[Seavox] To business....

[Roy Lowry]

Hi John,

First, note that I was never proposing a guideline of 5% of the water column - it was min (10m, 5% of water column depth).  So for 3000m and 6000m the depth would have been 10m.  The reduction in layer thickness only kicks in at 200m (i.e. for shelf waters).

Although we have quite a few data scientists with an emphasis on the science rather than data in BODC and BADC I spent last week widening scientific consultation on the definition of 'surface' and 'near-bed'.  I got about 10 replies and the results were interesting.

As far as 'surface' goes there seems to be reasonable agreement to use a fixed value, with a bimodal distribution peaking at 5m and 10m.  I ran a mixed-layer depth computation algorithm on about 50,000 CTD casts held by BODC and only found a handful where the mixed layer depth (0.02 C definition) lay between 5m and 10m and these were in areas of strong haloclines like estuarine plumes.  Thus oceanographically I think 5m and 10m are pretty much the same and so erring on the side of 'inclusion' I'd go for a fixed 'guideline depth' for the surface boundary layer - defined as the portion of the water column where there is significant interaction with the atmosphere - of 10m.

'Near-bed' or 'benthic boundary layer' is more interesting.  There was general agreement that the thickness of the layer - defined as the proportion of the water column that is affected by the presence of the bed - varies as a function of water depth, particularly in shallow water.  There was also a difference of opinion on the thickness of the layer by discipline.  Physicists tended to think of the layer as the water column affected by drag which is significantly thicker than the thickness of the layer in which biogeochemical effects are detectable.  I feel the guideline min(10m,5% of water column depth) represents a reasonable compromise of the opinions that came in.

Unless there are further objections,  I'll go with these.

Cheers, Roy. 

>>> John Graybeal <graybeal at mbari.org> 11/7/2006 7:12 pm >>>
Roy,

Thanks, this is helpful.

The following is verbose, for only the Really Interested -- I'm seeing if Roy and I are on the same page. (Conclusions at the bottom.)

At 4:13 PM +0000 11/7/06, Roy Lowry wrote:
>3) It provides a neat way of discovering near-surface and near-bed data sets.  To search for these using absolute depths needs measurement depth, water column depth and the desired relationship between them to be incorporated into the query in some manner, which I've never managed to do satisfactorily.  Keywords plus a guideline algorithm to generate keywords from data labelled with absolute values is the most satisfactory way I've found

Here's how my thought process works, with respect to previous emails about determining near-bed. I think this mostly matches your own, except for (4).

1) It would be really valuable to have an indication of near-bed data.  (Ideally on a per-record basis, but for this application, let's make it on a per-file basis.)  I assume the existence of the keyword will mean "at least some points in this data set are in the indicated vertical range."

2) I agree with your experience: There is no elegant system to enable the derivation of distance from sea bed (can I call this 'altitude', for this thread only?) from other parameters.  At MBARI we've done systematic analyses of large numbes of extremely well-documented data sets, where we have both pressure/depth/lat/lon data, and detailed bathymetric profiles (= water column depth, roughly),  to identify algorithmically whether the measurement was "on the bottom."  We succeeded, but only to an approximation, and it required great effort.  This is not a realistic computation for most situations, especially in that it must be performed at every data point.

3) Given there are no great answers, we have three choices:
 a) Get someone to manually characterize the data set based on their knowledge of it.
 b) Take advantage of the necessary values to compute these results, which can either be "measured altitude" (from a sounder), or "water column depth - measurement depth."  If the values are available, calculate the results for every point and set the keyword accordingly.
 c) A combination of (a) and (b). 
My preference is (c) -- the flag/keyword gets set if any of the above conditions indicate the altitude is less than X for any measurement in the data set.  (Unfortunately a single X can not meet all science uses, but we'll have to live with that. Human judgment could perhaps override the precise number, in that case.)  In which case, we specify that meaningful data ranges (for altitude, in the case of this particular keyword) are an ideal source for setting this flag, but manual characterization of the data set is needed otherwise.  (And we understand that the absence of the keyword could just mean a bad or missing characterization process.)

4) For the many scientific purposes to which "near-bed" data can be put, the distance from the sea bed is arguably more important than the percentage of the water column. In shallow waters, the fact the measurement is 3 meters from the floor is more significant than the fact it is 0.5 meters from the air.  This rule of thumb is not 100% true, but I claim it is more than 50% true.  And intuitively I don't want "near-bed" in 6000m of water to be twice the size of near-bed in 3000m of water.  Anyone who considers using the data will have to consider the reality of the data set, and it's better to be inclusive (include some that may not apply) than exclusive (exclude some that may apply). 

CONCLUSIONS

So if I got to set X, I might ignorantly prefer a slightly smaller number like 3 meters, but I wouldn't worry about a different value for shallow waters.  It's still near the sea bed and many similar processes will typically apply.

John

P.S. I think what makes this seem complicated is that we're conflating the concepts of "depth" and "altitude" into a single keyword field.  From a data management perspective, one set of depth keywords and another set of altitude keywords would be better.  But as long as you get to use multiple keywords in a description, algorithmically it is equivalent.
-- 
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John Graybeal   <mailto:graybeal at mbari.org>  -- 831-775-1956
Monterey Bay Aquarium Research Institute
Marine Metadata Initiative: http://marinemetadata.org   ||  Shore Side Data System: http://www.mbari.org/ssds