses
BioBase EcoSound
BioBase’s EcoSound is a subscription-based service and calculates water volume automatically on a whole-system level with the “premium” level subscription (EcoSound Habitat+). At all subscription levels, the user can use a polygon tool to digitize an area of interest and BioBase will clip and summarize waterbody statistics for that specific area. We used the polygon tool in this case to isolate analyses to the lower basin of California Park Lake since the BioBase survey originally covered both basins (Figure 4).
Multibeam Survey Methods 1 and 2
We used ESRI’s ArcPro’s raster analysis features and hydrographic capacity tool to calculate metrics from the other bathymetric surveys.
Table of Results
Table 1. Summary table of basic bathymetric statistics for the lower basin of California Park Lake.
Table 2. Summary of differences in water volume between the different methods. 
We anticipated we would see similar results among the different platforms, but the level of closeness in lake-wide depth measurements and water volume estimates was a surprise! The Lowrance BioBase system landed squarely in between the different multibeam systems with only a 0.5% difference in water volume estimates. Also surprising was that the differences between multibeam systems were larger than any BioBase difference.
BioBase’s EcoSound: Fit for the purpose of water volume analysis
This high-level study evaluating alternative bathymetric survey hardware and processing options clearly demonstrated that off-the-shelf consumer sonar and automated mapping technology has come a long way. In the case of volumetric studies in small waterbodies, consumer technology like Lowrance and automated cloud processing with BioBase can produce results of similar accuracy at a much lower cost compared with higher-end multibeam systems and advance hydrographic software. Multibeam systems are unmatched in terms of the visual detail and bottom coverage they can attain in large deep systems. In other words, these systems are fit for the purpose of mapping the ocean floor and extracting exquisite detail of bottom features/reefs, wrecks, and underwater construction. However, given the high equipment and software costs and level of knowledge to conduct the surveys and analysis, perhaps high end side-scan or multibeam systems are not fit for the purpose of water volume analysis of small, shallow waterbodies; or where data are needed more frequently in highly dynamic environments. For those interested in digging deeper into this topic, the earlier referenced Army Corps of Engineers Hydrographic manual presents a nice decision matrix of single- vs multi-beam systems.
Appendix
Excerpts from the 2013 Army Corps Manual EM 1110-2-1003 “Hydrographic Surveying.”
(p 54) 3-2. Discussion. No measured depth is without error. Unlike visual topographic or construction survey measurements, acoustic depths are indirectly measured using various forms and combinations of time difference (amplitude detection), phase detection, or phase difference (interferometric) measurements. These measurement methods contain varying magnitudes of acoustic reflectivity and signal/noise that must be resolved into a “best estimate” of the depth. Spatial variations in bottom soil materials will also vary the recorded depth returns. Thus, an absolute “accuracy” of a depth measurement at a given point is rarely, if ever, known. In addition, hydrographic survey depth measurements are referenced to the local water surface, which will typically have tidal or river stage modeling uncertainties (biases) relative to a distant reference gage and datum
(p 59) 3-6. Horizontal Accuracy Standards. Specifying that a hydrographic survey depth measurement meet some required horizontal accuracy standard is difficult in practice. The horizontal positioning uncertainty of a measured depth is a function of various factors, primarily the GPS positioning method (code DGPS or carrier RTK), multibeam outer beam array limits, acoustic footprint size, and vessel motion compensation employed. No specific Performance Test type procedure exists to quantify the resultant horizontal accuracy of a depth observation—positional uncertainties are buried within the error budget of the depth results in a traditional Performance Test, and any standards based on that test. Thus, horizontal accuracies can only be roughly estimated using TPU techniques described in Appendix D.
