[Qgis-user] Re: off topic, wgs 84 datum

[Gerry Creager]

In general terms, datums are well-defined mathematical constructs to 
describe a location, ultimately with respect the centroid of mass of a 
planet (like Earth, in this case). There are exceptions. While I listed 
Pulkovo and NAD27, both of these were subjective datums where a fair bit 
of inference went into their original adjustment. As a result, getting 
good long-term reproducability is problematic.

Newer datums such as WGS ITRF and NAD83 (and subsequent) are based on 
objective observational criteria and are adjusted for, among other 
things tectonic drift, as well as adjustments for significant plate 
movement as a result of, e.g., earthquake (the Taiwan seismic event of 
1999). Similarly, routine reobservation and adjustment allows refinement 
of the data. In the US, common datum information are archived officially 
at the National Geodetic Survey and generally are publicly available 
(although the National Geospatial-Intelligence Agency, if memory serves, 
is the responsible agency for WGS. In the UK, the Ordinance Survey are 
the responsible agency.

For truly long-term experiments, I prefer to use ITRF from the IERS: 
http://www.iers.org/IERS/EN/DataProducts/ITRF/itrf.html?__nnn=true

gerry

M.E.Dodd wrote:
> A naïve question, what if you are doing a long-term experiment and need to return to the same position say every 20 or 50 years to within a couple of cm how will this be affected?  Already positions we recorded 25 years ago have moved significantly due to continental drift but can account for this, how do all the little tinkerings with the same datum affect things? 
> 
> -----Original Message-----
> From: Gerry Creager [mailto:gerry.creager at tamu.edu] 
> Sent: 04 November 2010 11:13
> To: mtnbiketrail at zzz.com
> Cc: qgis-user at lists.osgeo.org
> Subject: Re: [Qgis-user] Re: off topic, wgs 84 datum
> 
> mtnbiketrail at zzz.com wrote:
>> this wiki entry says wgs 84 is to become obsolete??
>>
>> http://en.wikipedia.org/wiki/WGS84#cite_note-0
> 
> At the risk of humor, datums are ephemeral. However, NAD83, and for that 
> matter, NAD27 and Pulkovo 1942, are still with us. Periodic adjustments 
> may, or may not, result in renaming the datum in some way. For instance, 
> NAD83 was updated and adjusted once every 4 years until computational 
> and data management systems allowed it to be updated much more 
> frequently and less painfully. Now, the adjustment is annual, by 
> unnoticed by most users. Similarly, WGS84 has been adjusted annually. At 
> some point a new objective survey of control will be called for and a 
> new datum will be created. I don't anticipate a big change in our usage, 
> though.
> 


-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University
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