In an addendum to an earlier post, we continue to evaluate the accuracy and precision of BioBase depth outputs. Lowrance has been in the depth sounding business since 1957. They have tight factory calibration standards whereby depth should never be more than 2% different than the actual depth. Of course then we expect depths to be spot on on hard bottom surfaces where truth can be easily measured. But what about in mucky bottoms which are common place in many lakes, ponds, backwaters throughout the US and abroad? With this in mind, in late May of 2012, we traveled to Pool 8 of the Mississippi River near LaCrosse WI to do some testing in a mucky, moderately dense vegetated backwater (Figure 1). At some point we have to step back and ask, “what is the bottom of a body of water?”
|Figure 1. Vegetation cover and biovolume (% of water column occupied with vegetation) in Pool 8 of the Mississippi R. in LaCrosse WI on 5/29/2012. Average biovolume was 30% during the survey.|
The most difficult aspect of this testing was to get an objective estimate of the true depth. In other words, where exactly did the plants end and bottom start? Typically, investigators use a survey rod like that seen in Figure 2 to estimate actual bottom based on where they feel resistance on the survey rod. Piece of cake over sand. Not so easy over flocculant silt and muck or vegetative areas.
|Figure 2. Measuring bottom with a survey rod in a mucky Minnesota Lake. Typically, the survey rod will sink several inches into the bottom before the surveyor feels resistance and judges the depth to the bottom|
Many experienced surveyors will tell you that the rod will sink into the muck some distance before you feel resistance. There is a positive correlation in the distance it sinks and how mucky the bottom is. So, we went into this investigation expecting deeper rod depths measured than ciBioBase outputs.
Accurate and precise results in mucky, vegetated bottoms
After 30 points measured with the survey rod, we compared the results with the ciBioBase depths measured in the same location. We were pleased to see very high precision with a Coefficient of Determination (R^2) of 0.94 and a systematic difference in depth of only 4.9″ (Figure 3). The depth of 4.9″ was quite possibly the average depth where we first felt resistance of the survey rod. The upshot here is that ciBioBase depth outputs are highly precise, consistent and accurate even in mucky vegetated bottoms.
|Figure 3. Accuracy and precision of ciBioBase depths measured against depths collected with a survey rod in the mucky, vegetated backwaters of Pool 8 of the Mississippi River near LaCrosse, WI.|