Just because I say my parameter was measured relative to a particular
datum, says nothing about the accuracy or the measurement itself. I
don't think selection of a datum, whether as the de facto default or
explicitly, can be construed in any way to imply accuracy of the
coordinate data.
Since potential error will be introduced by the assumption of *any*
datum -- and the lack of assumption of a datum does nothing to get rid
of the potential error -- I'm not sure there is value in avoiding a
default value for datum. Well, other than saying in a loud voice
"Don't do that", which people will probably do anyway. (Hmm, maybe
there is value in this approach....)
There is a pretty good analogy to the use of accuracy metadata with
data. If this is truly important, than we accept no data without datum
(in the analogy case, we accept no data without accuracy metadata --
note how rare this practice is in the real world). But rather than
saying "data should not be accepted", do we want to say "data can be
accepted at the option of the receiving process, and should be assumed
in the default datum"?
John
On Apr 19, 2010, at 08:48, Nan Galbraith wrote:
Just want to point out that many of our older datasets, which are
still available - and actually still used - predate the routine use of
GPS on ships. These datasets are from buoys that were set using
dead reckoning.
And, although we now have pretty nearly exact anchor positions,
the buoys on our moorings deployed in about 5 km of water
typically have 5 km watch circle diameters.
So, although the datum specification should be encouraged, there's
a definite drawback to assuming a default. And, the position's
accuracy
is probably more significant than the datum spec for a lot of in
situ data-
so we should really encourage the use of QC terms for all coordinates.
Thanks - Nan
n Apr 19, 2010, at 08:51, Bentley, Philip wrote:
Hi Jon,
Indeed, the UM global climate model, for example, assumes a spherical
earth.
In the absence of specific datum information I think I would probably
caution against assuming any particular datum is applicable. I can
understand the temptation to use WGS 1984 as a default because it is
widely known and used. However, this implies a degree of accuracy in
the
coordinate data which in many instances may well not be justified -
indeed its use might even compound any relative location errors.
As you rightly suggest, if the end-use/application demands a
particular
spatial accuracy then any datasets which demonstrably do not not meet
those requirements should probably not be included in the analysis -
or
else should be with appropriate caveats, provisos and disclaimers!
Regards,
Phil
Mike Grant wrote:
On 19/04/10 15:43, Jonathan Blower wrote:
specification, which datum should be assumed? Spherical Earth?
WGS84?
If you're picking one at random, I'd go for WGS84 - that's a pretty
safe
bet for a lot of remote sensed and GPS related data.
Cheers,
Mike.
Hi all,
The CF conventions define a means to specify the datum used in
certain
map projections
(http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/cf-conventions.ht
ml#grid-mappings-and-projections). In the (common) case of a data
file
containing latitude and longitude axes, but no explicit datum
specification, which datum should be assumed? Spherical Earth?
WGS84?
The horizontal error resulting from use of an incorrect datum can be
significant for high-resolution and/or local dataset (hundreds of
metres).
Cheers, Jon
--
*******************************************************
* Nan Galbraith (508) 289-2444 *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
* Woods Hole, MA 02543 *
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