Hi Jonathan,
Just to chip in - WGS84 is an ellipsoidal approximation to the geoid,
so I don't think "depth_below_geoid" etc would be right. I would
prefer "depth_below_ellipsoid" where the ellipsoid is defined
somewhere, or perhaps even "depth_below_wgs84" considering that WGS84
is an extremely commonly-used ellipsoid.
Regards, Jon
On Wed, Sep 10, 2008 at 1:18 PM, Jonathan Gregory
<j.m.gregory at reading.ac.uk> wrote:
> Dear Dale
>
>> + Create a z dimension with a value of 1
>> + The vertical coordinate variable 'z' (i.e. z(z) ) will be
>> used to fix the location of the sensor in vertical space
>> (i.e. not the distance below the water's surface) referenced
>> to a datum.
>> + For the datum we use WGS84.
>
> By "WGS84" I presume you mean their geoid (I am not an expert in vertical
> datums). If so, as Olivier says, you could request a new standard name of
> depth_below_geoid or height_above_geoid. As an alternative to a dimension of
> size 1, you can use a scalar coordinate variable (CF 5.7).
>
>> + The depth variable (pressure sensor) gives depth in vertical
>> distance below surface (which varies with the tide for fixed
>> instruments).
>
> The standard name of "depth", which you have used, means depth below the
> (sea) surface, which I think is what you want. Since it is not a coordinate
> variable, it doesn't need a positive attribute.
>
> If I understand correctly, the offsets are giving the height_above_geoid of
> various tidal datums. I suggest that a more CF-like way to do this would be to
> provide these offsets as data variables, each with its own standard name; the
> standard names would be e.g. height_above_geoid_of_mean_lower_low_water. You
> could request such standard names. If they are all standard tidal datums, that
> might be unproblematic! The tidal datum epoch would then naturally be given as
> the bounds of the time coordinate variable for these various datums.
>
>> The instrument goes up and down with the tide and the
>> sensors are at a fixed distance below the surface. The vertical
>> values are all distance below the surface. How do we handle that?
>> One solution is to keep the vertical dimension of unity and have a
>> vertical coordinate variable referenced to depth below the surface.
>
> Yes, I think that is the right solution.
>
> Cheers
>
> Jonathan
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--
Dr Jon Blower
Technical Director, Reading e-Science Centre
Environmental Systems Science Centre
University of Reading
Harry Pitt Building, 3 Earley Gate
Reading RG6 6AL. UK
Tel: +44 (0)118 378 5213
Fax: +44 (0)118 378 6413
j.d.blower at reading.ac.uk
http://www.nerc-essc.ac.uk/People/Staff/Blower_J.htm
Received on Wed Sep 10 2008 - 08:18:01 BST
