[tdwg-content] Usage of elevation, depth & distanceAboveSurface

[Markus Döring]

Hi all,

I am notoriously confused how to use the dwc terms that define the vertical position of a location. 
For each of elevation, depth and distanceAboveSurface there is a minimum and maximum term defined. Taking min/max aside, how do those 3 terms play together?

From my common understanding elevation refers to the vertical position of the location relative to a reference point, usually the sea level. With negative values for places below the sea.
Wikipedia seems to follow the same definition: http://en.wikipedia.org/wiki/Elevation

The Darwin Core definition is a bit lose, but close: 
"The lower limit of the range of elevation (altitude, usually above sea level), in meters."

The use of depth in that view is then only to give an idea how far below the (water) surface the location is (think pressure) - especially for places not in the sea like lakes.
In the case of locations under the sea depth should then always be the negated value of elevation, is that true?

I can see the use of the third term distanceAboveSurface from the drilling core in the only example given, but that is rather exotic. Would it not replace the term depth as a more generic version unrelated to water, for example to refer to locations up in trees?

Would a use of the terms like this be correct?

(A) a fish found 800m below the sea:

dwc:minElevationInMeters = -800
dwc:minDepthInMeters = 800
dwc:minDistanceAboveSurfaceInMeters = -800



(B) a frog found on a tree, 20m above the ground which grows in 760m above the sea (measured from the ground):

dwc:minElevationInMeters = 780
dwc:minDepthInMeters = 
dwc:minDistanceAboveSurfaceInMeters = 20


When reading the definition for minimumDistanceAboveSurfaceInMeters I doubt my understanding is correct and instead elevation is used as the primary reference point on top of which depth and then distanceAboveSurface is added:
"The lesser distance in a range of distance from a reference surface in the vertical direction, in meters. Use positive values for locations above the surface, negative values for locations below. If depth measures are given, the reference surface is the location given by the depth, otherwise the reference surface is the location given by the elevation."


It would be good to have that documented a bit more explicitly.
Any advice appreciated.

best,
Markus